Compound Interest Calculator with Monthly Contributions

How to use this calculator

This compound interest calculator with monthly contributions shows how your money could grow from a starting amount plus regular deposits. It also shows the result in today's dollars, after inflation.

Fill in these inputs:

• Starting amount. What you have now. Enter $0 if you're starting fresh.
• Deposit and frequency. How much you'll add, and how often: weekly, every two weeks, monthly, quarterly or yearly.
• Interest rate. Your expected annual return. Use your account's APY for savings, or a conservative estimate for investments.
• Years. How long the money will grow.
• Compounding. Daily, monthly, quarterly or yearly. Check your account terms. If unsure, monthly is a fine default.
• Yearly deposit increase. Raise your deposit each year, say 3%, to match raises.
• Inflation and "today's dollars." Turn this on to see your balance in current buying power.
• Deposit timing. Start of each period or end. Deposits at the start earn a little more.

You'll see your final balance, total deposits, total growth, and a year-by-year chart. Few calculators combine yearly deposit increases with an inflation toggle, so you can test a realistic plan.

How we calculate compound interest

Compound interest means you earn interest on your interest. Each period, growth is added to your balance, and the next period's growth is based on that bigger balance.

The standard formula has two parts: growth on your starting amount, plus growth on your regular deposits.

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

P   = starting amount
PMT = deposit each compounding period
r   = annual rate (as a decimal)
n   = compounding periods per year
t   = years

This version assumes deposits at the end of each period. For deposits at the start, multiply the second part by (1 + r/n). When your deposit schedule differs from the compounding schedule, or deposits rise each year, the calculator works period by period instead.

Worked example: $10,000 at 7% for 20 years

With yearly compounding and no deposits:

• $10,000 × (1.07)^20 = $10,000 × 3.8697 = $38,697

Your money nearly quadruples. About $28,697 is growth.

Worked example: $500 a month for 30 years

At 6% compounded monthly, deposits at the end of each month:

• r/n = 0.06 ÷ 12 = 0.005, and nt = 360
• (1.005)^360 = 6.0226
• $500 × (6.0226 − 1) ÷ 0.005 = $502,258

You deposit $180,000 in total. About $322,258 is growth. If you deposit at the start of each month instead, you'd have about $504,769.

Compound vs simple interest

Simple interest pays only on your original amount. Compound interest pays on your original amount plus all the interest you've earned.

$10,000 at 7%	10 years	20 years	30 years
Simple interest	$17,000	$24,000	$31,000
Compounded yearly	$19,672	$38,697	$76,123

The gap grows over time. In year one, they're equal. By year 30, compounding produces more than twice as much.

The rule of 72

The rule of 72 is a quick way to estimate doubling time. Divide 72 by the annual rate.

• At 4%: about 18 years to double
• At 7%: about 10.3 years (the exact answer is 10.2)
• At 10%: about 7.2 years

It also works in reverse. At 3% inflation, prices roughly double in 24 years.

Why rate and time matter more than compounding frequency

Compounding frequency gets a lot of attention, but its effect is small. Here's $10,000 at 7% for 20 years:

Compounding	Final balance
Yearly	$38,697
Monthly	$40,387
Daily	$40,547

Going from monthly to daily adds only $160 over 20 years. Changing the rate or the time changes far more. $500 a month for 30 years grows to about $347,025 at 4%, $502,258 at 6%, and $745,180 at 8%.

The biggest lever you control is time. Starting five years earlier often beats chasing a slightly higher rate.

Nominal vs real returns

Your nominal return is the number on your statement. Your real return is what's left after inflation.

If your money earns 7% and prices rise 3%, your real return is about 4%. The "today's dollars" toggle makes this concrete. $38,697 in 20 years, at 3% inflation, buys what about $21,426 buys today.

That doesn't mean growth is small. It means future dollars buy less, so plan in today's dollars when setting goals.

Realistic return assumptions

The rate you enter drives everything, so choose it carefully.

• High-yield savings and CDs. Top accounts pay around 4% APY now, but rates move with the economy. These are FDIC-insured up to $250,000 per depositor, per bank, per ownership category. Compare terms with our CD calculator.
• Stock market investments. Stocks have historically returned more than savings over long periods. They also swing a lot, including losses in some years. Returns are never guaranteed, and past performance doesn't predict future results.
• Bonds and mixed portfolios. These typically fall between savings and stocks in both return and volatility.

The calculator assumes the same return every year. Real investments don't work that way. A good habit is to run a conservative case and an optimistic case and plan around the lower one.

How much will $10,000 grow in 20 years?

It depends mostly on the rate. Here's a $10,000 lump sum with yearly compounding and no added deposits:

Annual return	Balance after 20 years
4%	$21,911
6%	$32,071
7%	$38,697
8%	$46,610
10%	$67,275

A few points of return can more than double the outcome over two decades. That's why it pays to be realistic, not hopeful, about the rate you enter.

Putting compound growth to work

• Start now, even small. Time does the heavy lifting.
• Automate deposits. Regular contributions add up faster than occasional big ones.
• Raise deposits with your pay. The yearly increase input shows how much this adds.
• Use tax-advantaged accounts. Growth in a 401(k) or IRA may be tax-deferred or tax-free. See our 401(k) calculator and retirement calculator.
• Saving for something specific? Our savings goal calculator works backward from your target.