How to File Crypto Tax on Your UK Self Assessment for 2025/26

The 2025/26 tax year ended on 5 April 2026. If you bought, sold, swapped, staked, or earned crypto at any point between 6 April 2025 and 5 April 2026, this is the return that covers it. The online filing deadline is 31 January 2027, but the calendar that actually matters starts months earlier — first-time filers have to register with HMRC by 5 October 2026 or risk a penalty before they have even submitted a return.

2025/26 is the second tax year to use the dedicated crypto boxes on SA108 (boxes 13.1 to 13.8). It is also the first tax year to use the post-Autumn-Budget CGT rates of 18 percent and 24 percent throughout, with no split-year complication. That last point matters more than it sounds: most online guides you find right now will be 2024/25 content with the year number swapped, and they will tell you to fill Box 51 for the rate adjustment. For 2025/26, Box 51 stays empty.

This guide walks through the full process. Who needs to file, the deadlines that catch people out, the calculation rules, the SA100 and SA108 box entries, and the DeFi-specific gotchas that generic crypto tax guides miss. It is written for both first-time filers and people who filed last year and want to know what changed.

Who needs to file Self Assessment for 2025/26

You need to register for Self Assessment and file a return for 2025/26 if any of the following apply:

• Your total capital gains across all chargeable assets (crypto, shares, property other than your main home) exceeded the £3,000 annual exempt amount in 2025/26
• Your total disposal proceeds exceeded £50,000, even if your gains were below the exempt amount. HMRC requires a return to evidence the calculation
• You earned crypto income (staking rewards, lending interest, airdrops you actively claimed, mining rewards) above the £1,000 trading and miscellaneous income allowance
• You are already in Self Assessment for any other reason (self-employment, rental income, dividends above the allowance) and had any crypto activity at all in the year

The £1,000 trading and miscellaneous allowance is per-source. If your only miscellaneous income is from staking and it totals less than £1,000 across the year, you can claim the allowance and you do not need to declare it. If you have any other miscellaneous income (consultancy fees, side-hustle earnings) the £1,000 is a single shared allowance.

For more detail on the threshold tests, see do I need to report crypto to HMRC?

The five dates that matter for 2025/26

Most guides only mention the 31 January deadline, but four other dates are load-bearing for 2025/26 filers:

The 5 October registration deadline is the one that catches first-time crypto filers. HMRC has been issuing nudge letters to suspected non-filers throughout 2026, and from 1 January 2026 UK exchanges report your account data under CARF. If you owe tax for 2025/26 and never registered, the 'failure to notify' clock started on 5 October 2026.

Step-by-step: from registration to submission

Step 1: Register for Self Assessment (first-time filers only)

Go to gov.uk and search for 'register for Self Assessment'. You will need a Government Gateway user ID and password. HMRC posts your Unique Taxpayer Reference (UTR) by letter, which can take up to two weeks. You cannot file without a UTR, so register early.

For crypto-only filers (no other reason to be in Self Assessment) you register as 'not self-employed' — the return type is the standard SA100 with the SA108 capital gains supplement.

Step 2: Gather your records

HMRC expects you to keep records in pounds sterling for every transaction. The minimum dataset:

• Date of every acquisition and disposal
• Asset type (token contract address for ERC-20s, NFT collection address for NFTs)
• Number of units transacted
• GBP value at the moment of the transaction
• Wallet addresses involved
• Running balance of your Section 104 pool for each asset

Exchange CSV exports cover most of this for centralised exchange activity. For DeFi, you need on-chain transaction hashes plus a pricing source for each token at each timestamp. Most CEX exports use FIFO, which is wrong for HMRC — you have to recalculate on a Section 104 basis. See why your Coinbase report is not enough and how to handle Binance trades for HMRC.

Step 3: Calculate your gains and income

This is where most filers either spend a weekend in spreadsheets or use a tax tool. The mechanics are covered in the next section, but the output you need is two numbers per tax year: total taxable capital gain (after losses, after annual exempt amount) and total crypto-related miscellaneous income.

Step 4: Fill SA100 and SA108

SA100 is the main return. Crypto income (staking, lending, airdrops you claimed) goes in the 'Other taxable income' section with a brief description — for example, 'crypto staking rewards: £1,200'.

SA108 is the capital gains supplement, and since 2024/25 it has dedicated crypto boxes 13.1 to 13.8. The next section covers what goes in each.

Step 5: Submit and pay

File online via your Government Gateway account before 31 January 2027. The system calculates your tax liability automatically from the boxes you fill. Pay via bank transfer, debit card, or direct debit. HMRC's sort code and account number for Self Assessment payments are listed on the gov.uk payment page — double-check them before sending.

