Simple Interest Calculator

Calculate simple interest on an amount over a number of days at an annual rate.

How it works

Simple interest is principal × rate × days ÷ 365 — no compounding. Handy for a flat late fee, a short loan to a collaborator, or a quick "what would this be worth" check. For overdue invoices under the MSMED Act the interest actually compounds monthly — use the late-payment tool for that.

Frequently asked

When is interest simple vs compound?: Simple interest is charged only on the original amount. Compound interest is charged on the amount plus accumulated interest. Flat late fees are usually simple; statutory MSMED interest compounds monthly.

Retainer’s reminders and late-payment line mean you rarely have to compute interest by hand at all. Try Retainer free