Merizo
Track shared expenses and bills with your household or friends — simple, secure, and free.
Overview
Merizo is a shared household expense tracking app built for groups of people who split costs — housemates, friends, couples, or travel companions. It handles the full lifecycle of shared expenses: logging, splitting, tracking balances, and settling debts.
Unlike basic split calculators, Merizo introduces monthly periods for organized bookkeeping, a debt simplification engine that minimizes the number of transfers needed, and flexible split methods (equal, exact, percentage) with bulletproof floating-point handling.
Key Features
What makes Merizo useful
Smart Expense Splitting
Split costs equally, by exact amounts, or by percentage — with automatic rounding correction so totals always balance.
Group Management
Create groups, invite members via unique invite codes, and switch between multiple groups seamlessly.
Debt Simplification
A greedy matching algorithm minimizes the number of transfers needed to settle all balances within a group.
Monthly Periods
Organize expenses into monthly periods that can be finalized for clean bookkeeping.
Settlement Tracking
Record settlements with automatic distribution across open monthly periods, oldest first.
Activity Feed
Chronological timeline of all group expenses and settlements with per-member filtering.
Technical Challenges
Problems I solved while building Merizo
Floating-Point Precision in Expense Splits
Problem: Splitting an expense like $100 three ways produces $33.333…, and naive rounding causes totals to drift from the original amount.
Solution: Built a normalizeSplitsToTotal function that rounds each share individually, then distributes the residual cent(s) to the participant with the largest share — guaranteeing the sum always matches the expense total exactly.
Minimizing Settlement Transactions
Problem: With N group members, tracking pairwise debts can produce up to N×(N-1)/2 transactions, making settling confusing and tedious.
Solution: Implemented a greedy creditor-debtor matching algorithm that sorts participants by balance, then pairs the largest creditor with the largest debtor iteratively — reducing the typical settlement set to at most N-1 transfers.
Cross-Period Settlement Distribution
Problem: When a user pays off their balance, the payment needs to be allocated fairly across multiple open monthly periods.
Solution: Created a distributeSettlementAcrossPeriods function that allocates payments chronologically (oldest period first), supporting partial payments and handling overpayments gracefully.
Mapping Auth Concepts to the Domain Model
Problem: The app needs group-based access control, but Better Auth has no native "group" concept.
Solution: Mapped Better Auth's organization plugin to the app's group domain, extending the schema with custom fields (invite codes, currency, group type) while preserving all of Better Auth's role and permission capabilities.
Screenshots
A closer look at the interface
