The 30-day (bed and breakfast) rule for crypto

The trap most people miss

This rule looks forward. Most people assume a sale's gain is fixed the moment they sell — under UK rules it isn't. If you buy the same token back within 30 days, the sale is re-matched against that repurchase at the repurchase price, not against your old pool.

The classic scenario: your coin has crashed, you sell to "bank the loss" before the tax year ends, then buy back a week later because you still believe in it. The 30-day rule matches your sale to your buy-back — your loss is now measured against the buy-back price, usually wiping most of it out. The market dipped, but your allowable loss largely vanished.

How the matching works

After same-day matching, any remaining disposal quantity is compared against acquisitions of the same token dated 1 to 30 days after the disposal, earliest acquisition first, with cost apportioned by the quantity matched. Whatever the 30-day rule doesn't consume falls through to the Section 104 pool.

Because acquisitions consumed this way never enter the pool, a busy trading month can produce chains where a single purchase is split between several earlier disposals.

Example

Pool: 2 ETH at £1,500 average. On 1 Feb you sell 1 ETH for £1,000 — intending a £500 loss. On 10 Feb you buy 1 ETH for £1,050.

• 30-day rule: the sale matches the 10 Feb buy → proceeds £1,000, cost £1,050 → a £50 loss, not £500
• Your pool still holds 2 ETH at £1,500 average (the buy-back never joined it)
• To have kept the £500 loss you'd have needed to wait 31+ days before re-buying (or buy a different asset)

Provisional results

A consequence of the forward window: any disposal in the last 30 days isn't final yet. Our calculator flags these as provisional — a purchase you make tomorrow could legally change last week's gain.