Loan Calculator Conformance Test

Every loan calculator prints a monthly payment. Almost none of them lets you check it. This page publishes the test set Make It Exact uses on itself: 2688 fixed-rate loans with a payment that has been verified two independent ways. It is in the public domain, and it works against any calculator — ours included.

Download the corpus (JSON, version 1.0) — published 2026-08-20, CC0 1.0, no attribution required.

What it covers, and why so little

The scope is deliberately narrow. It holds only what has a single mathematically correct answer: an amortising loan at a fixed rate, level monthly payments, the nominal annual rate divided by twelve, payments at the end of each period, and nothing else — no fees, no insurance, no tax, no calendar dates, no early repayment.

The corpus is the product of 12 amounts, 14 rates and 16 terms, which is where the 2688 comes from. The edges are the point: a rate of exactly zero, where the closed form divides by zero unless somebody thought about it; a loan with a single instalment; forty years, where floating point starts to matter; and amounts with pennies in them, where premature rounding shows.

What it refuses to judge

This is the part that makes the test usable rather than argumentative. A difference is not an error, and the following are differences of model, not of correctness:

• nominal against effective against APR — different quantities, not different answers;
• 30/360 against Act/365 against Act/Act day counts;
• rounding every month against rounding at the end;
• an adjusted final payment;
• an irregular first period.

A calculator that disagrees with this corpus on any of those is not wrong. It has made a different, defensible choice — and a test that pretended otherwise would be measuring conventions while claiming to measure accuracy.

How the payments were verified

Twice, and the second way is the one that matters. The first is the closed form every textbook prints. The second uses no formula at all: it searches, by bisection, for the level payment that leaves nothing owing after the last instalment — the only property that actually defines an amortising loan.

Checking a formula against itself proves nothing, because both sides fail together. The two methods agree on all 2688 loans to within a billionth, which is why the figures in the file can be used as a reference rather than as an opinion.

Three further properties are checked, the kind no single reference value can reveal: doubling the amount doubles the payment, a higher rate never lowers it, and a longer term lowers the payment while raising total interest. An implementation can produce a plausible number on one case; it cannot stay monotone across the whole corpus by accident.

How to use it against a calculator

1. Take a case from the file — amount, rate, months.
2. Enter it in the calculator under test, with fees and insurance set to zero.
3. Compare the monthly payment. Treat it as conforming when it matches to within one cent.
4. Before concluding anything, check the list above. If a documented convention explains the difference, it is not an error.

What this page does not do

It does not name anyone, and it publishes no results for any other site. A public test set is worth more than a league table: it can be re-run, disagreed with, and used against the people who wrote it. That last part is the point — the same corpus runs against Make It Exact on every build, and a failure blocks the release.

Our own rules are on the methodology page, and the calculators this corpus checks are the loan payment and amortization pages.