Simple Interest Calculator
Simple interest grows at a flat rate on the original principal only. Enter the principal P, annual rate r and time t in years; the calculator returns the interest I = P·r·t and the final amount A = P + I.
Linear growth, and why it matters
Simple interest charges the rate on the original sum and nothing else. Interest already earned sits idle rather than earning in its turn, so the balance grows by the same amount every period — a straight line rather than a curve.
This calculator finds that interest and the final total from a principal, an annual rate and a time in years.
The absence of compounding is the whole content of the model. Doubling the term doubles the interest exactly, and the interest for the tenth year is identical to the interest for the first — neither of which is true once interest starts earning interest.
That makes simple interest the baseline the compound case is measured against. The gap between the two is small over a short term and enormous over a long one, and knowing the simple figure is what makes that gap visible.
How to use this calculator
- Enter the principal The original sum, in whatever currency you are working in. No conversion is performed, so the unit that goes in is the unit that comes out.
- Enter the annual rate as a percentage Five percent is entered as 5, not 0.05. The rate line echoes both the percentage and the decimal actually used.
- Enter the time in years Fractional values are meaningful and common — six months is 0.5, ninety days is roughly 0.2466.
- Read the interest and the total separately The interest is what the arrangement costs or earns; the total is the principal plus that. Which one a question wants is worth checking.
The formula, and where it comes from
I = P · r · t A = P + I r = rate% / 100
The formula multiplies three quantities and nothing more. The principal never changes, so the same product applies to every period identically — which is exactly what distinguishes this from compounding, where the base grows.
Because it is a plain product, the relationship is linear in every input. Doubling any one of principal, rate or time doubles the interest, and that proportionality is what makes simple interest easy to reason about and easy to rearrange for an unknown.
The percentage is divided by 100 before use, and the tool reports both forms. Entering 5 gives a decimal rate of 0.05, and the echoed line is the quickest confirmation that the field was read as intended.
The final amount is the principal plus the interest rather than an independently computed figure, so the two always reconcile exactly. Any discrepancy between them would have to come from a misread input, not from the arithmetic.
What each input means
- P Principal — form field “Principal P”
- The original sum, which never changes under this model. Both the interest and the total scale directly with it. Units: currency, unconverted.
- r Annual rate — form field “Annual rate r (%)”
- Entered as a percentage per year and divided by 100 internally. The rate line shows both values. Units: percent per year.
- t Time — form field “Time t (years)”
- In years, matching the rate's period. Fractional values are meaningful and are used directly. Units: years.
- I, A Interest and amount
- The interest accrued and the principal plus that interest. The second is derived from the first, so they always agree.
Worked examples
Every number below is produced by the same calculation engine the tool above runs. Nothing here is typed by hand, so the walkthrough cannot drift from what you get when you enter the same values yourself.
Three years at five percent
A principal of 1,000 at 5% for three years — the textbook shape, with a whole number of years.
Inputs Principal P = 1000, Annual rate r (%) = 5, Time t (years) = 3
- Principal P = 1000.00
- Rate r = 5% = 0.05 per period
- Time t = 3 periods
- Formula I = P · r · t, A = P + I
- Interest I = 1000.00 · 0.05 · 3 = 150.00
- Final amount A = 1000.00 + 150.00 = 1150.00
Result I = 150.00, A = 1150.00
The interest is 150, which is exactly three times the 50 that a single year produces. That exact multiplication is the signature of simple interest, and it fails immediately once compounding is introduced.
Under monthly compounding at the same rate the interest over three years would be noticeably higher. The gap is modest here and grows sharply with the term.
A term of eighteen months
5,000 at 8% for a year and a half, entered as 1.5 years rather than 18 months.
Inputs Principal P = 5000, Annual rate r (%) = 8, Time t (years) = 1.5
- Principal P = 5000.00
- Rate r = 8% = 0.08 per period
- Time t = 1.5 periods
- Formula I = P · r · t, A = P + I
- Interest I = 5000.00 · 0.08 · 1.5 = 600.00
- Final amount A = 5000.00 + 600.00 = 5600.00
Result I = 600.00, A = 5600.00
The time must be in years to match the rate's period. Entering 18 here would compute eighteen years of interest and overstate the result twelvefold, with nothing in the output to flag it.
