The one thing people get wrong: this isn't a monthly-payment loan calculator
"Simple interest calculator" is searched by two very different groups of people. Some want the growth on a lump sum that just sits for a fixed term — a promissory note, a short-term deposit, a bond held to maturity — which is exactly what this page computes. Others are actually looking for how much interest they'll pay on a loan or card they're paying down every month, where the balance shrinks with each payment. Those are genuinely different calculations: a fixed principal earning interest for a fixed time (this page) versus a declining balance recalculated every period. If that's what you need, the Amortization Calculator or Credit Card Payoff Calculator will give you the right number — plugging a payoff schedule's numbers into this formula would understate the interest, because it assumes the full principal sits untouched for the whole period.
The formula
Because it's linear, doubling the time exactly doubles the interest — there's no "interest on interest" the way a compounding balance builds it. That's also the fastest way to sanity-check any simple-interest figure by hand.
A full worked example
An $8,000 principal at a 5.5% annual rate for 30 months — the defaults already loaded into the calculator above:
| Step | Value |
|---|---|
| Time in years (30 months ÷ 12) | 2.5 |
| Interest: $8,000 × 0.055 × 2.5 | $1,100.00 |
| Total amount: $8,000 + $1,100.00 | $9,100.00 |
Enter these same numbers into the calculator above and the result matches exactly — $1,100.00 in interest, $9,100.00 total.
Simple interest vs. compound interest, in one example
Put $1,000 at 5% for 3 years under both rules:
This calculator only ever computes the simple-interest side of that comparison — for a balance that compounds (most savings accounts, credit cards, and long-term investments), the numbers will diverge from what's shown here the longer the money sits.
Frequently asked questions
What is simple interest, and what's the formula?
Interest calculated only on the original principal, for the whole time period, with no compounding: I = P × r × t, where r is the annual rate as a decimal and t is time in years. Total amount A = P + I = P × (1 + rt).
How is simple interest different from compound interest?
Simple interest is charged only on the original principal, so it grows in a straight line. Compound interest is charged on the principal plus interest already added, so it grows faster. On $1,000 at 5% for 3 years: $150.00 simple vs. $157.63 compounded annually — a gap that widens over time.
Does this calculator work for a loan I'm paying off with monthly payments?
No. This prices a lump sum held for a fixed period with no paydowns — a note, deposit, or investment held to maturity. A loan paid down monthly has a shrinking balance; use the Amortization Calculator or Credit Card Payoff Calculator for that.
Where is simple interest actually used in the real world?
Short-term promissory notes, many CDs and savings bonds, some bond coupon calculations, statutory interest on unpaid judgments, and day-count interest on a late invoice. It's also the first formula taught before compound interest in most finance courses.
Why does this calculator use a 365-day year, and does that matter?
It divides a day count by 365 (actual/365). Some lending agreements use a 360-day "banker's year," which accrues about 1.4% more interest for the same nominal rate and day count. Treat day-based results here as a close approximation if your agreement specifies 360 days.
How do I find the principal, rate, or time if I know the other values and the total?
Switch to Reverse mode and choose what to solve for. Rearranging A = P(1 + rt) gives principal = A ÷ (1 + rt), rate = (A ÷ P − 1) ÷ t, and time = (A ÷ P − 1) ÷ r.
Can the interest rate be 0%?
Yes — interest is $0 and the total always equals the principal. The one case that can't be solved is working backward for TIME at 0%, since the total never moves; the calculator explains that instead of dividing by zero.