Total interest over a loan's life

Calculate the cumulative interest across the entire life of a fixed-rate loan. This tool keeps the original balance, rate, and term together and reports total borrowing interest separately from the principal you repay. It also shows the combined amount paid across the scheduled installments.

Your interest schedule

Worked example: payments in the second year

The example inputs are from Microsoft CUMIPMT and Microsoft CUMPRINC: an original balance of $125,000, an annual rate of 9%, a 30-year term, and end-of-period monthly payments. The selected range is payments 13 through 24. Figures below are calculated from those inputs and rounded for display.

How it works

P is the original loan balance, N is the number of monthly payments, and A is the constant scheduled payment. For beginning-of-period payments, divide A by (1 + r); the first payment has no interest. With a zero rate, A = P / N and interest is zero. Sum the interest and principal separately across the requested payments.

Microsoft PMT documentation supports constant-payment calculation, and Microsoft CUMIPMT and CUMPRINC define the inclusive payment-range totals. The CFPB amortization explanation describes how the balance and interest portions change. Zero-rate results are an extension here; Excel CUMIPMT and CUMPRINC require a positive rate.

Read the lifetime total

Lifetime interest is the sum of every interest portion in the schedule. Principal repaid is the original amount borrowed when the modeled loan reaches its final payment. Total paid combines those two amounts. These results describe scheduled principal and interest only. They do not add taxes, insurance, origination charges, late charges, or other costs outside this model.

The annual table helps you see where the lifetime total comes from. Read down the interest column to compare earlier and later loan years. The cumulative column adds each year's interest to all preceding modeled payments. Its final value equals lifetime interest. If the term ends during a loan year, the last row contains only the payments that remain.

Compare terms without changing the question

To compare two possible schedules, enter the same original balance and rate, calculate one term, and record the lifetime interest. Change only the term and calculate again. This isolates the effect of the payment count inside the model. If you change the balance, rate, and term together, the difference combines all of those changes and does not tell you which input caused it.

A longer schedule spreads principal repayment across more periods. Interest is tied to the outstanding balance each period, so leaving principal outstanding affects the total cost. The CFPB explains this changing allocation in its amortization guidance. Use the actual contractual rate for a loan comparison. An advertised APR that includes other charges is not necessarily the nominal interest rate needed by this periodic model.

Keep the assumptions visible

Choose payment timing that matches the schedule you want to model. End-of-period timing accrues interest before each installment. Beginning-of-period timing pays the first installment immediately and adjusts the constant payment accordingly. Do not switch timing merely to make a lender statement match; first check the contract and the lender's calculation method.

This tool calculates with unrounded intermediate values and rounds only the displayed dollar amounts. A servicing schedule that rounds every payment or uses actual days can differ. Extra principal payments, skipped payments, changing rates, and balloon balances require a different schedule. This tool deliberately keeps the lifetime calculation fixed so that each annual subtotal can be traced back to the same assumptions.

For only part of the term, return to the cumulative interest calculator and select an inclusive range. For a closer view of one annual subtotal, use interest paid by year. For spreadsheet verification, the CUMIPMT tool shows the equivalent function inputs and signed results.

Frequently asked questions

Sources

• Microsoft: PMT function
• Microsoft: CUMIPMT function
• Microsoft: CUMPRINC function
• CFPB: amortization and payment allocation

Data as of 2026-10-05. Source examples and formula documentation only; no live market rates.

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