Documentation

Privacy & Security

Your financial data is sensitive. Here’s exactly what happens to it — and what doesn’t.

How Your Data Flows

On paid tiers, the calculation engine is compiled to WebAssembly (WASM) — a format that runs natively in your browser, just like JavaScript, but faster and more secure. Your broker file never leaves your device.

When you upload a broker CSV, four things happen — all inside your browser tab:

1. Your browser reads the file — the CSV is parsed locally. The raw file never leaves your device.
2. Exchange rates are fetched — HMRC’s official monthly rates are downloaded so foreign-currency trades can be converted to GBP.
3. The engine calculates your report — all matching rules, option strategies, corporate actions, and income calculations happen inside the WASM engine running in your browser.
4. You see the result — the finished report is rendered on screen. You can download it as a PDF.

Data Storage by Tier

On paid tiers (Fish & Chips and High Tea), the WASM engine runs entirely in your browser — your data never leaves your device. On free tiers (Museum and Tea with Milk), your CSV is sent to the server for calculation but is never stored.

Why WebAssembly?

We chose WebAssembly for three reasons:

Analytics & Cookies

We use Vercel Web Analytics to understand how many people visit the site. It collects page views and country-level geography — no personal data, no IP addresses, no fingerprinting. It is privacy-compliant by design and does not use cookies for tracking.

We do not use Google Analytics, Facebook Pixel, or any advertising tracker.

What We Don’t Do

• We do not sell, share, or send your data to any third party
• We do not store your broker files, transaction history, or calculated reports
• We do not use advertising cookies or tracking pixels
• We do not fingerprint your browser or device

Share Matching Rules

How HMRC matches your sell transactions to your buy transactions to calculate capital gains.

When you sell shares, HMRC doesn't let you choose which purchase to match the sale against. Instead, three rules are applied in a strict order. You can't skip or reorder them.

1. Same Day Rule

If you buy and sell the same stock on the same calendar day, those transactions are matched first. This prevents you from selling at a loss and immediately rebuying to "harvest" the loss.

2. Bed & Breakfast Rule (30-Day Rule)

After same-day matching, any remaining sold shares are matched against purchases made within the next 30 calendar days. This anti-avoidance rule stops you from selling shares and rebuying them shortly after to create an artificial loss. The 30-day window runs forward from the sale date, not backward.

3. Section 104 Pool

Any shares not matched by the first two rules come from your "pool" — a running total of all your purchases, with an average cost per share. Think of it like a bucket where shares go in at various prices, and the calculator works out the average.

Short Positions

If you sell shares you don’t own (short selling), the calculator tracks the short position separately. When you buy to cover, purchases are matched to the short sale in first-in-first-out (FIFO) order. Gain or loss is calculated as the difference between the sale proceeds and the covering purchase cost.

How Fees Are Handled

When you buy shares, any broker fees and commissions are added to your cost basis. When you sell, fees are deducted from your proceeds. The calculator handles this automatically.

Option traders: if you're assigned shares from a short put, the Same Day and B&B rules still apply. Sell assigned shares and rebuy within 30 days? B&B kicks in.

Corporate Actions

How stock splits, mergers, spin-offs and ticker changes affect your cost basis.

When a corporate action occurs, the calculator adjusts your Section 104 pool automatically. Here is what is supported:

Stock Splits

When a company does a stock split (e.g. 4:1), your share count increases but your total cost basis stays the same. You just have more shares, each worth less.

Note: Stock splits during a Bed & Breakfast 30-day window are handled automatically. The calculator adjusts matching quantities so pre-split disposals match correctly against post-split acquisitions.

Cash Mergers

When a company is acquired for cash, all your shares are exchanged for cash at the merger price. The calculator treats this as a disposal — you receive proceeds and a capital gain or loss is calculated against your cost basis.

Spin-Offs

When a company spins off a division into a new public company, your cost basis is split between the parent and the new company. By default, 10% of your cost basis transfers to the spun-off shares. The 10% default is a placeholder — you should adjust the percentage based on actual market values on the spin-off date. Check the company’s investor relations page for the official allocation and configure it in the Settings panel.

