Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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What does this calculator estimate?
Simple interest = principal × rate × time. $1,000 at 5% for 3 years earns $150 ($50 per year), for a total of $1,150. Unlike compound interest, simple interest never earns interest on interest — over 3 years, annual compounding would earn $157.63 instead.
- Simple interest = P × r × t; $1,000 × 5% × 3 = $150.
- Simple interest grows linearly; compound interest grows exponentially.
- Auto loans, some personal loans, and bond coupons commonly use simple interest.
Why does simple interest matter if most things compound?
Because some of the biggest loans people sign use it. Most auto loans charge simple interest — extra principal payments early in the loan directly cut future interest. Understanding the linear math tells you exactly how much an extra payment saves, with no compounding surprises.
Simple vs compound: how big is the gap?
The divergence is slow at first, then dramatic — which is why compound interest favors savers and simple interest favors borrowers.
- $1,000 at 5% for 3 years: $150 simple vs $157.63 compound
- $1,000 at 5% for 10 years: $500 simple vs $628.89 compound
- $1,000 at 10% for 20 years: $2,000 simple vs $6,727.50 compound
How do you spot simple-interest terms in a loan?
Look for 'interest accrues daily on the unpaid principal balance' — that is simple interest. It means paying early in the billing cycle reduces the balance interest is computed on. If the contract mentions a 'precomputed' loan or a Rule of 78s schedule instead, interest is front-loaded and early payoff saves less.
How this calculator works
Formula
simple interest = principal × annual rate × years; total value = principal + interest
Worked example
$1,000 at 5% simple interest for 3 years earns $150 — a total of $1,150 ($50 per year).
Sources
Sources support the method, terminology, or planning context. Editable assumptions remain your responsibility.
Assumptions to verify
- Rate is a nominal annual percentage
- No compounding — interest is calculated on the original principal only
- Fractional years are allowed (1.5 years = 18 months)
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is charged only on the original principal. Compound interest also earns interest on accumulated interest — $1,000 at 5% for 3 years earns $150 simple versus $157.63 compounded annually. The gap widens with time and rate.
Where is simple interest actually used?
Most auto loans, many personal loans, some mortgages (simple-interest mortgages), and bond coupon payments. Savings accounts and credit cards almost always compound instead.
Can I calculate for months instead of years?
Yes — convert months to years first (18 months = 1.5 years) and enter 1.5. The formula needs time in years when the rate is annual.
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