Interest-Only vs Amortized Calculator
Compare interest-only and fully amortized loan payments with payment shock analysis
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Interest-Only Calculator
Interest-Only or Fully Amortized?
Interest-only loans are common in business lending, commercial real estate, and some HELOCs. The lower initial payment provides cash flow flexibility, but the full principal must be repaid during the amortization period or as a balloon at maturity.
Key Takeaways
- A $100,000 interest-only loan at 8% costs $667/month during the 3-year IO period — $546/month less than the $1,213 fully amortized payment. But the balance stays at $100,000 the entire time.
- After the IO period ends, the payment jumps to $1,356/month to amortize the full $100,000 over the remaining 7 years — $143 MORE per month than if you had amortized from the start over 10 years.
- Interest-only loans cost more in total interest because the principal is not reduced during the IO period. On this example, total interest is approximately $3,700 higher than a fully amortized loan.
- The year-by-year comparison table shows the “cash saved” during the IO period and how it reverses afterward. During IO years, you save hundreds per month; after, you pay more. The total usually nets negative.
- Interest-only structures make sense when you need temporary cash flow flexibility — startup business phase, bridge financing, or when you expect income to increase significantly before the IO period ends.
How to Use This Calculator
This calculator compares two payment structures on the same loan: interest-only (IO) for an initial period followed by full amortization, versus fully amortized from day one. It shows the payment difference during and after the IO period, plus the total cost impact.
Step 1 — Enter the loan details. Amount, APR, and total term. The default $100,000 at 8% for 10 years represents a typical business loan or commercial line of credit.
Step 2 — Select the IO period. This is how long you pay interest only — 1, 2, 3, or 5 years. During this time, your payment covers only the interest charge; no principal is paid down. After the IO period, the full balance amortizes over the remaining term.
Step 3 — Compare the results. The VS panel shows the IO payment during and after the IO period versus the constant amortized payment. The verdict quantifies the tradeoff: lower payments now, higher payments later, and more total interest.
Step 4 — Review the schedule. The year-by-year table shows both paths side by side — IO payment, IO balance, amortized payment, amortized balance, and the “cash saved” column that shows how much you keep each year with IO (positive during IO, negative after).
Understanding the Results
The IO payment is simply principal times the monthly interest rate — no principal repayment. This is the lowest possible monthly cost for the loan. After the IO period, the full original balance must be amortized over fewer months, which pushes the payment higher than if you had amortized from the start.
The payment timeline bars make the structure visual. The IO bar has two colored segments: gold for the low IO period and red for the higher amortization period. The amortized bar is a single navy segment — the same payment for the entire term. The contrast shows the “payment shock” that occurs when the IO period ends.
The cash saved column in the year-by-year table is the decision metric. Positive values during the IO period show how much less you pay versus amortizing. Negative values after show how much more. If you invest or productively use the cash saved during the IO period (earning a return higher than your loan rate), IO can be financially advantageous. If the cash saved just gets spent, IO simply costs more.
Interest-Only vs Amortized: Full Comparison
| Factor | Interest-Only | Fully Amortized |
|---|---|---|
| Initial Payment | Lower (interest only) | Higher (principal + interest) |
| Payment After IO Period | Higher (payment shock) | Same (no change) |
| Total Interest Paid | Higher | Lower |
| Principal Reduction | None during IO period | Every month from day one |
| Cash Flow Flexibility | High during IO period | Moderate |
| Best For | Business startups, bridge loans, investors | Standard business/personal loans |
Frequently Asked Questions
What is an interest-only loan?
A loan where you pay only the interest charge for an initial period (typically 1–5 years), with no principal reduction. After the IO period, the full original balance amortizes over the remaining term. The IO payment is the lowest possible monthly cost, but the balance does not decrease until amortization begins.
Why would I choose interest-only?
When you need maximum cash flow flexibility in the short term. Common scenarios include business startups (revenue has not yet ramped), real estate investors (holding property short-term before sale), bridge financing (temporary funding before permanent financing is arranged), and borrowers expecting a significant income increase before the IO period ends.
Is the payment shock real?
Yes. When the IO period ends, the payment can jump 50%–100% because the full principal now amortizes over fewer months. On a $100,000 loan at 8% for 10 years with a 3-year IO period, the payment goes from $667 to $1,356 — a 103% increase. Plan for this increase before choosing IO.
Do I build any equity during the IO period?
No. Your balance remains exactly the same as the original loan amount throughout the IO period. No principal is paid, so no equity accumulates. If the underlying asset (property, business) appreciates in value, you gain equity from appreciation — but not from payments. This is the primary tradeoff of IO loans.
Are interest-only personal loans available?
Rarely. IO structures are most common in commercial real estate, business lending, and HELOCs. Standard personal loans from major lenders (SoFi, LendingClub, Prosper) are fully amortized. Some business lenders and SBA 504 loans offer IO periods. This calculator is most useful for business and investment loan comparisons.
Can I pay extra principal during the IO period?
Many IO loans allow voluntary principal payments (called “curtailments”) during the IO period without penalty. This gives you the best of both worlds: the low required payment of IO with the option to reduce the balance when cash allows. Confirm this feature with your lender before signing.
References & Further Reading
Keep Reading
- Best Business Loans — Compare IO and amortized options
- Business Loan Calculator — Standard amortized payments
- SBA Loan Calculator — SBA programs with IO options
- Equipment Loan Calculator — Equipment financing with residual
- All PrimeRates Calculators