Ticker Renames

When a company changes its ticker symbol, the calculator merges the old and new pools automatically. Your cost basis transfers to the new symbol with no tax event. The following renames are handled automatically:

Return of Capital

Sometimes a company returns capital to shareholders. This isn't income — it reduces your cost basis instead. If you later sell the shares, your gain will be higher because your cost basis is lower.

Stock Dividends

When a company pays a dividend in shares instead of cash, the new shares are added to your Section 104 pool. Their cost basis is the market value on the dividend date. This is not a disposal — no capital gains tax is due at the time you receive the shares.

Transfers In & Out

When you transfer shares between brokers, no CGT event occurs:

• Transfer In — shares transferred from another broker are added to your Section 104 pool with their original cost basis.
• Transfer Out — shares transferred to another broker are removed from the pool. The receiving broker inherits the cost basis.

Options Trading

How call and put options are taxed under UK rules — from simple covered calls to complex multi-leg strategies.

Options are taxed very differently from shares. The key difference: each option contract is a separate asset. They are never pooled together like shares — even if you have multiple contracts of the same series. HMRC also has a special rule: when you write an option, that date is the CGT event — not when it closes.

How Options Are Taxed

There are three ways an option position can end: you close it, it gets exercised/assigned, or it expires. Each has different tax treatment.

Writing (selling) an option — the premium you receive is treated as disposal proceeds. The date you wrote the option is the CGT date. Here is the simplest case — writing an option that expires worthless:

Buying an option — the premium you pay becomes your cost basis. Sell it later for a gain or loss.

Expiration — when an option expires worthless, the outcome depends on which side you were on:

Exercise and Assignment

This is the key concept: when an option is exercised or assigned, HMRC doesn't tax the option separately. Instead, the premium adjusts what you paid (or received) for the stock. Think of the premium as part of the stock transaction.

Exercise — you exercise your long option. The premium you paid gets added to the stock cost:

Assignment — your short option gets assigned. The premium you received reduces the stock cost:

Cross-Tax-Year Trades

If you write an option in one tax year and close it in the next, the calculator splits the tax correctly across both years. The premium is taxed in the year you wrote it, and the closing cost creates a repay entry in the closing year.

Supported Strategies

The calculator processes each leg of a strategy independently — HMRC treats each option as a separate asset regardless of how your broker groups them. Here is what we test and support:

Crypto

How UK Capital Gains Tax applies to cryptocurrency, and how we handle your Binance and Coinbase data.

How HMRC Taxes Crypto

HMRC treats crypto as property, not currency (CRYPTO22200). Selling, swapping, or spending crypto is a disposal — and Capital Gains Tax applies. The gain or loss is the difference between your disposal proceeds in GBP and the cost from your Section 104 pool.

The matching rules work the same way as for shares: Same Day first, then Bed & Breakfast (30 days), then Section 104 Pool. See Share Matching for details.

Crypto-to-Crypto Swaps

Swapping one token for another (e.g. BTC for ETH) counts as two events under HMRC rules: a disposal of what you gave up and an acquisition of what you received. We split these automatically, which is why your report may show more lines than trades.

Fees are allocated to the first side only, to avoid double-counting.

Stablecoins

Stablecoins are pegged to fiat, so we map them to their fiat equivalent. A BTC/USDT trade is treated as BTC/USD — and the USD amount is then converted to GBP at the HMRC daily rate.

We recognise USDT, USDC, BUSD, DAI, TUSD, FDUSD, USDP, UST, USD1, USDS, USDSOLD, RLUSD, PAX, and VAI as USD equivalents. AEUR and EURI map to EUR, GYEN to JPY. Anything not on the list is treated as a crypto-to-crypto swap and priced independently.

Both Binance (Trade History + Statements) and Coinbase Retail (all four header variants) are supported. See Supported Brokers for export instructions.

