Simple Interest Calculator
Created by: Daniel Hayes
Last updated:
Calculate simple interest with I = P × r × t, solving for interest, principal, rate, or time. See total repayment and a side-by-side comparison against compound interest at the same rate and term, with a chart of how the gap grows over the years.
Simple Interest Calculator
FinanceI = P × r × t with solve-for-any-variable support, total repayment, and a side-by-side compound interest comparison.
Use 0.5 for six months, 0.25 for a quarter.
What Is a Simple Interest Calculator?
A simple interest calculator applies the foundational interest formula I = P × r × t, charging interest only on the original principal with no compounding.
It solves for any of the four variables — interest, principal, rate, or time — and reports the total repayment.
It also puts simple interest side by side with compound interest at the same rate and term, because the practical question is almost always comparative: how much does compounding change the outcome?
For short terms the answer is "barely"; for long terms it is "completely," and the chart makes the divergence obvious.
How Simple Interest Works
Interest accrues linearly: each year adds the same charge, P × r, regardless of what has already accrued.
Three years at 6% on $10,000 costs exactly three times one year — $1,800.
Under annual compounding the same three years cost $1,910, because years two and three charge interest on prior interest.
The solve-for-any-variable feature rearranges the same equation.
Lenders use it to price notes (solve for interest), borrowers use it to back out the implied rate from a quoted fee (solve for rate), and savers use it to find how long a deposit must sit to earn a target (solve for time).
Real-world lending uses this math on auto loans, personal loans, and short-term notes where interest accrues daily on the outstanding principal only.
Simple Interest Formulas
Interest: I = P × r × t
Principal: P = I ÷ (r × t)
Rate: r = I ÷ (P × t)
Time: t = I ÷ (P × r)
Total repayment: A = P + I = P(1 + rt)
Compound comparison: A = P(1 + r)^t
Example Scenarios
Three-Year Personal Note
You lend a family member $15,000 at 5% simple interest for 3 years. Interest: $15,000 × 0.05 × 3 = $2,250; total repayment $17,250. Under annual compounding the same terms would return $17,364 — a $114 difference. Over a short term at a moderate rate, simple versus compound barely matters; documenting the rate and repayment date matters far more.
Solving for the Implied Rate
A lender offers $8,000 today for $9,200 repaid in 18 months. Interest = $1,200, so r = 1,200 ÷ (8,000 × 1.5) = 10% simple annual interest. Backing out the implied rate turns any "fee" quote into a comparable APR-style number — often revealing that a casual-sounding arrangement is more expensive than a bank loan.
Why Long Horizons Demand Compounding
$10,000 at 7% for 30 years earns $21,000 of simple interest ($31,000 total) but $66,123 of compound interest ($76,123 total) with annual compounding — nearly two and a half times as much. The comparison chart shows the straight line versus the curve splitting apart around year 10. This is why savings and retirement projections always use compound math while short-term notes can safely use simple math.
How People Use This Calculator
- Pricing or checking short-term promissory notes and private loans
- Backing out the implied annual rate from a flat-fee lending arrangement
- Understanding how auto loan and federal student loan interest accrues daily on principal
- Teaching the I = Prt foundation before introducing compounding
- Comparing simple and compound outcomes at the same rate and term before choosing an instrument
Simple Interest Tips
Confirm which math your loan actually uses. "Simple interest" on a contract means daily accrual on outstanding principal with no capitalization — good for borrowers who pay early.
Watch for precomputed interest or Rule of 78s language on older or subprime contracts, which front-loads interest and blunts the benefit of early payoff.
Time is the variable that changes everything.
At 3 years, the simple-versus-compound gap at 6% is about 6% of the interest; at 30 years it is more than triple.
Any decision with a horizon beyond roughly 5 years should be evaluated with compound math — our compound interest calculator handles the full frequency options.
For daily-accrual loans, the daily cost is principal × rate ÷ 365.
On a $20,000 auto loan at 7%, that is $3.84 per day.
Seeing the per-day number makes the case for early payments concrete: paying two weeks early on each installment saves about $54 per year on that loan at no cost to you.
Frequently Asked Questions
What is simple interest and how is it calculated?
Simple interest is interest charged only on the original principal — never on previously accrued interest. The formula is I = P × r × t: principal times the annual rate times the time in years. A $10,000 loan at 6% simple interest for 3 years costs $10,000 × 0.06 × 3 = $1,800, for a total repayment of $11,800. Because interest never compounds, the cost grows in a straight line rather than a curve.
What is the difference between simple and compound interest?
Simple interest is charged only on the principal; compound interest is charged on the principal plus all previously accrued interest, so the balance grows exponentially. At 6% for 10 years, $10,000 accrues $6,000 of simple interest but about $7,908 compounded annually. The gap widens with time and rate — small over 2–3 years, enormous over 20. As a borrower you prefer simple interest; as a saver or investor you want compounding.
Which real loans actually use simple interest?
Most auto loans, many personal loans, and short-term promissory notes use simple interest computed on the outstanding balance — interest accrues daily on principal only and does not capitalize. Federal student loans also accrue simple interest, though unpaid interest can capitalize at specific events like leaving deferment. Credit cards, by contrast, effectively compound because unpaid interest joins the balance. Mortgages amortize monthly, which behaves like monthly compounding on the outstanding balance.
How do I solve for the rate, time, or principal instead of interest?
Rearrange I = P × r × t. Rate: r = I ÷ (P × t) — the annual rate implied by a known interest charge. Time: t = I ÷ (P × r) — how long money must be lent to earn a target amount. Principal: P = I ÷ (r × t) — the loan size a given interest budget supports. This calculator has a solve-for selector that handles the algebra: choose the unknown, fill the other three values, and calculate.
Is simple interest calculated on 360 or 365 days?
It depends on the day-count convention. Consumer loans typically use 365 days (actual/365), while some commercial and older banking conventions use a 360-day year (bankers’ year), which slightly increases the daily rate. On large short-term notes the difference is real: $1 million at 6% for 90 days costs $15,000 under actual/360 but $14,795 under actual/365. Check the note’s stated convention; this calculator uses annual periods, matching either convention when time is entered in years.
Does a simple-interest auto loan penalize early payoff?
No — that is one of its advantages. Because interest accrues daily on the outstanding principal, paying early or paying extra immediately reduces future interest, and there is no unearned interest built into the payoff amount. This differs from precomputed-interest loans (the Rule of 78s, now heavily restricted), where the interest schedule is fixed up front. With a simple-interest loan, every early dollar saves real money — verify your contract says "simple interest."
Sources and References
- Consumer Financial Protection Bureau: What is the difference between a simple interest rate and precomputed interest? — consumerfinance.gov
- Federal Student Aid (U.S. Dept. of Education): How interest is calculated on federal student loans — studentaid.gov
- Truth in Lending Act, Regulation Z (12 CFR Part 1026) — interest accrual and disclosure conventions