Interest-Only Mortgage Calculator
Calculate your interest-only mortgage payment, the post-IO amortizing jump, and how much extra interest an IO loan costs compared to a conventional mortgage.
🏦 What is an Interest-Only Mortgage?
An interest-only (IO) mortgage is a home loan where your monthly payment covers only the interest charged on the outstanding balance for a fixed period, typically 5 to 15 years. During this interest-only phase you pay nothing toward the principal, so your loan balance stays exactly the same from the day you sign to the last day of the IO period. When the IO period ends, the loan converts to a fully amortizing structure and your monthly payment increases, sometimes dramatically.
The primary appeal is lower payments upfront. A $400,000 loan at 6.5% has an IO payment of about $2,167 per month versus a conventional 30-year payment of about $2,528 per month, a savings of $361 per month during the IO phase. For real estate investors maximizing rental cash flow, or high-income professionals who expect larger future earnings, this can be attractive. Buyers in expensive markets sometimes use IO loans to afford a home that would otherwise be out of reach on a conventional payment schedule.
The significant trade-off is cost. Because no principal is repaid during the IO phase, the full original balance must be amortized over the remaining shorter term when the IO period ends. On a 30-year loan with a 10-year IO period, you repay the original principal over just 20 years instead of 30, producing a post-IO payment that is substantially higher than the conventional mortgage payment. The same $400,000 at 6.5% would require a post-IO payment of about $2,988 per month versus the conventional $2,528, an increase of $460 per month.
IO mortgages also cost more in total interest over the life of the loan. A conventional 30-year mortgage begins reducing the principal from day one, steadily shrinking the balance on which interest accrues. An IO loan keeps that balance at its maximum for the entire IO phase, accumulating extra interest with every passing month. This extra interest cost can easily exceed $50,000 to $100,000 on a typical jumbo IO loan. This calculator shows you the exact comparison so you can make an informed decision about whether the IO structure suits your financial situation.
📐 Formula
IO Payment = P × r
P = Loan principal (original balance)
r = Monthly interest rate = Annual Rate ÷ 12 ÷ 100
Post-IO Payment = P × r × (1+r)n ÷ [(1+r)n − 1]
n = Remaining months after IO period = (Total Term × 12) − (IO Years × 12)
Conventional Payment = P × r × (1+r)N ÷ [(1+r)N − 1], where N = total term × 12
Total Interest (IO) = (IO Payment × IO Months) + (Post-IO Payment × Amortizing Months) − P
Example: $300,000 at 6%, 30-yr term, 10-yr IO. r = 0.005. IO = $1,500. Post-IO over 240 months = $2,149. Conventional over 360 months = $1,799.
📖 How to Use This Calculator
Steps
1
Enter your loan amount — Type or slide to your home loan balance. For purchase loans, subtract your down payment from the home price to get the loan amount.
2
Enter the annual interest rate — Enter the rate your lender has quoted. IO mortgage rates are often 0.25% to 0.50% higher than conventional rates. Use the same rate for a fair comparison.
3
Select the interest-only period — Choose 5, 7, 10, or 15 years. After this period, the loan converts to a fully amortizing payment schedule.
4
Select the total loan term — Choose your full loan term: 20, 25, or 30 years. The amortizing phase covers the years remaining after the IO period ends.
5
Click Calculate — See your IO payment, the post-IO payment jump, the conventional comparison, and the total extra interest cost of the IO structure.
💡 Example Calculations
Example 1 — $300,000 Loan, 6%, 10-Year IO, 30-Year Term
Classic IO mortgage: modest loan, moderate rate
1
Monthly rate r = 6% ÷ 12 ÷ 100 = 0.005. IO Payment = $300,000 × 0.005 = $1,500/month for 120 months.
2
After 10 years, $300,000 still owed. Amortize over 240 remaining months: PMT = $300,000 × 0.005 × 1.005²&sup4;&sup0; ÷ (1.005²&sup4;&sup0; − 1) = $2,149/month.
3
Conventional 30-yr: PMT = $300,000 × 0.005 × 1.005³&sup6;&sup0; ÷ (1.005³&sup6;&sup0; − 1) = $1,799/month.
4
Total interest IO = ($1,500 × 120) + ($2,149 × 240 − $300,000) = $180,000 + $215,760 = $395,760. Total interest conventional = $1,799 × 360 − $300,000 = $347,640. Extra cost = $48,120.
IO saves $299/mo for 10 years but costs $48,120 more in total interest
Try this example →Example 2 — $600,000 Jumbo Loan, 7%, 7-Year IO, 30-Year Term
Jumbo IO: high-cost market investor scenario
1
Monthly rate r = 7% ÷ 12 = 0.005833. IO Payment = $600,000 × 0.005833 = $3,500/month for 84 months.
2
After 7 years: amortize $600,000 over 276 months at 7%. Post-IO payment = $4,427/month.
3
Conventional 30-yr at 7%: PMT = $3,993/month. Post-IO payment is $434 per month MORE than conventional.
IO saves $493/mo for 7 years; post-IO payment is $434/mo above conventional
Try this example →Example 3 — $450,000 Loan, 6.5%, 5-Year IO, 25-Year Term
Short IO period on a 25-year mortgage
1
Monthly rate r = 6.5% ÷ 12 = 0.005417. IO Payment = $450,000 × 0.005417 = $2,438/month for 60 months.
