Most loan calculators give you a number. This one shows you the math behind it — and why the difference between a 15-year and 30-year mortgage is more dramatic than most people realize.
What a loan calculator actually does
When you enter a loan amount, interest rate, and term into a loan calculator, it's solving a version of this formula:
M = P × [r(1+r)^n] / [(1+r)^n − 1]
Where:
- M = monthly payment
- P = principal (amount borrowed)
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of payments (term in years × 12)
Most people stop there. The interesting part is what happens when you start adjusting variables.
Why a 15-year mortgage costs less than it looks
Take a $300,000 mortgage at 6.5%:
- 30-year term: $1,896/month, total cost: $682,000
- 15-year term: $2,586/month, total cost: $465,000
The 15-year costs $690 more per month. But over the life of the loan, you save $217,000 in interest. That's a 36% reduction in total interest paid.
The reason is compounding in reverse — you're paying down principal faster, which means less interest accrues on a shrinking balance each month.
The extra payment trick
Most banks allow you to make extra principal payments without penalty. The impact is outsized:
On that 30-year $300,000 mortgage at 6.5%:
- Adding $200/month to payments cuts ~8 years off the loan
- Saves approximately $78,000 in total interest
- Equivalent to finding a 0.6% lower rate
This is why financial advisors often recommend the 30-year mortgage with the 15-year payment target — you get flexibility if cash is tight, and the option to redirect the difference to principal when it's not.
Amortization is the part nobody looks at
Early in a loan's life, most of your payment goes to interest, not principal. By the end, it's the opposite.
For the $300,000 / 30-year / 6.5% loan:
- Month 1: $1,625 goes to interest, $271 goes to principal
- Month 180 (year 15): $1,098 goes to interest, $798 goes to principal
- Month 360 (final): $10 goes to interest, $1,886 goes to principal
This matters if you're considering refinancing or making extra payments — the further along you are, the less benefit you get from those moves.
The loan calculator I built
I needed a loan calculator that handled the real edge cases: amortized schedules, extra payment scenarios, and the ability to compare scenarios side by side without a spreadsheet.
Built one that covers:
- Mortgage, personal loan, car loan, and student loan scenarios
- Extra payment support with full amortization schedule
- Side-by-side comparison of two scenarios
- No signup, no data stored
→ Loan Calculator — with amortization and extra payment scenarios
The amortization table is the feature I use most — it's the clearest way to see exactly where your money goes each month.
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