Two directions, one formula. Payment mode answers "what will this loan cost per month?"; affordability mode answers the more useful budgeting question — "what loan does my monthly budget support?"
Formula & methodology
Payment mode uses standard amortization (see the loan calculator for the derivation). Affordability inverts it: P = M·((1+r)ⁿ − 1) ÷ (r(1+r)ⁿ); at zero rate it is simply M × n. Rounding follows the same half-up, 2-decimal rule.
Worked example
Budget of 1,000/month at 4% over 15 years → supports a loan of about 135,191.75 (total paid 180,000, of which ~44,808 is interest).
Limits
Lenders also apply income ratios, fees, and stress tests — affordability here is arithmetic, not approval. Estimates only, not financial advice.