Compound Interest Calculator 🔒 Your data never leaves your browser.
Project investment growth with compounding and optional monthly contributions.
About this tool
This compound interest calculator projects how a lump sum grows over time once interest is added back to the balance and starts earning interest itself. The compounding frequency you choose — annually, quarterly, monthly or daily — controls how often that happens: the more often interest compounds, the sooner each bit of it starts generating its own interest, so a daily-compounding account edges out an otherwise identical annual one over the same period.
Add an optional monthly contribution to see what regular saving does to the total. Rather than approximating contributions with a single formula, the calculator simulates the balance month by month, adding your contribution and applying the effective monthly rate implied by your chosen compounding frequency at every step — so the future value, total contributed and total interest earned figures reflect what actually happens to the money over the whole period. Everything runs locally in your browser; no numbers are ever sent anywhere.
Frequently asked questions
Why does a higher compounding frequency give a slightly higher result?
Compounding more often means interest gets added to the balance sooner, and that added interest itself starts earning interest sooner too — the classic "interest on interest" effect. At the same nominal annual rate, daily compounding earns slightly more than monthly, which earns more than quarterly, which earns more than annual — the difference is usually small unless the rate is high or the time horizon is very long.