Simple Interest Calculator
Free simple interest calculator using the I = PRT formula. Find interest earned, final amount, or solve for rate, time, or principal. Step-by-step math shown, no signup.
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Simple vs Compound Comparison
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Step-by-Step Worked Solution
How This Is Calculated
Simple interest uses I = P × R × T where R is the annual rate as a decimal and T is time in years. Rearrangements: P = I ÷ (R × T), R = I ÷ (P × T), T = I ÷ (P × R). Compound comparison uses monthly compounding: A = P(1 + r/12)12t.
Everything behind the Simple Interest Calculator
Formulas, reference charts, and detailed answers — expand any section you need.
How to Calculate Simple Interest: Worked Examples
To calculate simple interest, convert the rate to a decimal, convert time to years, and multiply the three values. The calculator writes out these steps for any inputs; these examples show the pattern.
| Principal | Rate | Time | Interest (I = PRT) |
|---|---|---|---|
| $2,000 | 5% | 2 years | $200 |
| $5,000 | 6% | 3 years | $900 |
| $10,000 | 4.5% | 18 months | $675 |
| $15,000 | 8% | 5 years | $6,000 |
Simple Interest Calculator: I = PRT Made Instant
This simple interest calculator runs the I = PRT formula in every direction: enter principal, rate, and time to get the interest and final amount, or enter the interest and solve for the missing principal, rate, or time. Every answer comes with the worked solution written out step by step, the formula, the substitution, and the arithmetic, so the simple interest calculator shows not just the result but the math behind it.
Example: $5,000 at 6% for 3 years earns $900 in simple interest, for a final amount of $5,900.
Simple Interest Formula: How I = PRT Works
The simple interest formula is I = P × R × T: principal times the annual rate (as a decimal) times the time in years. Interest is charged on the original principal only, never on accumulated interest, which is what separates simple from compound interest. Time must be in years, so 18 months enters the formula as 1.5, and the calculator handles the conversion automatically with the step shown.
Simple Interest Calculator: Solve for Rate, Time or Principal
The reverse directions are where a simple interest calculator earns its keep, since homework and loan checks rarely ask the forward question twice. Rearranged: P = I ÷ (R × T), R = I ÷ (P × T), and T = I ÷ (P × R). Select the unknown, enter the three known values, and the worked solution shows the rearranged formula with your numbers substituted.
Example: $750 of interest on $5,000 over 3 years solves to R = 750 ÷ (5000 × 3) = 5%.
Simple Interest vs Compound Interest
Simple interest pays on the principal only; compound interest pays on the principal plus everything already earned. On short terms the gap is small, on long terms it dominates: $10,000 at 6% for 20 years earns $12,000 in simple interest but $23,102 in compound interest (monthly compounding). The comparison row runs your exact inputs through both formulas, and the Compound Interest Calculator handles the full compounding side.
Simple Interest Loan Calculator: Auto and Personal Loans
A simple interest loan calculator matters because most auto loans and many personal loans accrue interest daily on the remaining principal, the simple method, which means every early or extra payment cuts the interest that accrues afterward. Checking a payoff quote against the formula takes seconds here, and the daily time unit handles per-diem interest questions directly.