30-day rule
checker

If you dispose of shares and acquire the same shares within the following 30 days, those two are matched with each other — not with your Section 104 pool. The rule is section 106A of the Taxation of Chargeable Gains Act 1992, and it is usually met by people trying to do something perfectly sensible.

The classic case: you are sitting on a loss in December, you sell to crystallise it against gains made earlier in the year, and you buy back a fortnight later because you still want to own the thing. The loss you banked is not banked. Your disposal is matched against the repurchase, and the loss follows the new holding instead of landing in this year's return.

• It is the 30 days after, not before. A purchase in the 30 days running up to a sale is an ordinary pool acquisition. Only what comes after is pulled back.
• Day 30 is inside the window; day 31 is outside it. The boundary is exact, and this checker gets it from the engine, whose tests pin both sides of it.
• Partial matches are normal. Sell 1,000 and buy back 300 and the rule takes 300 of them; the other 700 meet the pool as usual, and one disposal ends up with two different cost bases.
• Same-day comes first. Shares bought and sold on the same day are matched with each other before the 30-day rule is reached at all.
• A spouse buying back is not caught by this rule — s.106A applies to acquisitions by the same person. Other anti-avoidance rules may still be relevant, and this is the point at which to ask an accountant rather than a web page.

The rule does not delete the loss. It moves it: the disposal is computed against what you paid on the repurchase, and the difference ends up in the cost of the shares you now hold — so it surfaces whenever you eventually sell those. What it does destroy is the timing, and timing is the whole point of a December sale.

To see the actual figures, you need what the shares cost. The Section 104 calculator takes those and shows the gain each rule produces.