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Splitwise LLD: Where Every Rounding Paisa Must Find a Home

A low-level design walkthrough of a Splitwise expense sharing system: immutable expenses, derived balances, split strategies with the rounding paisa rule, and greedy debt simplification.

By fiveyearsdevJune 12, 202610 min read

"Design Splitwise." The app is beloved; the design question is feared, because it hides three separate exams in one trench coat: a data-modeling exam (what's the source of truth — debts or expenses?), an arithmetic exam (split ₹100 three ways and account for the missing paisa, because money must sum exactly), and an algorithms exam (the famous "simplify debts" feature — turn a web of IOUs into the fewest transfers). Most candidates fail the middle one. Floating point and money, ₹33.33 × 3 = ₹99.99, and somewhere a paisa has vanished into the void — in a system whose only job is that money sums…

What’s inside

Let's start nowhere near a computer

Where the fridge notebook scales up

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — Money first: paise, never doubles

Step 4 — Splits are strategies, and the paisa needs a rule

Step 5 — The ledger: expenses in, balances out

Step 6 — Simplify debts: the algorithm everyone came for

Step 7 — Trade-offs (each one keeping an NFR)

The complete implementation

The interview corner

Where to go from here

Read this one free

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