The HMRC rule — Section 104, same-day, B&B, then SA108

UK CGT on crypto uses three matching rules in a fixed order, defined by section 104 of the Taxation of Chargeable Gains Act 1992:

1. Same-day matching. If you bought and sold the same asset on the same day, those transactions match against each other first.
2. 30-day bed and breakfasting rule. Disposals match against any acquisitions of the same asset in the 30 days immediately following the disposal.
3. Section 104 pool. Anything left over uses the weighted average cost basis of all your remaining holdings of that asset.

You apply these rules per asset symbol, not per wallet or per exchange. All your USDC across Coinbase, Binance, MetaMask, and a Ledger sit in the same pool. Same for ETH, same for every other token.

For the deep mechanics of how these rules interact, see Section 104 pooling explained and the same-day and 30-day rules.

Once you have a list of disposals with their matched cost bases, the SA108 boxes are straightforward. Boxes 13.1 to 13.8 are dedicated to cryptoassets:

For the full breakdown of every box including worked numbers, see SA108 boxes 13.1 to 13.8 explained. That post covers 2024/25 specifically (with Box 51), but the box meanings are unchanged for 2025/26.

Worked example — a 2025/26 DeFi filer's return

DeFi-specific gotchas for 2025/26

Generic Self Assessment guides assume CEX-only activity. If you used DeFi at any point in 2025/26, four classification rules trip up almost every first-time filer:

Bridges are not disposals

Moving USDC from Ethereum to Arbitrum via a bridge is not a disposal for HMRC purposes. The cost basis transfers across chains. If your tax tool is treating bridges as sells, every cross-chain move is being double-counted as a CGT event. See is bridging crypto taxable in the UK?

Wrapping ETH into WETH is not a disposal

WETH is the same beneficial asset as ETH, just an ERC-20 wrapper for DeFi compatibility. HMRC treats wrap and unwrap as no-op for CGT. Same logic applies to wstETH around stETH, and most other wrap contracts where economic ownership is unchanged. See wrapped tokens and UK tax.

Staking rewards are income on receipt, then CGT later

When you receive staking rewards, the GBP fair market value at that moment is taxable as miscellaneous income on SA100. That same value becomes the cost basis for those tokens in your Section 104 pool. When you eventually sell, CGT applies on the gain above that cost basis. Two separate tax events, two separate forms. See how to report staking rewards to HMRC and DeFi income vs capital gains.

Liquidity pool entries and exits are composite events

Adding tokens to a Uniswap V2 or V3 LP position is a disposal of those tokens (entering the pool) and an acquisition of the LP position. Removing liquidity reverses it. The income earned (LP fees) is a third event, taxable as miscellaneous income. Most tools either miss the LP token entirely or treat the whole flow as a single swap. See how HMRC taxes Uniswap LP positions.

2025/26 vs 2024/25: the Box 51 difference

The most common mistake on 2025/26 returns will be people copying their 2024/25 SA108 entries verbatim and filling Box 51 again. Box 51 was the split-year CGT rate adjustment. It existed because the Autumn Budget 2024 changed the main CGT rates from 10/20 percent to 18/24 percent on 30 October 2024, mid-way through the 2024/25 tax year. Disposals before the boundary were taxed at the old rates and disposals after at the new rates, so a manual adjustment was required.

For 2025/26 the entire tax year applies the new 18/24 percent rates. There is no split, no boundary, no adjustment. Box 51 stays empty.

For everything else, the 2024/25 box-by-box guide remains accurate. See SA108 boxes 13.1 to 13.8 explained for the full breakdown of each crypto box.

How ChainTax handles your 2025/26 return

ChainTax is built specifically for UK filers. Every classification, cost basis calculation, and SA108 box value is generated for HMRC rules first, not adapted from a US tool:

1. Centralised exchange and DeFi in one place. Import Coinbase, Binance, and Kraken CSVs alongside on-chain activity from Ethereum, Arbitrum, Optimism, Base, and Polygon. All disposals of the same asset land in one Section 104 pool, as HMRC requires.
2. Section 104 with same-day and 30-day matching. Every disposal is matched against same-day acquisitions, then 30-day bed and breakfast acquisitions, then the S104 pool, in that order. Each match is shown explicitly so you can trace the calculation. Classified as Disposal, Income, or Transfer with confidence ratings.
3. Bridges, wraps, and LP positions handled correctly. Bridge transfers are recognised as non-disposals. Wrap and unwrap events are recognised as non-disposals. LP entries trigger the composite disposal-plus-acquisition treatment automatically.
4. Staking rewards split across SA100 and SA108. Staking, lending interest, and claimed airdrops are recognised as Income at fair market value on the receipt date, with that value becoming the cost basis for subsequent disposals.
5. SA108 box values pre-calculated for 2025/26. Boxes 13.1 to 13.8 are populated automatically. Box 51 is left empty for 2025/26 returns. The PDF report mirrors the SA108 layout so you can transcribe directly into your Government Gateway submission.

Every classification is traceable. You can paste any transaction hash into the public explainer to see how the engine categorises it before you sign up.