Penny

Penny is an English word for the smallest coin. Here it means splitting shared costs to the last one.

Four friends go to Lisbon. One pays the flights in euros, another the apartment, one a surf lesson in dollars. Splitting 100.00 three ways leaves a cent that belongs to nobody. By the end there are seven receipts and a tangle of who owes whom.

Penny turns that into a short list. You add what each person paid. Penny splits every expense down to the smallest unit of its currency, converts foreign ones once at a rate you can see and change, and works out the fewest payments that bring everyone to exactly zero. There is no account to make: a group is opened by its private link.

The Lisbon trip is built in. Press "Try a sample trip" on the home page and you get all seven expenses and a plan of three payments: Cy pays Ana 13.81, Cy pays Ben 81.34 and Dev pays Ana 141.93. The line under the plan reads "3 payments settle 7 expenses."

Four promisesCopy Anchor Link

Each promise is a rule the product keeps on every screen, and each one has tests behind it.

1. Every amount is exact. Amounts are stored as whole numbers of the currency's smallest unit. A split always adds up to the expense, in its own currency and in the base currency. Nothing is rounded except a currency conversion, which rounds once per expense, half up.
2. Everyone ends at exactly zero. The balances of a group add up to exactly 0.00. Applying the settle-up payments brings every member to zero, using the fewest payments possible for up to 16 people with a balance.
3. Nothing is recorded twice. Pressing "Add expense" twice, or a dropped connection that retries, never adds an expense twice. "Mark as paid" on an out-of-date page, such as a second phone, never records a payment twice.
4. A link opens one group. One group's link can never read or change another group. After "Reset link", the old link opens nothing.

Where to go nextCopy Anchor Link

• Quick start: try the sample trip in the live app, then start a group of your own.
• How it works: the path a typed amount takes to become a balance and a payment, with the Lisbon numbers.
• The self-fixing loop: how a planted bug went from red to green in 4 minutes 44 seconds, and why a person still merges every robot fix.

The rest of the sidebar covers concepts, step-by-step guides, the test suite, the API, security and the FAQ.

One pull request, five linesCopy Anchor Link

Pull request #10 plants a one-line bug on purpose: the leftover cent of a split goes to the last person listed instead of the first, so 100.00 split three ways comes out as 33.33, 33.33 and 33.34. All times are UTC, 20 October 2026.

• 05:02 The bug is pushed and the pull request opens. At 05:23 Vercel's preview is live and TestSprite starts its 21 browser tests against it.
• 05:27:08 TestSprite posts TestSprite V3: failure, "18/21 passed, 3 failed". The branch rule requires that check, so GitHub will not merge.
• 05:27:09 The self-fix run starts. Its commit lands at 05:28:29, 81 seconds after the red, and changes one line to undo the planted flip.
• 05:31:52 TestSprite tests the new preview: "All 21 tests passed". Red to green took 4 minutes 44 seconds, with no person involved.
• 05:48:49 The maintainer reads the one-line change and merges.

The full story, with the four jobs and the caps, is on the self-fixing loop page. The live version is on the checks page.