Fees

Exchange fees can be paid in different currencies. We handle each case:

• Fee in the quote currency (e.g. USDT) — used directly
• Fee in the traded asset (e.g. you buy ETH, fee charged in ETH) — converted via the trade price. If you’re selling, fee tokens are also a disposal from your pool per CRYPTO22280 — so the disposed quantity increases to include them
• Fee in a third token (e.g. BNB) — converted to GBP at the daily price. If the price isn’t available, the fee is recorded as zero with a warning

Staking & Airdrops

If you upload a Statements CSV (see Supported Brokers), staking rewards and airdrops are also processed:

• Staking rewards — reported as miscellaneous income at the GBP value on the date received, and added to your Section 104 pool at the same value. Covers earn products, staking interest, referral commissions, and card cashback
• Unsolicited airdrops — added to your Section 104 pool but not treated as income on receipt (CRYPTO22500). Airdrops received in return for a service may be taxable as miscellaneous income (CRYPTO22400). The CGT event happens when you later sell or swap those tokens

How Prices Are Converted to GBP

Every transaction needs a GBP value. The conversion path depends on how the pair is quoted:

1. GBP pair (BTC/GBP) — already in GBP, no conversion
2. Other fiat pair (ETH/USD, SOL/EUR) — converted to GBP at HMRC’s daily exchange rate
3. Stablecoin pair (BTC/USDT) — USDT is mapped to USD, then converted to GBP via the HMRC rate
4. Crypto pair (ETH/BTC) — the quote asset (BTC) is priced in GBP using daily data from CryptoCompare and CoinGecko. The GBP value of the trade is then derived as: BTC amount × BTC/GBP rate

If a price isn’t available for a given date, we check ±1 day. If still missing, the transaction is skipped and a warning lists the affected coins and dates so you can review them.

FX Trading

How foreign exchange trades (like GBP/USD) are taxed as "Other Assets" under UK capital gains rules.

If you trade spot foreign exchange (forex) through your broker, those trades are subject to capital gains tax. HMRC classifies foreign currencies as "Other Assets" — they go on a different section of the SA108 form than your share disposals.

How Pairs Work for UK Tax

Since GBP is your home currency, it can't be an "asset" for CGT purposes. When you trade a pair like GBP/USD:

• Selling GBP/USD = acquiring USD (the foreign currency is the asset)
• Buying GBP/USD = disposing of USD

The calculator automatically flips the perspective so the foreign currency is always treated as the asset being bought or sold.

Matching Rules

FX trades use the same three matching rules as shares — Same Day, Bed & Breakfast, and Section 104 Pool — applied in the same order and with the same logic.

ERI / Offshore Funds

Excess Reportable Income — the extra tax you owe on offshore funds even if you didn’t sell anything.

What is ERI?

If you hold offshore accumulation funds (like Irish-domiciled ETFs from iShares or Vanguard), you may owe tax on “phantom income” — income the fund earned and reinvested, even though you never received any cash. This is called Excess Reportable Income.

Under the UK’s offshore fund rules, funds with UK reporting status must publish how much income they earned per unit each year. If that income exceeds what the fund actually distributed, the excess must be reported as taxable income by UK investors — regardless of whether you sold anything.

How ERI Works

Each year, the fund manager publishes an ERI figure per unit. The tax works in two stages:

1. Cost basis adjustment (on the fund’s reporting date) — your Section 104 pool cost increases by the ERI amount. This prevents you from being taxed again when you eventually sell.
2. Taxable income (6 months after the reporting date) — the same amount becomes reportable income, usually taxed as dividends or interest depending on the fund type.

ERI and the Matching Rules

ERI applies to any shares you held on the fund’s reporting date — including shares matched under the Bed & Breakfast rule. If you sold shares before the ERI reporting date and repurchased within 30 days (B&B), those shares are treated as held on the reporting date.

