Do your own tax.
On your own machine.
Longstop Software is a UK company building tools that help individuals prepare their own tax records — accurately, without handing a year of financial history to a web service.
GainPool output — 2025/26
Tax due
£1,518.92
47 disposals · £8,438.47 taxable after the exempt amount
HMRC reporting
Must report
By 31 Jan 2027
Headroom
£7,261.53
Before the higher rate
Illustrative figures from a sample portfolio.
One pool per security.
Every broker in it.
HMRC pools a share across everything you own of it. Hold the same ETF at two brokers and that is one Section 104 pool — working each account out separately gives you two plausible figures that are both wrong, and the error lands on the cost of every disposal. GainPool merges every export you give it into the pool HMRC actually asks for, then applies the same-day and 30-day rules across all of them at once.
GainPool — UK Capital Gains Tax
GainPool reads the transaction exports your broker already gives you and works out the Capital Gains position for your Self Assessment return, showing the computation behind every figure so you — or your accountant — can check it.
How GainPool works →
Disposals view — every sale with the rule it matched by
01
Reads your broker exports
Import the CSV files your broker already produces. A mapping step handles formats that aren't recognised automatically.
02
Applies the matching rules
Section 104 pooling, same-day and thirty-day matching, foreign-currency trades, and exclusion of ISA and SIPP holdings.
03
Shows its working
Every figure traces back to the disposals behind it, exportable as a computation pack rather than a single unexplained number.
04
Stays on your computer
Your transaction data is read, computed and stored locally. It is not uploaded to us, and we operate no server that receives it.
Why a desktop app
and not a website
The input is your entire trading history. A web service would need a copy of it. GainPool doesn't: files are read, computed and stored on your machine, sessions are encrypted at rest, and there are no accounts and no telemetry. We operate no server that receives your transactions.
Will it read your export?
Drop your broker CSV in and find out what we make of it — which platform, how many rows, which would need attention — before downloading anything.
Check a file →Section 104 calculator
Enter buys and sells and get the pool, the matching and the chargeable gain, with the working shown. Load HMRC’s own worked example and check it.
Open the calculator →30-day rule checker
Sold and bought back? Two dates and two quantities tell you whether the bed-and-breakfast rule caught it, and how much of the disposal it took.
Check a repurchase →GainPool is being prepared for the 2025/26 Self Assessment season and is not yet available to buy. It produces an estimated Capital Gains position from the records you give it; it is not a filing service, it does not submit anything to HMRC, and it is not a substitute for advice from a qualified accountant. Its scope and limitations are stated in the application itself, before any figures are shown.
The engine is built.
Break it with your real exports.
Work in progress covers code signing, automatic updates, licensing and support, ahead of a public release in time for the 2025/26 deadlines.
Opens your email app, addressed to us. No list, no newsletter. A person replies.