A fractional term works without any special handling, because the formula is a plain product. Half a year simply produces half a year's interest.
A term measured in days
2,500 at 4% for ninety days, converted to years as a decimal.
Inputs Principal P = 2500, Annual rate r (%) = 4, Time t (years) = 0.2466
- Principal P = 2500.00
- Rate r = 4% = 0.04 per period
- Time t = 0.2466 periods
- Formula I = P · r · t, A = P + I
- Interest I = 2500.00 · 0.04 · 0.2466 = 24.66
- Final amount A = 2500.00 + 24.66 = 2524.66
Result I = 24.66, A = 2524.66
Ninety days is roughly 0.2466 of a year, and that conversion is the user's responsibility — the field takes years and nothing else. The precision of the answer inherits the precision of that decimal.
Different conventions divide by 360 or by 365, and they give slightly different results. Short-term interest calculations often hinge on which convention applies, and the tool takes whichever number you supply.
Reading the result
The interest is the same every year
Nothing accumulates onto the base, so each period contributes an identical amount. A chart of the balance is a straight line, and the tenth year earns exactly what the first did.
Every input scales the answer proportionally
Doubling the principal, the rate or the term each doubles the interest. That makes rearranging for an unknown straightforward: any one of the three can be recovered by dividing the interest by the product of the other two.
Where simple interest is actually used
Short-term instruments, some bonds and many legal or statutory interest calculations use it. Most ordinary savings and lending arrangements compound, so applying this model to them understates the outcome.
When you would use this
Costing a short-term arrangement
Over months rather than years the difference between simple and compound interest is slight, so the simple figure is a reasonable estimate and much quicker to compute.
Establishing a baseline for comparison
Computing the simple interest alongside a compound figure quantifies exactly what the compounding is worth, which is otherwise hidden inside a single final balance.
Assumptions and limitations
What this calculator assumes
- The rate applies to the original principal only, with no compounding at any point.
- The rate is annual and expressed as a percentage; the time is in years to match.
- No payments, withdrawals or rate changes occur during the term.
- The final amount is the principal plus the computed interest, so the two always reconcile.
Where it stops being the right tool
- No compounding, so this understates any arrangement where interest is credited and then earns.
- No fees, taxes or inflation adjustment.
- A single lump sum only: regular contributions need an annuity model.
- No day-count convention is applied — a term in days must be converted to years beforehand.
Common mistakes
Entering the term in months or days
Why it happens. The rate is annual and the term is a duration, so any unit feels plausible. Entering 18 for eighteen months computes eighteen years of interest.
How to avoid it. Convert to years first. If the interest looks implausibly large, this is almost always the reason.
Entering the rate as a decimal
Why it happens. The formula uses r as a decimal, so 0.05 looks like the correct input. The field expects a percentage, and 0.05 is read as a twentieth of a percent.
How to avoid it. Enter 5 for five percent. The rate line shows both the percentage and the decimal used, which confirms the field at a glance.
Using simple interest where compounding applies
Why it happens. The formula is the one most people remember, and it gives a plausible answer for any inputs at all.
How to avoid it. Check whether interest is credited to the balance during the term. If it is, the compound interest calculator is the right model and this one understates the result.
Frequently asked questions
How is simple interest different from compound interest?
Simple interest applies the rate to the original principal only, so each period earns the same amount and growth is linear. Compound interest reapplies the rate to the accumulated balance, so interest itself earns interest and growth accelerates.
Can the time be a fraction of a year?
Yes, and it often is. Six months is 0.5 and ninety days roughly 0.2466. The formula is a plain product, so a fractional term needs no special treatment — it simply produces a proportional share of a year's interest.
Should I enter the rate as a percent or a decimal?
As a percent — 5 for five percent. The calculator divides by 100 internally and reports both the percentage you entered and the decimal it used, so the conversion is visible.
How do I find the rate or the term instead?
Rearrange the product. Since the interest is the three quantities multiplied, any one of them equals the interest divided by the product of the other two — a consequence of the relationship being linear throughout.