Reads your broker exports
Import the CSV files your broker already produces. A mapping step handles formats that aren't recognised automatically.
Desktop software for people who do their own tax.
Longstop Software is a UK company building tools that help individuals prepare their own tax records — accurately, on their own machine, without handing a year of financial history to a web service.
First product
GainPool — UK Capital Gains Tax, computed on your own machine.
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.
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Applies the matching rules
Section 104 pooling, same-day and thirty-day matching, foreign-currency trades, and exclusion of ISA and SIPP holdings.
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Shows its working
Every figure traces back to the disposals behind it, exportable as a computation pack rather than a single unexplained number.
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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.
GainPool is in development. It is being prepared for the 2025/26 Self Assessment season and is not yet available to buy. GainPool 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.
Current status
The tax engine is built and under test. Work in progress covers code signing, automatic updates, licensing and support, ahead of a public release in time for the 2025/26 Self Assessment deadlines. If you file your own Capital Gains and would be willing to test an early build against your real broker exports, we would like to hear from you.
Contact
Get in touch
General enquiries, press, and early-testing offers all reach the same inbox, and a person reads it.
Written enquiries may also be sent to our registered office.