Compound Interest Calculator

What does $200 a month actually become? Left alone for ten years at 5%, it turns into more than the $24,000 you put in, because every bit of interest starts earning interest of its own. That snowball is compound interest, and this calculator shows what it does to your money.

Add regular deposits on top and growth speeds up further. Type in your numbers below to see the end result and the year-by-year shape of it.

Enter your numbers

Without JavaScript: A = P(1+r/n)^(n x t) + PMT x [((1+r/n)^(n x t)-1)/(r/n)], n = 12 monthly.

Example: $10,000 + $200/month at 5% for 10 years grows to about $47,527.

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Result

We put the year-by-year breakdown under the headline result.

Start with the end in mind

Four inputs drive everything. Your starting balance is the lump sum you begin with, or 0 if you are starting from scratch. The yearly rate is what your savings account, CD, or investment is expected to earn, treated here as a fixed average. Compounding frequency, monthly, quarterly, yearly, or daily, sets how often interest joins the balance. And the monthly deposit is what you plan to add. The calculator answers with future value, total deposits, and total interest earned.

The math, in one line

With regular monthly deposits, the future value is:

A = P × (1 + r/n)^nt + PMT × [((1 + r/n)^nt − 1) ÷ (r/n)]

A is the future value, P the starting balance, r the yearly rate as a decimal, n the compounding periods per year, t the number of years, and PMT the monthly deposit. See Compound Interest Formula for each part explained. The first part grows your starting balance; the second part grows your deposits. Deposit nothing and the second part is zero, leaving the plain compound growth formula. APY, the annual percentage yield, is the yearly return after compounding is included, slightly higher than the stated rate when compounding happens more than once a year.

Ten years of $200

Take $10,000 at 5%, compounded monthly, with $200 going in each month for 10 years. The monthly rate is 0.05 ÷ 12 = 0.00416667, and there are 12 × 10 = 120 periods. The growth factor (1 + 0.00416667)¹²⁰ comes to 1.647009. The starting balance grows to $10,000 × 1.647009 = $16,470.09. The deposits grow to $200 × ((1.647009 − 1) ÷ 0.00416667) = $200 × 155.282 = $31,056.46. Add them: $16,470.09 + $31,056.46 = $47,526.55. You put in $10,000 + ($200 × 120) = $34,000, so the interest earned is $47,526.55 − $34,000 = $13,526.55. All figures rounded to the nearest cent.

The deposits alone turn into $31,056.46, more than the $24,000 actually deposited, because each deposit earns interest for the rest of the period.

Why the curve bends late

Three numbers tell the story. Future value is the total balance at the end, deposits plus interest. Total deposits is the money you actually put in, starting balance plus monthly deposits. Interest earned is the difference, the part compounding created for you.

The year-by-year table is where the pattern shows. Growth looks slow in the first few years, then the curve steepens. That is normal: the balance is largest late in the period, so the interest earned each year is largest then too. Starting earlier usually beats saving more later, because compounding has more time to work.

A few things this leaves out. The rate stays constant here; real returns move up and down. Each deposit is assumed to arrive at the end of the month and starts earning interest the next month. Withdrawals cut the balance and the interest it would have earned. Taxes on interest and account fees are not included. And the result is in nominal dollars; inflation reduces what the money will buy.

Two more runs

Starting from $5,000 at 4%, $100 a month for 5 years: future value $12,734.88.

Starting from $0 at 7%, $300 a month for 20 years: future value $156,278.00. No starting balance at all, just steady deposits and time.

One more comparison: on $10,000 at 5% for 10 years with no deposits, daily compounding gives a few dozen dollars more than yearly. Frequency matters, but rate and time matter far more.

Three things people ask

How does compound interest work?

You earn interest on your savings and on the interest you have already earned. See Compound Interest Formula for worked examples.

How do monthly deposits change the result?

Monthly deposits add two things: the deposits themselves, and the interest each deposit earns for the rest of the period. In the ten-year run above, $200 a month became $31,056.46 after 10 years, $7,056.46 more than the $24,000 deposited, purely from compounding.

Does compound interest apply to loans?

Yes, and on loans it works against you. On a loan, unpaid interest can be added to the balance, so you end up paying interest on interest. That is one reason paying a loan down early saves so much. See our Compound vs Simple Interest guide for the full comparison.

Also worth a look

Savings Goal Calculator, work backwards from a target to the monthly deposit you need
Retirement Calculator, compound growth over a working lifetime
Simple Interest Calculator, interest on the original amount only
Percentage Calculator, rate changes and growth percentages
Loan Calculator, compounding working against you on a loan
Compound Interest Formula, the math explained with examples
Compound vs Simple Interest, which one applies, and when
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General information and planning only. This is not financial, investment, tax, or legal advice. Real returns vary, and past growth says nothing about future growth, so check with a qualified professional before making investment decisions.