Mortgage Calculator — Monthly Payment, Total Interest & Amortization

About this calculator

Enter loan amount, annual interest rate and loan term (years) to calculate your monthly principal & interest payment, total interest paid and a full month-by-month amortization breakdown — updated instantly. Add an optional extra monthly principal payment to see how much interest it saves and how much sooner the loan is paid off, then export the schedule as CSV or print it.

How to use

1. Enter the loan amount (USD), annual interest rate and loan term in years.
2. Review the monthly payment, total paid and total interest, then open “Show month-by-month breakdown” for the per-payment principal/interest split.
3. Optionally add an extra monthly principal payment to see how much interest it saves and how much sooner the loan is paid off, then export the schedule as CSV or print it.

How it works — formula and rules

• Core amortization formula (fixed-rate, fully-amortizing loan):

M = P * (r * (1 + r)^n) / ((1 + r)^n - 1)
Where:
- M = monthly payment (principal + interest)
- P = loan principal (USD)
- r = monthly interest rate = annual_rate / 12
  * Normalization: if a user supplies annual_rate > 1, it is treated as percent and divided by 100 before dividing by 12 (e.g., 5 -> 0.05). If the rate is given as decimal (0.05) it is used directly.
- n = total number of monthly payments = loan_term_years * 12

• Totals:

• Total paid = M * n
• Total interest = Total paid − P

• Per-period amortization (for month k starting at 1), computed in whole cents:

• interest_k = round(balance_{k-1} * r)
• scheduled_principal_k = min(M − interest_k, balance_{k-1})
• extra_k = min(extra_monthly, balance_{k-1} − scheduled_principal_k) — the extra never overshoots the remaining balance
• balance_k = balance_{k-1} − scheduled_principal_k − extra_k
• Initial balance_0 = P. The loop ends the month the balance reaches exactly 0, which is why an extra payment shortens the schedule.
• Because every row is rounded to the cent as it is computed, the principal and interest columns sum exactly to the reported totals — there is no floating-point drift to reconcile.

• Zero-interest fallback (edge case):

- If annual_rate == 0, use M = P / n (no interest). Total interest = $0.00.

• Accepted input ranges:

• Loan amount: must be > 0 (upper bound: $100,000,000)
• Annual rate: must be >= 0 and ≤ 100 (%) by default; if input > 1, treat as percent
• Term: years > 0 and ≤ 100 years (n ≤ 1200 months)
• Extra monthly principal: must be >= 0 (leave at 0 for a plain schedule)
• If inputs exceed bounds or are invalid, the tool will show a clear validation message and prevent calculation.

• Display and precision:

- All displayed monetary values use USD with two decimal places and thousands separators. The monthly payment is derived in floating point from the formula above, then the schedule is amortized in integer cents so the rows reconcile with the totals exactly.

FAQ

How is a monthly mortgage payment calculated?

The tool uses the standard amortization formula: M = P[r(1+r)^n]/[(1+r)^n − 1], where r = annual_rate/12 and n = years×12. For 0% use M = P/n.

What does my monthly mortgage payment include?

Principal and interest (P&I) only. Property taxes, homeowners insurance, PMI and HOA dues are not part of this calculator, so your real escrowed payment (PITI) will be higher than the figure shown here.

How much interest will I pay over the life of the loan?

Total interest = (monthly payment × number of payments) − loan amount. The tool shows this total and the full amortization schedule.

What happens if I make extra payments?

Enter an amount in “Extra monthly principal” and it is applied to the balance every month on top of the scheduled payment. The balance falls faster, so each later payment carries less interest and the loan finishes early. The tool reports how many months you save and how much interest that avoids, and the schedule is rebuilt to match.

What's the difference between APR and interest rate?

This calculator uses the nominal annual interest rate (r = annual_rate/12). APR additionally folds in lender fees and closing costs, so it is usually a little higher than the note rate. There are no fee inputs here, so the results reflect the note rate only.

Can I enter a 0% interest rate?

Yes. At 0% the payment is simply loan amount divided by number of payments (M = P/n) and total interest is $0.00.

How do I read the amortization schedule?

Each row is one monthly payment and shows the payment number, the total paid that month, how much went to principal, how much went to interest, any extra principal, and the balance left afterwards. Early payments are mostly interest; the principal share grows every month.

Can I export the schedule?

Yes. “Export CSV” downloads every row (payment number, payment, principal, interest, extra principal, remaining balance) as a spreadsheet-ready file, and “Print” opens your browser's print dialog with the full schedule expanded and the input form hidden.

Are these results exact?

The schedule is computed in whole cents, so the principal and interest columns sum exactly to the totals shown. It models a fixed-rate loan with no fees, taxes or insurance, and assumes every payment arrives on schedule — your servicer's figures can differ by a few cents depending on their day-count convention.