Simple Interest Calculator

Calculate simple interest on a principal amount. Compare with compound interest to understand the difference.

This calculator applies the simple interest formula, Interest equals Principal times Rate times Time (line 9), directly — no compounding — so Total Interest is a plain product of Principal Amount, Annual Interest Rate, and Time Period, and all three move it by exactly the same 20% across a ±10% probe. Monthly Interest looks like it should depend on Time Period, since the formula divides by Time Period times 12 (line 11), but the Time Period in the interest calculation and the Time Period in that denominator cancel out algebraically: Monthly Interest reduces to Principal times Rate divided by 12, exactly Annual Interest divided by 12, and is completely unaffected by Time Period — confirmed by a 0% measured effect, not just a coincidence of the default 5-year term. Compound Interest Would Earn compares the same principal and rate against monthly compounding (line 30) rather than simple interest, and it is driven almost equally by Time Period and Annual Interest Rate, each with a ±10% span of about 42% — both let compounding accumulate more, while Principal Amount alone moves it by a comparatively flat 20%, the same share it contributes everywhere else. This calculator does not model any withdrawals, additional contributions, or a rate that changes partway through the term — it assumes a single flat rate applied to a single lump sum for the whole period.