How to fill the crypto tax forms SA100 and SA108 in the UK?

Reporting cryptocurrency to HMRC under Self Assessment revolves around two specific schedules: the main tax return, known as the SA100, and the capital gains summary pages, known as the SA108. Filing accurately means dividing your activity correctly between these two schedules.

What are the main differences between the SA108 and the SA100?

When reporting cryptoassets on your UK Self Assessment return, HMRC draws a firm line between income and capital. Earning tokens through protocol participation is treated as personal income, while selling, trading, or spending tokens triggers Capital Gains Tax (CGT). The SA1 is purely administrative; it alerts HMRC that you have untaxed liabilities and initiates your account.

When completing your return online via HMRC’s portal, you do not need to download standalone paper forms. During the initial “Tailor your return” section, answering “Yes” to receiving other taxable income adds the relevant SA100 fields, and answering “Yes” to disposing of chargeable assets automatically attaches the SA108 schedule.

Reporting Crypto Income on the SA100

Crypto rewards that do not amount to a commercial trade count as miscellaneous income. In the SA100, this sits on Page TR 3 under “Other UK income not included on supplementary pages”.

For individual investors, four boxes matter:

• Box 17 (Other taxable income): The gross sterling value of all crypto rewards received across the tax year. Each receipt is valued using the spot price at the exact moment the tokens became accessible in your custody.
• Box 18 (Total allowable expenses): Direct costs incurred to produce that income (such as hardware electricity for hobby mining). Alternatively, if you do not claim actual expenses, you can use the £1,000 Trading and Miscellaneous Income Allowance. If your total casual income across all sources is under £1,000, you pay no tax on it.
• Box 19 (Any tax taken off): Enter £0 or leave blank. Crypto platforms and protocols do not deduct UK tax at source.
• Box 21 (Description of income): A concise label for HMRC. A description like “Cryptocurrency staking yield and mining rewards” provides the necessary context.

(Note: If your mining or trading operation runs with significant capital, dedicated staff, and commercial premises, HMRC will expect you to file on the self-employment schedule, SA103, rather than the SA100.)

Declaring Disposals on the SA108 (Boxes 13.1 to 13.8)

HMRC provides a dedicated cryptoassets section on the SA108 (Page CG 2). Do not enter your numbers in the generic “Other property, assets and gains” boxes (boxes 14 to 22), as HMRC uses the crypto boxes specifically to monitor digital asset compliance.

Critical SA108 Rules to Avoid Rejections

1. Never net boxes 13.4 and 13.5: Enter total gains in 13.4 and total losses in 13.5 separately. HMRC’s software handles the netting calculation automatically.
2. Do not deduct the £3,000 Annual Exemption yourself: Enter your gross gains and losses. The portal automatically applies the annual exempt amount.
3. Handle prior-year losses in Box 45: Losses brought forward from earlier tax years belong in box 45 of the adjustments section—never in box 13.5. They only offset gains down to the £3,000 threshold. Any remaining unused losses go into box 47 to carry forward.

Determining Allowable Costs: HMRC Matching Rules

The figure in Box 13.3 cannot be calculated using accounting methods like FIFO or LIFO. UK tax law requires disposals of identical tokens to be matched in a strict order:

1. Same-Day Rule: Disposals are matched first against tokens of the same asset acquired on the exact same calendar day.
2. Bed and Breakfast Rule (30-Day Rule): Remaining tokens are matched against acquisitions made within the following 30 days.
3. Section 104 Pool: Any volume not matched by the first two rules is matched against your Section 104 holding pool, which represents the rolling average acquisition cost of that token.

Legitimate trading fees and blockchain gas fees directly associated with the purchase or sale are added to your allowable costs.

A Worked Example

Suppose during the 2025/26 tax year you executed two disposals:

• Disposal 1: You sold 0.5 BTC for £30,000. Under your Section 104 pool, that half-Bitcoin had an allowable cost of £18,000, and the exchange charged £100 in trading fees.
  • Result: £11,900 gain (£30,000 proceeds − £18,100 costs).
• Disposal 2: You swapped £8,000 worth of ETH for SOL. Under your pool, that ETH had an allowable cost of £10,000.
  • Result: £2,000 loss (£8,000 proceeds − £10,000 costs).

Your SA108 entries will look like this:

HMRC automatically deducts the £2,000 loss from your £11,900 gain, leaving £9,900 in net gains. Applying the £3,000 annual exemption leaves £6,900 taxable. At the 24% higher rate for capital gains, your final tax liability on these disposals is £1,656.

Computations and Documentation

HMRC explicitly states that writing “see attached” inside return boxes is not acceptable. You must enter your summary numbers into boxes 13.1 to 13.8 and supply a detailed computation schedule alongside the return.

Most online tax software allows you to upload this computation as a PDF attachment. If your filing software lacks an upload feature, summarize your figures in Box 54 (“Any other information”). Your computation schedule should show:

• Dates and asset types for every disposal.
• Gross proceeds in sterling.
• Associated acquisition costs and fees.
• The specific matching rule applied to each transaction (Same-Day, 30-Day, or Section 104 Pool).

If you must estimate any figure—for example, if an exchange collapsed and took your historical ledger with it—tick Box 53 and outline your estimation methodology in Box 54.

What is the best platform to report crypto taxes to HMRC?

Calculating rolling Section 104 pools across hundreds of decentralized swaps, multi-chain transfers, and staking distributions is where manual spreadsheets break down.

Waltio connects with read-only access to over 700 exchanges, hardware wallets, and blockchains to automate this process. It tracks wallet-to-wallet transfers, applies HMRC’s same-day and 30-day matching rules, and produces the exact figures needed for your SA100 income fields and SA108 boxes. It also generates the complete, disposal-level computation PDF required by HMRC to accompany your submission.