To calculate interest-only mortgage payments, multiply your current principal balance by your annual interest rate (expressed as a decimal), then divide by 12. On a $400,000 loan at 6.5%, that works out to $2,166.67 a month. During the interest-only period, none of that payment chips away at what you owe. Every dollar covers the cost of borrowing, and the balance sits exactly where it started until the loan converts to full amortization.
The Two Numbers You Need
You need just two inputs: your current principal balance and your annual interest rate. The principal balance is the total you owe the lender right now, which appears on your most recent mortgage statement or the closing documents you signed when the loan funded.
The interest rate lives on your promissory note, and you want the right number off that page. Your loan documents also list an Annual Percentage Rate, which lenders must disclose under federal consumer protection law.1Office of the Law Revision Counsel. 15 USC 1637a – Disclosure Requirements for Open End Consumer Credit Plans The APR folds in origination fees, points, mortgage insurance, and other closing costs to express the total cost of the loan over its lifetime. It does not drive your monthly payment. The note rate does. Plug the APR into the formula by mistake and you’ll overestimate the payment, because the APR is almost always higher than the note rate.
Convert the rate from a percentage to a decimal by dividing by 100. A rate of 7.25% becomes 0.0725. A rate of 6.5% becomes 0.065. This step matters: mixing up 6.5 and 0.065 throws the result off by a factor of a hundred.
The Formula
Monthly Payment = Principal Balance × Annual Interest Rate ÷ 12
Walk through it with a $400,000 loan at 6.5%:
- Step 1: Convert the rate to a decimal. 6.5% ÷ 100 = 0.065.
- Step 2: Multiply the balance by the decimal rate. $400,000 × 0.065 = $26,000. That’s the annual interest cost.
- Step 3: Divide by 12. $26,000 ÷ 12 = $2,166.67 per month.
That $2,166.67 is interest only. It doesn’t include property taxes, homeowners insurance, or any other escrow charge your servicer collects alongside the mortgage payment. The actual bill will run higher once those items are added.
The payment stays flat each month as long as two things don’t move: the balance and the rate. On a fixed-rate interest-only loan, both are locked in for the interest-only term. On an adjustable-rate loan, the rate can shift at specified intervals, and the payment moves with it.
Annual and Cumulative Interest
Interest-only periods on residential mortgages typically run between 3 and 10 years, inside a loan with a 30-year or 15-year total term. Multiplying your monthly payment by 12 gives you the annual interest. On the example above, that’s $2,166.67 × 12 = $26,000.04 per year.
For the cumulative cost, multiply the monthly payment by the number of months in the interest-only phase. Over a 5-year (60-month) interest-only term at $2,166.67 per month:
$2,166.67 × 60 = $130,000.20.
All of it goes to the lender. None of it reduces the $400,000 you borrowed. A traditional 30-year fixed mortgage at the same rate would have paid down roughly $30,000 of principal over those same five years.
Recalculating When an ARM Adjusts
Many interest-only mortgages are adjustable-rate loans. If yours is an ARM, the rate won’t hold steady for the whole interest-only period. It resets at intervals set in your loan agreement, and the payment changes each time it does.
When an ARM adjusts, the new rate is the sum of two numbers: an index (a benchmark the lender doesn’t control, such as the Secured Overnight Financing Rate) and a margin (a fixed percentage set in your loan documents that doesn’t change).2Consumer Financial Protection Bureau. For an Adjustable-Rate Mortgage (ARM), What Are the Index and Margin, and How Do They Work? Your documents also specify rate caps that limit how far the rate can move at a single adjustment and over the life of the loan.
Once you have the new rate, the recalculation is the same formula. Say the rate adjusts from 6.5% to 7.25% on a $400,000 balance:
- New annual interest: $400,000 × 0.0725 = $29,000.
- New monthly payment: $29,000 ÷ 12 = $2,416.67.
A 0.75-point rate increase lifted the payment by $250. Rerun the formula after every reset to keep your budget accurate.
What the Payment Becomes After the Interest-Only Period
When the interest-only phase ends, the loan converts to a fully amortizing schedule. You now have to repay the entire original principal plus interest over whatever time remains, which means the full balance is compressed into a shorter window because none of it has been paid down. The Office of the Comptroller of the Currency warns that payments at reset can jump to double or even triple the interest-only amount.3OCC (Office of the Comptroller of the Currency). Interest-Only Mortgage Payments and Payment-Option ARMs
The amortization formula is more involved:
M = P × r × (1 + r)n ÷ ((1 + r)n − 1)
Where:
- M = monthly payment.
- P = remaining principal balance (the full original amount, since nothing was paid down).
- r = monthly interest rate (annual rate ÷ 12).
- n = number of monthly payments remaining.
Take the same $400,000 at 6.5% with 20 years (240 months) left after a 10-year interest-only period:
- Monthly rate (r): 0.065 ÷ 12 = 0.005417.
- (1 + r)n: 1.005417240 ≈ 3.6537.
- Numerator: $400,000 × 0.005417 × 3.6537 ≈ $7,920.
- Denominator: 3.6537 − 1 = 2.6537.
- Monthly payment: $7,920 ÷ 2.6537 ≈ $2,984.
The payment climbs from $2,166.67 to about $2,984, an increase of roughly $817 per month, or 38%. A shorter remaining term or a higher rate makes the jump steeper. Federal rules require lenders to verify that you can afford the fully amortized payment before approving an interest-only loan, not just the lower initial amount.4Consumer Financial Protection Bureau. 12 CFR 1026.43 – Minimum Standards for Transactions Secured by a Dwelling
Adding Escrow to Get the Actual Bill
The interest-only formula gives you the interest portion. Your mortgage bill almost certainly includes more. Most lenders collect monthly escrow deposits for property taxes and homeowners insurance alongside the loan payment, and servicers must follow federal procedures when managing those accounts.5Consumer Financial Protection Bureau. 12 CFR Part 1024 – Real Estate Settlement Procedures Act (Regulation X)
To estimate the full monthly housing cost, add your annual property tax and annual homeowners insurance premium, each divided by 12, to the interest-only payment. Include private mortgage insurance if you pay it. On a $400,000 loan with $5,000 in annual property taxes and $2,400 in annual insurance:
- Interest-only payment: $2,166.67.
- Property taxes: $416.67.
- Homeowners insurance: $200.00.
- Total monthly bill: $2,783.34.
Your Loan Estimate, which lenders must provide after you submit a mortgage application, breaks out the escrow amounts along with the interest rate and projected payments.6Consumer Financial Protection Bureau. What Information Do I Have to Provide a Lender in Order to Receive a Loan Estimate?
Using the Formula to Check Your Statement
Running the calculation yourself each month is one of the easier ways to catch a servicer mistake. If your figure doesn’t match the statement, you have the right to submit a written notice of error to your servicer under federal mortgage servicing rules. The servicer must acknowledge the notice and investigate within specific timeframes.5Consumer Financial Protection Bureau. 12 CFR Part 1024 – Real Estate Settlement Procedures Act (Regulation X)