The calculator handles this automatically:

• Pool shares — ERI based on your Section 104 pool quantity on the reporting date
• B&B shares — ERI applied separately for shares matched under the 30-day rule

Bundled ERI Data

The calculator includes ERI data from multiple fund providers. If your fund is covered, ERI is applied automatically when you upload your broker CSV — no extra steps needed. Matching works via ISIN codes in your broker data.

VIEW ERI DATA →

Vanguard Data Sources

Source: Vanguard UK Tax Information

iShares (BlackRock) Data Sources

Source: BlackRock Reporting Fund Status

Dividends & Interest

How dividend income and interest income are taxed, including allowances and double taxation relief.

Dividend Tax

Dividends from UK and foreign shares are taxed at special rates, separate from your normal income tax. You get a tax-free dividend allowance each year:

After the allowance, dividends are taxed at:

The report shows tax for all three bands — use whichever matches your income level. Here is how the calculation works:

Foreign Dividends

Foreign dividends are converted to GBP at the exchange rate on the payment date. When a country withholds tax at source, the UK has treaties that let you offset some or all of that withholding against your UK tax bill.

Treaty relief is calculated automatically for supported countries:

Dividend reinvestment (DRIP): When dividends are automatically reinvested in shares, the calculator counts the dividend as income and the reinvested shares as a stock purchase — both are tracked separately for tax purposes.

Capital gain distributions: Some funds distribute capital gains to shareholders. These are treated as dividend income for UK tax purposes and included in your dividend total.

Interest Income

Interest earned on cash balances at your broker is taxed at your normal income tax rate. You get a Personal Savings Allowance (PSA) that depends on your tax band:

Note: Interest is grouped by month and broker in the report for easy cross-referencing with your broker statements.

Interest fund tickers: Some funds pay interest-like distributions that your broker labels as dividends. If you hold such funds, you can add their ticker symbols in the Settings panel under Interest Fund Tickers. Their income will then be taxed at interest rates instead of dividend rates.

Residency Status

How your UK tax residency affects which disposals are subject to Capital Gains Tax.

If you were a UK resident for the entire tax year, you don’t need to configure anything — the calculator treats you as fully resident by default. But if you left the UK, returned from abroad, or spent part of the year as a non-resident, your residency status determines which disposals are subject to CGT and which are excluded.

How It Works

You define one or more residency periods in the Settings panel. Each period has a start date, an optional end date, and a status. The calculator checks each disposal against these periods to decide if it’s taxable.

There are three residency statuses:

Taxability Rules

The calculator doesn’t just check the disposal date — it also considers when you acquired the shares. This matters for temporary non-residents, where gains on assets bought while you were UK resident can still be taxed when you return.

The key rule: if you sell during a temporary non-resident period but originally bought while UK resident, the gain is still taxable. HMRC’s anti-avoidance rules prevent you from leaving the UK briefly to sell assets tax-free.

Effect on Share Matching

Residency status also affects how HMRC’s share matching rules work:

• Same Day Rule — still applies, but the taxability of the match depends on your residency at disposal
• Bed & Breakfast Rule — has a special exception: if you were non-resident when you reacquired shares (after 22 March 2006), the B&B rule doesn’t apply to that match
• Section 104 Pool — non-taxable matches still consume shares from the pool but don’t generate a CGT entry

Mixed-Year Residency

If you were resident for part of the year and non-resident for the rest, the calculator handles each disposal individually based on its date. Your report will show:

• A “Mixed residency” status indicator in the summary
• The number of days spent as resident vs non-resident
• A separate table of excluded disposals showing each sale that fell outside your UK-resident periods, with the gain or loss that was excluded

Configuring Residency

Open the Settings panel (gear icon) and switch to the Residency Status tab. Add a period for each time your status changed:

• Start date — when this status began (required)
• End date — when this status ended (leave blank if ongoing)
• Status — Non-Resident or Temporary Non-Resident

You only need to add periods where you were not UK resident. Any dates not covered by a period are treated as UK resident by default. Your residency configuration is saved in your browser and applied automatically to future calculations.

