Interest Calculator
A $300,000 30-year loan at 6.625% costs $391,000 in interest on top of the principal. This calculator shows the full lifetime cost and what an extra payment saves.
Add an extra monthly payment to see how much interest you can eliminate.
Methodology & Assumptions
How this calculator works, what it assumes, and where it falls short. Every calculator on TruePITI documents its math.
Formula
- Monthly payment = L × r(1+r)ⁿ / ((1+r)ⁿ − 1). Monthly interest = balance × rate ÷ 12; total interest = sum of every month's interest over the full amortization. Extra-payment mode pays principal directly and re-solves the payoff month.
Assumptions
- Fixed rate for the full term — ARM and balloon loans differ.
- Extra payments applied every month at the entered amount.
- No prepayment penalty (standard for most FHA/VA/conventional loans).
Limitations
- Refinancing resets the schedule — early extra payments are the most valuable precisely because they shorten the highest-interest years.
- Lenders must apply extra to principal — otherwise it may sit as a credit toward future payments.
- The payment shown is P&I only — taxes, insurance, and PMI are separate (use the PITI calculator).
Worked Example
- $300,000 at 6.625% for 30 years: payment $1,921, total interest $391,500. Adding $200/month cuts interest to about $285,000 and pays off 7 years early — a $107,000 saving.
Sources
- CFPB — Owning a home
- Standard amortization formula (verified)
Interest Calculator FAQ
How much interest will I pay on a $300,000 mortgage?
At 6.625% over 30 years, about $391,000 in interest — more than the loan itself. At 5.625% it drops to $321,000; at 7.625% it rises to $468,000. The rate matters more than most buyers realize: each 0.5% is roughly $35,000-40,000 of lifetime interest on this loan size.
How is monthly mortgage interest calculated?
Monthly interest = current balance × annual rate ÷ 12. In month 1 of a $300,000 loan at 6.625%, that is $1,656 of interest; your $1,921 payment applies just $265 to principal. Every month the interest portion shrinks as the balance falls — that is amortization.
How much does an extra $100 per month save?
On a $300,000 loan at 6.625%, an extra $100/month saves about $63,000 in interest and pays the loan off 4 years early. $200/month saves about $107,000 and 7 years; $500/month saves about $185,000 and 12 years.
Interest calculator vs mortgage calculator — what is the difference?
The interest calculator focuses on lifetime interest cost and extra-payment savings. The mortgage calculator (PITI) adds property tax, insurance, and PMI for the full monthly cost. Use both: PITI for budgeting, interest calculator for payoff strategy.
Does paying extra toward principal actually reduce interest?
Yes — because interest is charged on the remaining balance each month, every dollar of extra principal permanently removes itself (plus all future interest on that dollar) from the schedule. The earlier you pay extra, the more it saves: an extra $200 in year 1 saves roughly twice what the same $200 saves in year 15.
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Last updated: 2026-09-17 | Estimates only, not financial advice.