Compound Interest Calculator

What compound interest means for your balance

Compound interest is interest that gets calculated on your original balance and on every bit of interest that has already been added to it. That second part is what separates it from simple interest, which only ever applies to the amount you started with. Once interest starts earning its own interest, growth speeds up on its own, and the effect becomes more noticeable the longer the money stays invested.

This calculator handles two directions of that same question. Growth Calculator mode takes a starting amount, a rate, and a time period and projects a future balance. Goal Calculator mode works backward from a target balance and reports how long it would take to get there given the same rate and deposit plan.

The formulas behind the numbers

More frequent compounding means interest gets added to the balance more often, so each round of interest starts earning its own interest sooner. The difference between monthly and daily compounding is usually small at typical savings rates, while continuous compounding represents the mathematical limit of how much more frequent compounding can help.

Why deposit timing and tax change your final number

A deposit made at the beginning of a period earns interest for that entire period. The same deposit made at the end of the period earns nothing until the next one starts. Over many years of regular contributions, that timing difference adds up to a noticeably larger balance for deposits made at the beginning of each period, even though the amounts being deposited never change.

Turning on the tax option applies your chosen rate to each period's interest before it gets added to your balance, rather than taxing the whole account only at the end. That matches how many taxable accounts are actually taxed in practice, and it tends to lower long term growth more than a single year of the same tax rate would suggest, since the reduced interest also loses out on future compounding.

Worked examples

Reading your results and what to try next

The milestone tracker shows roughly when your balance is projected to cross common round numbers, which is often a more motivating way to watch progress than staring at a single final figure. The year by year table breaks the same growth down into yearly deposits, interest, tax, and ending balance, useful for spotting exactly which years contribute the most growth once compounding really takes hold.

This calculator focuses on the growth phase, building a balance up over time. Once that balance exists, a separate question worth asking is how long it can support regular withdrawals later on, which our savings withdrawal calculator covers in detail, using the same underlying rate and compounding logic from the opposite direction.