Split amounts without losing a cent

How to do it

Never round each part and hope the sum matches. Share the total by ratio in integers, then give the remainder one unit at a time to the first parts. For tax, round once at a documented step, per line or per invoice, and use the same step as your invoices and tax service.

Example

function allocate(totalMinor, ratios) {
  const ratioSum = ratios.reduce((sum, ratio) => sum + ratio, 0)
  const parts = ratios.map(ratio => Math.floor((totalMinor * ratio) / ratioSum))
  let remainder = totalMinor - parts.reduce((sum, part) => sum + part, 0)
  for (let i = 0; remainder > 0; i++, remainder--) parts[i] += 1
  return parts
}
// allocate(10000, [1, 1, 1]) gives [3334, 3333, 3333]

def allocate(total_minor: int, ratios: list[int]) -> list[int]:
    ratio_sum = sum(ratios)
    parts = [total_minor * ratio // ratio_sum for ratio in ratios]
    for i in range(total_minor - sum(parts)):
        parts[i] += 1
    return parts

How to check

A property test splits 10,000 random totals by random ratios. The parts always sum to the total, and no part is a whole minor unit or more away from its exact share.