What is an interest-only mortgage?

An interest-only mortgage lets you pay just the interest for an initial period (typically 5–10 years), keeping payments lower. After the IO period, you pay principal and interest on the full original balance over the remaining term, causing a payment spike.

What happens when the interest-only period ends?

When the IO period ends, your payment increases significantly because you now pay principal and interest on the full original loan balance, compressed into fewer remaining years. A 10-year IO on a 30-year loan means 20 years to pay off 30 years of principal — much higher monthly payment.

How much more interest do I pay with an interest-only loan?

Significantly more. On a $400,000 loan at 6.875% with a 10-year IO period on a 30-year term, you pay roughly $85,000–$100,000 more in interest compared to a fully amortizing 30-year loan because you make no progress on principal during the IO phase.

Who should consider an interest-only mortgage?

IO mortgages make sense for: high-income borrowers with irregular income who want flexibility, investors who plan to sell before the IO period ends, and buyers in rapidly appreciating markets who plan to refinance. They're risky for buyers who need the forced savings of principal paydown.

Can I make extra payments on an interest-only loan?

Yes, most IO loans allow extra principal payments. Paying extra during the IO period reduces your loan balance, resulting in a lower payment when the loan converts to fully amortizing. This can significantly reduce payment shock.

Interest Only Calculator

Calculate your interest-only mortgage payment and see the full cost comparison vs. a standard amortizing loan — including payment shock when the IO period ends.

Loan Amount

Interest Rate

Interest-Only Period

Total Loan Term

Interest-Only Monthly Payment

$2,292

First 10 years · interest only, no principal reduction

After IO: Monthly Payment

$3,071

Payment Increase

+$780 (34%)

Amortizing Period

20 years

IO Phase Interest

$275,000

Amort Phase Interest

$337,101

Total Interest (IO Loan)

$612,101

Comparison: Fully Amortizing Loan (30 years)

IO Loan Total Interest: $612,101

Extra vs. Full Amort: $66,124

Full Amort Monthly

$2,628

Full Amort Total Interest

$545,977

Extra Interest (IO Loan)

+$66,124

IO Monthly Savings (during IO)

$336/mo

Advanced

Advanced Calculator

Payment shock analysis at IO reset, loan balance trajectory over time

Payment shock is the sudden increase when your interest-only period ends. Your payment jumps from $2,292 to $3,071 — a 34% increase.

During IO Period (Yrs 1–10)

$2,292/mo

Interest only · no equity built

Annual interest: $27,500

Low payment — good cash flow

After IO Reset (Yrs 11–30)

$3,071/mo

Full P&I · 20 years to pay off same principal

Yr 1 amort interest: $27,200

+$780/mo shock (+34%)

Payment Shock Amount

$780

Increase at IO reset

Payment Shock %

34%

% increase over IO payment

IO Monthly Savings

$336

vs. full amort from day 1

Extra Lifetime Interest

$66,124

IO loan costs more total

Pro

Professional Simulator

Invest IO savings analysis, total interest comparison across loan structures

During the IO period you save $336/mo vs. a fully amortizing loan. If invested, does that investment offset the extra interest cost of an IO loan?

Investment Return

Home Value

Annual Appreciation

Net Benefit of IO + Investing

-$7,959

Investment portfolio $58,165 minus extra interest $66,124 over loan life

Monthly Savings (IO vs Full Amort)

$336

Over 10-year IO period

Total Savings if Invested

$40,326

Without investment growth

Portfolio Value at IO End

$58,165

At 7.0%% annual return

Extra Interest Cost

$66,124

IO loan vs full amort (lifetime)

Equity at IO End (IO loan)

$340,122

Only from appreciation

Equity Gap vs Full Amort

$57,767

Less equity built during IO period

At 7.0%% return, the extra interest outweighs investment gains. You would need a higher investment return for the IO strategy to pay off financially.

How to Use This Interest Only Calculator

Enter your loan amount, interest rate, IO period length, and total loan term to see both your interest-only payment and the full amortizing payment you'll make after the IO period ends.

IO Period vs. Full Term

The IO period is a subset of the full loan term. A 10-year IO on a 30-year loan means you pay interest only for 10 years, then principal + interest for the remaining 20 years — compressed amortization over a shorter period.

Payment Shock

The most important figure is the jump from IO payment to the full amortizing payment. The calculator shows both the dollar increase and percentage increase. Qualifying borrowers must often be able to afford the higher payment even if they're not paying it yet.

Interest-Only Loan Formula

During IO Period:
Monthly Payment = Loan Amount × (Annual Rate ÷ 12)

After IO Period:
Remaining Years = Full Term − IO Period
Monthly Payment = P × [r(1+r)^n] / [(1+r)^n - 1]
Where P = original loan amount (unchanged during IO)
r = monthly rate, n = remaining months

Extra Interest vs. Full Amort:
IO Total Interest − Standard 30-yr Interest = Additional Cost

Example: $400,000 at 6.875% — IO payment = $2,292/mo. After 10-year IO, full payment for remaining 20 years = $3,076/mo (+$784, +34%). Standard 30-year payment from day 1 = $2,628/mo.

Example: High-Income Professional Uses IO Loan

Dr. Chen's IO Mortgage Strategy

A physician buying a $550,000 home with 20% down. She's in training for 5 more years and will have significantly higher income after. She chooses a 7-year IO period.

Loan Amount	$440,000
Interest Rate	6.875%
IO Period	7 years
Full Term	30 years
IO Monthly Payment	$2,521
Post-IO Monthly Payment	$3,318
Payment Increase	+$797/mo (+32%)
Total IO Phase Interest	$211,764
Total Interest (IO Loan)	$435,680
Total Interest (Standard)	$341,520
Extra Interest Cost	+$94,160

Dr. Chen saves $107/month vs. a standard 30-year loan during her lower-income training period. After completing training, her income triples and the higher payment is easily manageable. She plans to pay extra principal after year 3 to reduce the post-IO payment shock.

Frequently Asked Questions

An interest-only mortgage lets you pay only the interest portion for an initial period — typically 5, 7, or 10 years. Your loan balance doesn't decrease during this time. After the IO period, the loan converts to fully amortizing: you pay principal and interest on the original balance over the remaining term. This causes a significant payment increase, sometimes called "payment shock."

When the IO period ends, your monthly payment increases substantially. On a 30-year loan with a 10-year IO period, you must pay off the full original principal in just 20 years instead of 30 — meaning each payment is much larger. The payment shock can be 30–50% higher than your IO payment. This is why lenders qualify IO borrowers based on the fully amortized payment amount.

Not inherently — it depends on your situation. IO loans make sense for: (1) High earners with irregular income who want payment flexibility; (2) Real estate investors who plan to sell before IO period ends; (3) Buyers in high-appreciation markets who will have equity when they sell or refinance. They're problematic if you rely on the low payment long-term or if home values fall, leaving you with no equity and a massive payment increase.

Generally yes, by 0.125–0.375% compared to standard loans. Many IO loans are also ARM products (adjustable-rate mortgages), adding rate risk on top of payment shock. Fixed-rate IO loans do exist and are preferable from a risk management standpoint, though they're less common. Always understand whether your IO rate is fixed or adjustable.

Yes — most IO loans allow voluntary principal payments. Paying extra during the IO period reduces your balance, which lowers the post-IO amortizing payment. For example, if you reduce your $400,000 balance to $350,000 during the IO period, your post-IO payment will be based on $350,000 — meaningfully reducing the payment shock. This is one of the best strategies if you choose an IO loan.