Principal vs interest
Each payment covers interest accrued since the last payment plus a slice of principal. When principal falls, future interest charges shrink—this is why extra principal payments can shorten the loan dramatically.
Why APR is not the whole story
Closing costs, mortgage insurance, escrow for taxes/insurance, and adjustable features change total cost of homeownership beyond the quoted note rate.
Sanity-checking results
If you change rate slightly and the payment jumps unexpectedly, verify whether you entered annual rate as percent or decimal and whether the term is in years.
FAQ
Does a bigger down payment always reduce monthly payment linearly?
Principal borrowed drops, but PMI thresholds and lender pricing tiers can create nonlinear effects.
Should I compare 15-year vs 30-year only by payment?
Also compare total interest paid and liquidity needs—lower payments can create flexibility but increase lifetime interest.
Are taxes and insurance included?
Unless your model explicitly adds escrow, many calculators show principal+interest only.