You're dealing with a fundamental issue in financial rounding: individual rounding vs. cumulative rounding.

Key points:

• round(a) + round(b) ≠ round(a + b) In many cases. This is expected and unavoidable.

• Always perform calculations in full precision. Only round once, at the final step—when displaying, storing to DB, or sending to payment systems.

• Use deterministic rounding strategies:

  • ROUND_HALF_UP It is standard for most currencies.

  • Avoid always rounding up/down—this introduces bias.

  • Use ROUND_DOWN only when legally required (e.g. specific tax laws).

• Use BCMath or a similar high-precision library for all internal calculations. Define precision per currency—e.g. USD should always enter, process, and leave your system with exactly two decimal places.

• For invoices or receipts, if totals don’t align due to rounding, adjust the last line item to match the correct total.

You're dealing with a fundamental issue in financial rounding: individual rounding vs. cumulative rounding.

Key points:

• round(a) + round(b) ≠ round(a + b) In many cases. This is expected and unavoidable.

• Always perform calculations in full precision. Only round once, at the final step, when displaying, storing to DB, or sending to payment systems.

• Use deterministic rounding strategies:

  • ROUND_HALF_UP It is standard for most currencies.

  • Avoid always rounding up/down, this introduces bias.

  • Use ROUND_DOWN only when legally required (e.g. specific tax laws).

• Use BCMath or a similar high-precision library for all internal calculations. Define precision per currency, e.g. USD should always enter, process, and leave your system with exactly two decimal places.

• For invoices or receipts, if totals don’t align due to rounding, adjust the last line item to match the correct total.