Kostos is a free group expense splitter that lives on your device. Create a group, share its token, and split bills with friends, family, or housemates. No accounts, end-to-end encrypted, syncs live across devices, works offline.
Every sync message is AES-GCM encrypted in your browser before it leaves. The sync server only sees opaque ciphertext; the secret token never touches the network.
A group is just a shareable token. Send it; everyone joins. No password reset emails, no profile to maintain, no third-party identity provider.
Every device with the token sees the same expenses and balances in real time. Add a bill on a plane; it saves locally and syncs the moment you're back online.
Inspect the SvelteKit + Y.js codebase, run the sync server yourself, or fork it. No vendor lock-in and nothing hidden behind a paid tier.
Pick a name, an emoji, a currency, and the people involved. The app generates a secret token for the group locally on your device.
Send the share link to anyone you want in the group. They open it, pick their member, and join. Multiple devices per person are fine.
Add expenses with even, share-based, or precise per-person splits. Kostos computes the minimum number of transfers needed to bring everyone back to zero.
Yes. Kostos is free and open source. There is no paid tier, no ads, and no upsell.
No. Each group is identified by a secret token. Share the token to invite others; anyone who has it joins the group. No email, no password, no sign-up.
Every sync message is AES-GCM encrypted in your browser before it reaches the sync server. The server only stores opaque ciphertext and never has access to the secret token, so it cannot read your expenses even if compromised.
Yes. Add expenses while disconnected; they save to your device immediately and sync to the group the next time you come online. Kostos is a PWA, so it installs to your home screen and runs without a browser tab.
Pick a split mode: evenly between selected members, by weighted shares, or by precise per-person amounts.
Math expressions like 50/3 or (120+5)/4 work inside any amount
field, so you can write the calculation instead of pre-computing.
Yes. Anyone with the shared token can join from another device. Expenses, settlements, and member changes sync live between every device that holds the token, with conflict-free merging powered by Y.js CRDTs.