Cumulative Interest Calculator

A cumulative interest calculator adds up the interest inside a chosen range of loan payments, exactly like Excel's CUMIPMT function. On a $300,000 loan at 6.5% over 30 years, the first 12 payments total $22,754.45, of which $19,401.27 is interest and only $3,353.18 reduces the balance. Pick any start and end period to see the same split for that window.

This page is the between-two-dates tool. If you want the monthly payment on a brand-new loan, start with the Loan Calculator; for the full payment-by-payment schedule of a loan, use the Amortization Calculator; to see how extra payments shorten the loan and cut the total, use the Loan Payoff Calculator; and for plain interest on a balance without amortization, the Simple Interest Calculator is the right fit. Come here when the question is "how much interest did I pay between payment 13 and payment 24?"

How cumulative interest is calculated

A fixed-rate loan has a level monthly payment, but the mix of interest and principal inside each payment changes every month. Interest is charged on the balance that is still outstanding, so the first payment carries the most interest and the last carries almost none. The calculator first finds the payment with the standard amortization formula:

where P is the loan amount, r is the monthly rate (annual rate ÷ 12) and n is the number of monthly payments. It then walks the loan forward one month at a time. For month k:

There is also a closed-form shortcut that Excel's CUMIPMT uses internally. Because every payment is the same, the interest in a window is simply everything you paid in that window minus the amount by which the balance dropped:

B_k is the balance remaining right after payment k, and B₀ is the original loan amount. Both routes give identical results.

Worked example: interest in the first year of a $300,000 loan

Take a $300,000 loan at 6.5% over 30 years and ask for periods 1 to 12. The monthly rate is 0.065 ÷ 12 = 0.0054167, and (1 + r)³⁶⁰ works out to 6.9918, so the payment is $1,896.20. Now walk through the first year:

• Payment 1: interest = $300,000 × 0.0054167 = $1,625.00; principal = $1,896.20 − $1,625.00 = $271.20; balance falls to $299,728.80.
• Payment 12: the balance has slipped just enough that interest is $1,608.40 and principal is $287.81; the balance after payment 12 is $296,646.82.
• Sum of the 12 interest charges: $19,401.27 (cumulative interest, CUMIPMT).
• Sum of the 12 principal portions: $3,353.18 (cumulative principal, CUMPRINC), which equals $300,000 − $296,646.82.
• Check: 12 × $1,896.20 = $22,754.45 = $19,401.27 + $3,353.18.

Over the whole 360 payments the lifetime interest is $382,633.47, so year one alone accounts for about 5.1% of it, and 85.3% of every dollar paid that year went to interest rather than debt reduction. In Excel the same result is =CUMIPMT(0.065/12, 360, 300000, 1, 12, 0), which displays as −19,401.27.

Second example: a window in the middle of the loan

The tool is most useful for windows that do not start at payment 1. Ask for periods 61 to 120 (years 6 through 10) on the same loan. The balance right after payment 60 is $280,832.93 and after payment 120 it is $254,328.38, so the balance drops by $26,504.55 during those 60 payments: that is the cumulative principal. The 60 payments total 60 × $1,896.20 = $113,772.24, and subtracting the principal leaves $87,267.69 of cumulative interest. Even in years 6 to 10, 76.7% of what you pay is still interest.

Cumulative interest year by year: $300,000 at 6.5% for 30 years

This is the same schedule the calculator prints, condensed to milestone years. Every figure comes from the month-by-month amortization above.

Two milestones stand out. Half of all the interest you will ever pay is gone by payment 127, barely a third of the way through the loan, while half of the principal is not repaid until payment 257. And principal does not overtake interest inside a single payment until payment 233, when the split is $946.50 of interest against $949.70 of principal.

Cumulative interest by five-year window

Slicing the same $300,000, 6.5%, 30-year loan into six five-year windows shows how the interest share of each window declines. This is the kind of comparison CUMIPMT was designed for.

Each window costs the same $113,772.24 in payments, yet the interest inside it falls from $94,605 to $16,860.

First-year interest by loan amount and rate (30-year term)

The interest in the first 12 payments is the number most people look up, because it is roughly what shows on the first Form 1098 for a mortgage. All figures use the 30-year amortization formula.

First-year interest is very close to loan amount × rate, slightly less because the balance shrinks a little during the year. It scales in exact proportion to the loan amount, so a $250,000 loan at 6.5% costs about $16,168, halfway between the $200,000 and $300,000 rows.

How the term changes first-year interest and principal

People often assume a shorter term means less interest in year one. It does, but only slightly, because the first month's interest depends on the balance, not the term. What changes dramatically is the principal. For $300,000 at 6.5%:

• 30 years: payment $1,896; year-one interest $19,401; year-one principal $3,353; lifetime interest $382,633.
• 20 years: payment $2,237; year-one interest $19,277; year-one principal $7,563; lifetime interest $236,813.
• 15 years: payment $2,613; year-one interest $19,140; year-one principal $12,220; lifetime interest $170,398.
• 10 years: payment $3,406; year-one interest $18,851; year-one principal $22,026; lifetime interest $108,773.

The extra $717 a month on the 15-year loan reduces first-year interest by only $261, but it repays $8,867 more principal, and every future month's interest is charged on that smaller balance. That compounding effect is where the $212,235 lifetime saving comes from.

