What rate should I use?

One that's realistic for what you'll do with the money. For long-term projections, a real rate above inflation is best.

Are contributions added at the start or end of the month?

In this calculator, at the end of each month, after that month's interest.

Does it include taxes or fees?

No. If your investment has fees or taxes, subtract them from the rate.

How long does it take to double my money?

A quick rule: 72 divided by the annual rate. At 8% a year it doubles in about 9 years; at 12%, in about 6.

Compound interest calculator

Compound interest is earning interest on your interest: what your money earns is added to the principal and earns too in the next period. With a starting amount, monthly contributions and an annual rate, this calculator shows how much you'll have, how much you put in and how much was interest, year by year.

How it works

Every month the balance earns the monthly rate (the annual one divided by 12) and that month's contribution is added. At first it's almost all what you put in; over the years, interest becomes the biggest part. That's why starting earlier matters more than contributing more.

An example with numbers

You start with $1,000 and add $500 a month at 8% a year. In 10 years you put in $61,000 and have about $93,700: over $32,000 is interest.

The same over 20 years: you put in $121,000 and have about $299,000. The interest is already more than what you put in.

The formula

Without contributions, the final amount is P × (1 + i)ⁿ, where P is the starting amount, i the rate per period and n the number of periods. With monthly contributions, each one is added with the interest of the months it has left.

Example without contributions: $10,000 at 0.5% a month for 10 years (120 months) gives 10,000 × 1.005¹²⁰ ≈ $18,194. Not 60% but almost 82%: the difference is interest on the interest.

Nominal (APR) or effective (APY)?

A nominal annual rate (APR) is paid as a twelfth every month. An effective one (APY) already includes compounding: what you really earn in a year if you reinvest everything.

A 5% APR compounded monthly is a 5.12% APY. When comparing two options, compare APY with APY. In the calculator you pick which one you are entering.

Inflation: what it's worth in today's money

With high inflation the numbers look huge but don't buy more. Enter the annual inflation you expect and the calculator adds a column with the balance in today's money: what it will really buy.

How Busimy does it

In Busimy you add your investments (deposits, funds, stocks, crypto) and see how much you put in, what they're worth today and their return, alongside a compound interest calculator to project.

Frequently asked questions

What rate should I use?: One that's realistic for what you'll do with the money. For long-term projections, a real rate above inflation is best.
Are contributions added at the start or end of the month?: In this calculator, at the end of each month, after that month's interest.
Does it include taxes or fees?: No. If your investment has fees or taxes, subtract them from the rate.
How long does it take to double my money?: A quick rule: 72 divided by the annual rate. At 8% a year it doubles in about 9 years; at 12%, in about 6.