Supported Brokers

Which brokers are supported, what file format to export, and what each broker covers.

Interactive Brokers (IBKR)

The most feature-rich integration. Supports all transaction types including options, corporate actions, FX trades, and dividends. You must use the Activity Statement CSV format — the simple trades export does not support options.

Trading 212

Supports stocks, ETFs, dividends, interest, and stock splits. Both 2020 and 2024 export formats are supported. Currency conversion is built into the export. Not available for CFD accounts (Invest/ISA only).

Trading 212 exports calendar year data (Jan–Dec), so to cover a full UK tax year (6 Apr – 5 Apr) you need two calendar year exports.

Charles Schwab

Supports stock trades, dividends, interest, RSUs, and corporate actions (cash mergers). If you have equity awards (RSUs), you need both the Brokerage CSV and the Equity Awards CSV — without the awards file, vest events will be missing.

Freetrade

Supports stock trades, dividends (with withholding tax), and interest. Stamp duty and FX fees are extracted automatically. Corporate actions are not included — contact support for a full statement.

Morgan Stanley (MSSB)

For RSU/equity award holders. You need two files: the Releases Report (RSU vest events at Fair Market Value) and the Withdrawals Report (share sales and cash transfers). Without the Releases Report, you’ll see “no purchase record found” because vest events are stored separately.

Revolut

Supports stock trades, dividends (with withholding tax), and stock splits. Revolut is commission-free — the FX spread is embedded in the exchange rate, not shown as a separate fee. Revolut exports statements as Excel (.xlsx) files, which are converted automatically on upload.

Binance

Supports cryptocurrency trades from two CSV formats. Covers fiat-quoted pairs, stablecoin pairs (USDT mapped to USD), and crypto-to-crypto swaps. See Crypto for details on how swaps, fees, stablecoins, staking rewards, and airdrops are handled.

We accept two export formats:

• Trade History — spot buy/sell trades. Simplest export, covers most users
• Statements — includes trades, staking rewards, airdrops, and BNB dust conversions. Use this if you have staking income or airdrops

Upload both and duplicates are removed automatically — we keep the Statements version when a trade appears in both.

Coinbase

Supports the Coinbase Retail transaction history CSV — the export any Coinbase.com user can generate from their account. Covers fiat Buy/Sell, Advanced Trade, Convert (crypto–to–crypto swaps split per HMRC CRYPTO22100), Receive (referrals, Earn, staking rewards), Send, and Rewards/Staking Income. Four header variants (V4 current, V3/V2/V1 legacy) are auto-detected. See Crypto for details on how swaps, fees, stablecoins, staking rewards, and airdrops are handled.

Multi-Broker Support

Manual Transaction Entry

If your broker isn’t listed, you can add transactions manually. Available fields include date, ticker, action (buy/sell), quantity, price, fees, and currency. Manual entries appear alongside CSV transactions and follow the same HMRC matching rules.

This is useful for unsupported brokers, one-off corrections, or historical trades that aren’t in your broker export.

Report Output

Understanding your CGT report — what each section means and how to use it for Self Assessment.

What’s in Your Report

The report is organised into sections. Not all sections appear every time — you only see the ones relevant to your data.

Self Assessment (SA108)

The Self Assessment Summary at the top of your report gives you the numbers for HMRC’s SA108 Capital Gains page. SA108 splits disposals into separate sections by asset type — you enter the totals for each section into a different group of boxes:

Box numbers are taken from the SA108 form for 2025-26 (year ending 5 April 2026). The Cryptoassets section (boxes 13.1–13.8) was added from 2024-25 onwards — before that, crypto disposals went into “Other property, assets and gains”.

For dividends and interest, use the Income Summary to fill in the relevant boxes on your SA100 main return.

Downloading Your Report

Click Download PDF to open a print-ready view of your report. Use Ctrl+P (Windows) or Cmd+P (Mac) to save as PDF. Print styles hide navigation and format tables for paper.