Monthly principal and interest, the interest across the whole term, and what one extra payment a month saves.
Optional add-ons are display only. They never enter the loan maths.
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The monthly rate is the annual rate divided by twelve; the number of payments is the term in years times twelve. The payment is the standard amortizing formula: principal × monthly rate × (1 + monthly rate)^payments, over (1 + monthly rate)^payments − 1. At a 0% rate there is no interest to solve for, so the payment is simply the loan divided by the number of months.
Total interest comes from running the loan one month at a time: interest is the balance × the monthly rate, the rest of the payment cuts the balance, and the count stops at zero or at 1,000 months. This is principal and interest only — escrow, closing costs, mortgage insurance and the tax deduction rules are outside it. The optional add-ons above only feed the estimated total monthly outlay line.
$300,000 at 6.5% over 30 years: $1,896.20 a month in principal and interest, $382,633 of interest over 360 payments, $682,633 paid in total — more interest than you borrowed. The first payment is $1,625.00 interest ($300,000 × 0.065 ÷ 12) and $271.20 principal, 86% interest. Pay $200 extra each month and the loan retires in 277 months (23 years 1 month) with $279,185 of interest: 83 months (6 years 11 months) shorter, $103,449 saved. At 0% the same loan over 30 years is $833.33 a month and no interest.
The payment is fixed but the split inside it is not. Early months are mostly interest, so an extra dollar in year one buys far more than an extra dollar in year twenty-five.
It assumes one fixed rate and one payment a month: no points, no balloon, no ARM reset, no prepayment penalty, no servicing fee, and nothing about inflation or your tax situation.
A refinancing decision turns on total cost over the remaining term, closing costs rolled in, not the headline rate — a lower rate on a fresh 30-year term can add interest while the payment falls.
Arithmetic only: not a rate quote or a loan offer. Check rates, terms and fees against your own disclosures.