How simple interest works
Simple interest is calculated only on the original principal, never on the interest already earned. The formula is I = P × r × t, where P is the principal, r is the annual rate as a decimal and t is the time in years. Because the base never changes, the interest is the same every year — a flat line rather than the accelerating curve of compound interest. This makes simple interest easy to compute by hand and common on short-term loans, car finance, some bonds and many informal or personal loans where the maths needs to stay transparent.
Simple vs compound interest
The difference between simple and compound interest is where the interest is applied. Simple interest always uses the original principal, so $10,000 at 5% earns exactly $500 every year. Compound interest adds each period's interest to the balance, so the next period earns slightly more — and over long horizons that gap becomes enormous. For a saver, compound interest is far better; for a borrower, simple interest is usually cheaper. Use this tool alongside the compound interest calculator to see exactly how much compounding adds or costs over the same rate and period.
Where you'll meet simple interest
Simple interest shows up more often than people expect. Many car loans, personal loans and short-term instalment plans quote it, as do some certificates of deposit and government bonds that pay a fixed coupon on face value. It is also the basis of everyday calculations like the interest on a late invoice or a short bridging loan. Knowing the flat-interest figure helps you sanity-check a lender's numbers and compare a simple-interest offer against a compounding one on equal footing.
Reading the results
The total interest is what the principal earns or costs over the whole period; the total amount is principal plus interest, the sum you would repay or receive at the end. The per-month figure breaks the flat interest into equal monthly slices, useful for budgeting a loan repayment or estimating monthly income from a fixed-rate deposit. Remember that real products may compound, charge fees or deduct tax, so treat this as the baseline flat-interest case. These calculators use standard financial formulas and the figures you enter. Real interest rates, lender fees and tax treatment vary by bank and country, so treat the results as planning estimates and confirm exact numbers with your lender or a qualified adviser before committing.
Frequently asked questions
What is the simple interest formula?
Interest equals principal times the annual rate times the time in years: I = P × r × t. The total amount owed or earned is the principal plus that interest.
What is the difference between simple and compound interest?
Simple interest is charged only on the original principal, so it stays flat each year. Compound interest is charged on the principal plus accumulated interest, so it grows faster over time.
Is simple interest better for a loan?
For a borrower, simple interest is usually cheaper than compound interest at the same rate, because interest never accrues on unpaid interest. For a saver, compound interest is better.
Does simple interest compound monthly?
No. Simple interest never compounds — it is calculated once on the principal for the full period. The per-month figure here just divides that flat interest evenly across the months.