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.
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 →
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.
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.