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Price and tax math should live in one shared place so Android and iOS can never disagree on a total. 💰

When each app rounds tax its own way, the same cart shows two prices. A single shared calculator kills that whole class of bug.

This calculator belongs in commonMain, the source set both platforms compile, so there is exactly one source of truth.

Start with a tiny data class that holds the amount in plain numbers. Keep it pure Kotlin so it works on every target.

Tax is just the amount times the rate. Expressing it as one function means both apps run the exact same line.

The total is the base amount plus the tax you just computed. Compose small functions instead of one giant one.

Group the rules in a small class so both apps get a clean, named entry point instead of loose functions.

Floating point can drift by a cent. Decide your rounding rule once in shared code so totals are identical everywhere.

Real carts have discounts. Apply the discount before tax so the saving feels honest to the customer.

A sensible default tax rate keeps simple call sites short while still letting either app override it when needed.

Android and iOS each call the same total function. No copied math, no drift, one trusted answer for the price.

Where should the price and tax calculator live in a KMP project?

You modeled price, computed tax, totaled it, and handled rounding in commonMain. One calculator, zero price drift between apps. ✅