Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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Use this result
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What does this calculator estimate?
A simple interest calculator computes interest on a principal without compounding. Enter the principal, rate, and years to see the interest and total.
- Interest = principal × rate × time
- $5,000 at 4% × 3 years = $600
- No compounding — flat, predictable
How simple interest works
Simple interest accrues only on the original principal — no interest on interest. It's used for some loans, bonds, and short-term notes. The formula is flat and predictable: I = P × r × t.
Limitations to watch for
Most bank accounts and long-term loans compound — the balance formula differs. Simple interest on a loan often understates the true annual cost if payments are structured differently. The tool assumes the rate is annual and time is in years.
How to use it in practice
Use it for short-term loans, personal notes, and understanding the base cost of borrowing. For savings and mortgages, use the compound interest or amortization calculators instead.
- Enter the principal.
- Enter the annual rate and the time in years.
- Read the interest, per-year figure, and total.
How this calculator works
Formula
Interest = principal × rate × time (I = P×r×t). Total = principal + interest. Interest per year = principal × rate.
Worked example
$5,000 at 4% for 3 years: interest = $5,000 × 0.04 × 3 = $600; total = $5,600.
Assumptions to verify
- The rate is annual and time is in years.
- Interest is not compounded.
- No payments are made during the term.
Frequently asked questions
What is simple interest?
Interest calculated only on the original principal: I = P × r × t. $5,000 at 4% for 3 years = $600.
What's the difference from compound interest?
Compound interest earns interest on interest; simple interest doesn't. Over long periods the gap is large.
Where is simple interest used?
Short-term loans, personal notes, bonds, and some consumer loans with flat-rate terms.
How do I calculate interest per year?
Principal × rate: $5,000 × 4% = $200/year.
Is simple interest better for me?
As a borrower, compounding costs more; simple interest is cheaper on long terms. As a saver, you want compounding.
Does this include payments?
No — it assumes the principal is untouched for the whole term.
What about monthly rates?
Convert: a monthly rate × 12 gives the annual rate, then use years.
Cite this tool
BoringToolsKit. “Simple Interest Calculator.” boringtoolskit.com/simple-interest-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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