Why your calculator's money math is wrong (and how to fix it)

Quick Calcify — Sat, 14 Nov 2026 05:39:40 +0000

If you have ever built a loan or mortgage calculator, you have probably shipped this bug: 0.1 + 0.2 in JavaScript is 0.30000000000000004, not 0.3. Display it raw and your calculator tells a user their payment is $313.36000000000003. Round only at display time and the error compounds across an amortisation schedule, so the final balance comes out a few cents off and the schedule never quite reaches zero.

Three rules that fix it in practice:

Do money in integer cents (or a decimal type) until the very last step. Parse the input once, convert dollars to cents, and keep every intermediate value in cents. Only format back to dollars for display.
Round each month's split, not just the total. Interest for a month is balance x monthly rate, rounded to the cent; principal is payment minus that rounded interest. If you round only the payment, the schedule drifts.
Force the last payment to clear the balance. Take a $10,000 loan at 8% over 3 years: the payment is $313.36 a month, first month's interest is $66.67 and the total interest is $1,281.09. Compute the schedule month-by-month with the rules above and the final payment lands a few cents short of $313.36 — set it to whatever balance remains, and say so in the UI, instead of showing a phantom final balance.

The same discipline applies to the formula itself: use the standard amortisation formula, payment = P x r / (1 - (1 + r)^-n), with r the monthly rate and n the number of payments, and show the schedule so a user can check your arithmetic — a calculator nobody can audit is a calculator nobody trusts. Live examples of the rounding done this way: https://quickcalcify.com/loan-calculator and https://quickcalcify.com/mortgage-calculator

DEV Community: Quick Calcify

Why your calculator's money math is wrong (and how to fix it)