Simple Interest Calculator

How this is calculated

Interest = P × (rate ÷ 100) × years · Final = P + Interest

Worked example: R5,000 at 7% for 4 years → 5,000 × 0.07 × 4 = R1,400 interest, final value R6,400. The third result line shows the same terms compounded monthly — the honest comparison, because most savings products compound and most contract penalties don't.

Simple and compound diverge slowly, then dramatically

Simple interest is charged on the original principal only: I = P × r × t. Compound interest is charged on principal plus accumulated interest, so the base grows. Over one year at the same rate they are nearly identical. Over thirty years they are not remotely comparable — R100,000 at 8% simple returns R240,000 of interest, while the same at 8% compounded annually returns about R906,000.

That divergence is the entire argument for starting to invest early, and the entire danger of leaving debt to accumulate. The direction of the compounding is all that differs.

Where simple interest is genuinely used

It is less common than compound interest but far from extinct. Many short-term and instalment loans quote simple interest over the term. Car finance in some markets uses it. Certain bonds pay simple interest coupons rather than reinvesting. Late-payment interest on invoices and statutory interest on judgments are frequently calculated simply. And it remains the standard for very short periods, where the difference is negligible and the arithmetic is easier to audit.

Watch for interest quoted on the original balance

A quoted "simple" rate on an instalment loan can be considerably more expensive than it sounds, because interest is calculated on the full original amount for the whole term even though you are repaying it monthly. By the final month you owe almost nothing but are still being charged as though you owed everything. A 10% "simple" rate applied this way over four years is roughly equivalent to an 18% conventional rate — which is why the APR, not the quoted rate, is the number to compare.

Day-count conventions matter over short periods

For anything shorter than a year, the fraction of a year used changes the answer. Actual/365 divides by 365; actual/360 divides by 360, which produces slightly more interest and is common in commercial lending; 30/360 treats every month as 30 days for tidiness in bond markets. On a 90-day facility the difference between conventions is small but real, and on a large sum it is worth reading which one applies.