1,000 at 5% for 3 years
1,000 × 0.05 × 3 = 150
Interest is 50 for each year, so the full period adds 150. The total is 1,150. Annual compounding would instead produce 1,157.625; the two models use different bases for later interest.
Interest on a fixed principal
Simple Interest Calculator
Calculate interest without compounding, or work backward to find the principal, annual rate, or time. Compare the interest with the total principal plus interest.
Calculate simple interest
Simple interest
Choose what to calculate, enter the known values, then select Calculate.
- Principal
- Interest amount
- Total: principal + interest
- Annual interest rate
- Duration
≈ marks display rounding. No currency or bank rounding rule is assumed. Calculations run in your browser. Press Enter to calculate. Successful results appear in this tab's Recent calculations.
Worked examples
180 interest at 6% for 18 months
180 ÷ (0.06 × 1.5) = 2,000
Eighteen months equals 1.5 years. A fixed principal of 2,000 generates 180 of simple interest at this rate and duration; principal plus interest is 2,180.
240 interest on 3,000
100 × 240 ÷ (3,000 × 2) = 4%
Over 2 years, the required annual rate is 4%. If the annual rate is instead 8%, the same interest takes 1 year, or 12 months. Neither result includes fees.
Formula, precision, and limits
I = P × (R ÷ 100) × t · A = P + I
P is the unchanged principal, I is the interest amount, R is the annual rate entered as a percentage, t is elapsed time in years, and A is principal plus interest. To find the principal, divide I by (R ÷ 100) × t. To find the annual percentage, divide 100 × I by P × t. To find years, divide I by P × (R ÷ 100).
Principal, interest, and annual rate must be nonnegative. A supplied duration must be positive; both supplied and solved durations are limited to 1,000 years. Months are divided by exactly 12. There is no calendar-date input, leap-year adjustment, or choice of 360-day or 365-day financial convention.
Zero principal or zero rate gives zero interest when calculating interest directly. When solving backward, a zero divisor cannot determine one answer: zero interest leaves multiple possibilities, while positive interest is impossible with that zero divisor. The calculator explains the situation instead of returning Infinity or choosing an arbitrary value. Zero interest with positive principal and rate gives a solved duration of exactly zero.
Inputs accept up to 64 characters, 15 integer digits, and 10 decimal places. The annual rate has no separate percentage cap; values above 100% are mathematical inputs, not a product recommendation. Results may have more integer digits than inputs. Calculations retain exact integer fractions, including fractional months, without intermediate decimal or currency rounding.
Only the final display is rounded to the selected maximum decimal places, with halfway values rounded away from zero and trailing zeros removed. The ≈ symbol identifies a changed display value. A notice appears if a positive value rounds to zero. Choosing two decimal places changes the display; it does not apply a bank's contractual rounding rules. Changing precision or the unit of a solved duration does not create a new history entry.
This is a constant-principal mathematical model, not a loan repayment schedule or a quoted savings return. It excludes compounding, deposits, withdrawals, changing rates, fees, taxes, inflation, currency conversions, and repayments during the term. For an actual account or loan, use the provider's terms and day-count rules.
Reference: OpenStax — Solve Simple Interest Applications describes I = Prt with an annual decimal rate and time in years, including solving for an unknown quantity.
Common questions
Does the interest amount include the principal?
No. Interest is the added amount. The total shown separately equals principal plus interest. Enter only the interest amount when finding an unknown principal, rate, or duration.
How do I enter a partial year?
Use a decimal duration in years or enter months. Nine months and 0.75 years are equivalent here. This conversion does not count actual days between calendar dates.
Can simple interest be used for a monthly payment loan?
This tool assumes the principal stays fixed throughout the entered duration. Regular repayments reduce an outstanding loan balance, so a repayment schedule needs a model that tracks those payments and the lender's interest rules.
Why can zero interest produce an error?
Finding duration from zero interest with a positive principal and rate gives zero time. But if the principal or rate is also zero, many durations produce zero interest and no unique duration exists. Similar ambiguity occurs when solving for principal or rate with a zero divisor.
Is the annual rate an APR or APY?
The rate is the annual simple percentage used in this formula. The tool does not calculate a regulated APR that includes applicable fees or an APY based on compounding. Those terms depend on the product and its rules.