2
After 5 years: amortize $450,000 over 240 months (20yr remaining) at 6.5%. Post-IO payment = $3,358/month.
3
Conventional 25-yr at 6.5%: PMT = $3,081/month. Post-IO is $277 above conventional. Total extra interest = approximately $37,000.
IO saves $643/mo for 5 years; post-IO payment is $277/mo above conventional
Try this example →❓ Frequently Asked Questions
What is an interest-only mortgage?
An interest-only mortgage is a home loan where monthly payments cover only the interest on the outstanding balance for a set period, typically 5 to 15 years. During this IO phase, your principal balance does not decrease at all. When the IO period ends, the loan converts to a fully amortizing mortgage and your payment increases to cover both principal and interest over the remaining term.
How is the interest-only mortgage payment calculated?
The IO payment is simply: Monthly Payment = Loan Balance times (Annual Rate divided by 12 divided by 100). For a $300,000 loan at 6%, the monthly rate is 0.5%, so the IO payment is $300,000 times 0.005 equals $1,500 per month. This is lower than the conventional mortgage payment because it includes no principal reduction. The formula applies for the entire IO period regardless of how many months pass.
What happens after the interest-only period ends?
When the IO period ends, the loan converts to a fully amortizing mortgage. Your remaining balance (still the full original loan amount) must now be repaid over the remaining, shorter term. The post-IO payment uses the standard amortization formula applied to the original balance over the remaining months. This is always higher than the conventional mortgage payment originated on the same day, because the remaining term is shorter.
Why is the post-IO payment higher than a conventional mortgage?
A conventional 30-year mortgage spreads principal repayment evenly over all 360 months. An IO loan with a 10-year IO phase only begins repaying principal at month 121, leaving just 240 months to repay the same original balance. Compressing the same debt into fewer payments mathematically requires larger payments. The shorter the remaining term relative to the original, the larger the jump.
Do interest-only mortgages cost more in total interest?
Yes, always more than a conventional mortgage at the same rate. Because the principal balance does not shrink during the IO phase, you pay interest on the maximum possible balance for the entire IO period. A conventional mortgage immediately begins reducing the balance, slowly shrinking the interest accrual. The difference in total interest typically ranges from $30,000 to over $100,000 depending on loan size, rate, and IO period length.
Who benefits most from an interest-only mortgage?
Real estate investors who plan to sell within the IO period benefit from maximum cash flow without excess interest cost. High-income earners with irregular income (commissions, bonuses, self-employment) who need lower mandatory payments in slow months but can pay extra in good months also benefit. Buyers in very high-cost markets where a conventional payment would exceed their budget sometimes use IO loans to enter homeownership, planning to refinance as their income grows.
Can I make principal payments during the interest-only period?
Most IO mortgages allow voluntary principal payments during the IO phase even though they are not required. Paying down principal during the IO period reduces your outstanding balance, which lowers both your IO payment and your post-IO amortizing payment. This is one of the most effective ways to manage IO loan risk: pay principal when cash is available and maintain the flexibility of low required payments when cash is tight.
What credit score is needed for an interest-only mortgage?
IO mortgages are classified as non-QM (non-qualified mortgage) products in most cases, meaning they are considered higher risk. Most lenders require a minimum credit score of 700 to 720, with the best rates above 740. They also typically require at least 20% down payment (80% LTV or lower), 6 to 12 months of cash reserves, and documented income sufficient to qualify for the post-IO amortizing payment, not just the lower IO payment.
Is an interest-only mortgage risky?
IO mortgages carry specific risks. You build no equity through principal repayment during the IO phase (only through property value appreciation). If home prices fall, you could owe more than the home is worth with no equity cushion. The payment jump at IO period end can be a budget shock if you have not prepared. Refinancing risk exists if rates have risen significantly by the time you want to exit the IO structure. These risks are manageable with careful planning but are real.
What is the difference between an IO mortgage and a balloon mortgage?
Both offer low initial payments, but they differ in what happens at the end. An IO mortgage converts to a fully amortizing loan after the IO period and continues normally. A balloon mortgage requires the entire remaining principal to be paid in one lump sum at the balloon date, typically 5 to 7 years. If you cannot sell or refinance in time, a balloon mortgage can force default. IO mortgages are structurally less risky because they continue as ongoing loans without a balloon payment.
Can I refinance an interest-only mortgage?
Yes. Many IO borrowers refinance before the IO period ends, converting to a conventional fixed-rate mortgage. However, refinancing requires qualifying at prevailing rates, which may be higher than when you originated the IO loan. If your home value has not appreciated and you have made no voluntary principal payments, you may have the same balance and limited equity, which can restrict your refinancing options. Planning your exit strategy before signing an IO loan is strongly recommended.
Are interest-only mortgages available in 2025?
Yes. IO mortgages are available in 2025 primarily through non-QM lenders, jumbo mortgage lenders, and some portfolio lenders (banks that hold loans on their own books rather than selling them to Fannie Mae or Freddie Mac). They are not available through conventional conforming loan programs. Rates on IO loans are typically 0.25% to 0.75% higher than comparable conventional rates to compensate lenders for the additional risk.