Stablecoin Tax UK: Why Swapping to USDC or USDT Is a Disposal (2025/26)

One of the most common UK crypto tax mistakes: assuming that swapping ETH or SOL into USDC during a market downturn is "going to cash" and therefore tax-neutral. It is not. HMRC treats every stablecoin as a token, and every swap into one is a disposal of whatever you swapped from. This guide covers what that actually means at filing time.

The disposal rule

Under HMRC's CG12100 guidance, swapping one crypto asset for another is treated as a disposal of the first and an acquisition of the second. The fact that USDC, USDT and DAI are pegged to the dollar is irrelevant — they are crypto-assets, not fiat. So if you swap £8,000 worth of ETH into USDC at a profit over your Section 104 pool, you owe CGT on that gain in the year of the swap, even if you never converted to GBP.

Why this catches people out

In a bear market, UK holders often rotate into stables to "lock in" their position. Many do this multiple times per year, generating dozens of disposals without realising it. At Self Assessment time, they report only their fiat off-ramp events and miss the on-chain swaps entirely. CARF reporting from 2026 onwards means HMRC will increasingly see the on-chain volume even if you do not.

Stablecoin-to-stablecoin swaps

Swapping USDC for USDT (or DAI for USDC) is also a disposal — of negligible CGT impact if both prices were ~£0.79/$1.00 at both points, but still a row on your SA108. Bridge fees, swap fees and gas form part of the cost basis. Most UK holders show a tiny loss on these because of fees, which is mildly useful as it offsets gains elsewhere.

The price-feed problem

A surprising number of tax tools price USDC and USDT as exactly $1.00, ignoring the real GBP/USD rate at the moment of swap. HMRC requires the actual GBP value at disposal, which means a swap done at GBP/USD = 1.27 versus 1.26 produces a different (small) gain. Tools that hardcode $1 are slightly wrong on every stablecoin disposal — CryptoLens uses the published GBP/USD spot at the block timestamp.

Stablecoin yield

Earning yield on USDC via Aave, Compound, or a CeFi platform is miscellaneous income at the GBP value of each yield distribution. That income value then becomes the cost basis of the new USDC for any future swap.

Filing efficiently

The simplest way to clean up stablecoin disposals is a single SA108 export from a wallet scan — every swap is on a row, every fee is captured, every GBP value is pulled at the correct block. CryptoLens generates the full pooled report including all USDC, USDT, DAI and FRAX activity for one wallet on the free tier.