Offline Bill Splitter Android App for Roommates with UPI Settlement Tracking

Roommates and hostel-mates split rent, groceries, electricity, Wi-Fi and food bills every month, yet most groups still track it all in chat messages or notebooks. The arithmetic drifts, someone always feels they overpaid, and month-end reconciliation turns into an argument. Existing splitting apps fix the maths but demand accounts and a constant internet connection — a real barrier in hostels with patchy Wi-Fi — and they treat the actual repayment as somebody else's problem: the UPI reference that proves a payment happened lives in a different app and is forgotten within days. SplitBill takes the opposite approach. It is an offline-first Android app where a group of roommates records every shared bill on one device, watches live balances update after each entry, and gets a settle-up plan containing the fewest possible repayments. Money itself moves over any UPI app; SplitBill records the UPI transaction reference against each settlement, so the ledger stays complete, private and auditable without ever touching the network.

1. Roommates are added to a group (name and share identity), stored as Member records in the local Room database.
2. Each shared bill is recorded with a description, amount, payer and split type — equal across selected members, or custom per-person amounts.
3. The split engine derives every member's net balance: total paid minus total share owed, adjusted by all recorded settlements (balances always sum to zero).
4. A greedy minimum-cash-flow algorithm pairs the largest creditor with the largest debtor, emitting the smallest possible list of repayments.
5. Money moves over any UPI app outside SplitBill; the payer then records the settlement in the app with the UPI transaction reference.
6. The ledger appends the settlement permanently, balances update instantly, and the settle-up plan shrinks until everyone is even.

Tech stack:

• Kotlin
• Android SDK (minSdk 24)
• Room · SQLite (on-device database)
• Material Components for Android
• Android Studio
• HTML · CSS · JavaScript (companion web demo)
• localStorage (web demo persistence)
• Git

• [Offline-first ledger] Every member, expense and settlement is stored in an on-device database — the app works in airplane mode with zero network calls and no login.
• [Equal and custom splits] Split a bill equally across the group, or assign custom per-person shares for expenses not everyone took part in (the app validates that custom shares total the bill).
• [Live who-owes-whom balances] Net balance per member is recomputed after every entry as (total paid − total share owed) adjusted by settlements, so the group always sees the current truth.
• [Minimum-transfer settle-up plan] A greedy cash-flow engine pairs the largest creditor with the largest debtor repeatedly, reducing the whole history to the fewest repayments.
• [UPI settlement ledger] Each repayment is recorded with payer, receiver, amount, date and an optional UPI transaction reference, forming a permanent, dispute-proof record.
• [One-tap settlement recording] Every suggested repayment carries an Autofill button that pre-fills the settlement form — confirm the UPI reference and the debt clears instantly.
• [Private by design] No accounts, no servers, no analytics — the group's financial data never leaves the device it was entered on.

• Complete Android source code (Kotlin + Room) with setup instructions
• Debug APK installable on any Android 7.0+ phone
• Single-file web demo implementing the same split engine and screens
• Room database schema and data-model documentation
• Project report PDF (background, methodology, architecture, screens)
• PPT presentation for final review
• Viva Q&A preparation document (local-first design, split algorithm, Room vs SQLite, UPI tracking scope)
• Downloadable project abstract PDF