How to use this cumulative interest calculator

1. Loan amount: enter the original principal, not today's balance. The period numbers count from the first payment of that original loan.
2. Annual interest rate: the note rate on your loan documents, not the APR. The APR includes fees and would overstate the interest.
3. Loan term: click 30, 20, 15, 10 or 5 years, or type a custom term. The calculator shows the resulting number of monthly payments.
4. Start and end period: type the payment numbers, or use the presets for year 1, year 2, years 1 to 5, or the whole loan. Set both to the same number to isolate a single payment.
5. Read the results: the green card shows cumulative interest, cumulative principal, and the balance before and after the window. Below it, the interest share and the highlighted rows in the year-by-year table put the window in context.

If you know calendar dates rather than payment numbers, count forward from your first due date. A loan whose first payment was due in March 2025 has payments 1 to 10 in 2025 and payments 11 to 22 in 2026.

Who this calculator is for

• Homeowners checking a Form 1098 who want an independent estimate of the mortgage interest reported for a tax year.
• Spreadsheet users who need CUMIPMT or CUMPRINC without opening Excel, or who want to sanity-check a formula's arguments.
• Borrowers deciding whether to refinance or sell who want to know how much interest they have already paid and how much remains.
• Landlords and small-business owners allocating loan interest to a fiscal year or to a specific project period.

Key terms explained

• Period: one scheduled payment. With monthly payments, period 1 is the first payment and period 360 is the last on a 30-year loan.
• Cumulative interest (CUMIPMT): the sum of the interest portions of all payments from the start period to the end period, inclusive.
• Cumulative principal (CUMPRINC): the sum of the principal portions over the same window, which equals the drop in the balance.
• Type 0 vs type 1: Excel's last argument. Type 0 means payments fall at the end of each period, which is how US loans work; type 1 means payments at the beginning. This calculator uses type 0.
• Amortization: the gradual repayment of a loan through level payments that shift from mostly interest to mostly principal.
• Note rate: the contractual interest rate used for the monthly interest charge, as opposed to the APR, which folds in fees.

It works for any amortizing loan, not just mortgages

The math is identical for shorter consumer loans; only the scale changes. A $35,000 auto loan at 7% over 60 months has a payment of $693.04. In payments 1 to 12 the interest is $2,258.07 and the principal is $6,058.43, leaving a balance of $28,941.57. In payments 13 to 24 the interest drops to $1,820.11. In the final year, payments 49 to 60, it is only $306.93. Lifetime interest is $6,582.52. A $25,000 personal loan at 11% over 3 years has a payment of $818.47 and carries $2,382.35 of interest in the first 12 payments against $7,439.26 of principal. Because these loans are short, the interest share falls quickly: 27% in year one of the auto loan compared with 85% in year one of the 30-year mortgage.

What changes the result the most

• Where the window sits: the same 60 payments cost $94,605 in interest at the start of the $300,000 loan and $16,860 at the end.
• The rate: first-year interest rises almost one-for-one with the rate; each 0.5% adds about $1,500 on a $300,000 loan.
• The loan amount: cumulative interest scales exactly with principal. Double the loan, double the interest in every window.
• The term: a longer term barely changes early interest but stretches the number of high-interest years, which is why lifetime interest more than triples from 10 to 30 years.
• Extra payments: not modeled here, but any prepayment lowers the balance and therefore the interest in every later period.

Tips for getting an accurate answer

• Use the original loan amount and count periods from the first payment. If you only know today's balance, enter that as the principal with the remaining term and treat the next payment as period 1.
• Match the window to your statement or tax year by counting due dates, not the months you actually sent money.
• If you have made extra principal payments, expect your lender's figure to be lower than the calculator's, since a smaller balance means less interest each month.
• For an adjustable-rate loan, run each rate period separately with the balance at the start of that period as the principal.

Limitations and assumptions

• Assumes a fixed rate, level monthly payments made exactly on the due date, and no extra, late or skipped payments.
• Uses a monthly rate of annual rate ÷ 12. Some lenders accrue interest daily, which produces small differences from month to month.
• Excludes escrow, taxes, insurance, PMI, origination fees and prepayment penalties; none of these are interest.
• The last payment is capped so the balance ends at exactly zero; lenders may round the final payment slightly differently.
• Results are for planning. Your lender's statement or Form 1098 is the authoritative record of interest paid.

How it compares to related calculators

This page answers "how much interest falls between payment s and payment e?" For neighboring questions, a sister tool fits better:

• To see every payment's split from first to last, use the Amortization Calculator.
• To size a new loan and find the payment, use the Loan Calculator.
• To see how extra payments shorten the loan and shrink the lifetime interest, use the Loan Payoff Calculator or, for a home loan, the Mortgage Payoff Calculator.
• For interest on a fixed balance with no amortization, use the Simple Interest Calculator; for savings that grow, the Compound Interest Calculator.
• To model the full monthly housing cost with taxes and insurance, use the Mortgage Calculator.

Sources

• Internal Revenue Service (IRS) - Publication 936, Home Mortgage Interest Deduction (how interest reported on Form 1098 is treated).
• Consumer Financial Protection Bureau (CFPB) - What is amortization and how could it affect my loan?
• Standard fixed-rate amortization formula (deterministic financial mathematics; equivalent to Excel CUMIPMT / CUMPRINC with type 0).