Compound Interest Calculator with Monthly Deposits
Compound interest is what happens when growth earns growth of its own. Enter a starting deposit, a monthly contribution, the annual rate and how often interest compounds, and this free compound interest calculator with monthly deposits projects the future value, alongside exactly how much you put in and how much is pure interest. It doubles as a savings goal calculator with monthly contributions and shows simple interest vs compound interest side by side: the fastest way to see why starting early beats saving more later. Shows how monthly, quarterly or annual compounding grows your principal over time, with a year by year breakdown of interest earned.
Related reading: Guide: how compound interest grows your moneyRetirement Calculator
Last updated: October 22, 2026
Your result
Contributions are assumed at the end of each compounding period. Figures are before tax and inflation.
The formula
Rate per period = annual rate ÷ compounding periods per yearPeriods = periods per year × yearsFuture value of deposit = P × (1 + r/n)^(n×t)Future value of contributions = PMT × (((1 + r/n)^(n×t) − 1) ÷ (r/n))Interest earned = future value − total contributions
A worked example
Put $10,000 in at 7% compounded monthly, add $200 every month, and after 10 years you’re looking at $54,713.58. You put in $34,000; interest did the rest, $20,713.58 of the total came from compounding alone.
More about the Compound Interest Calculator
Why the early years feel pointless (and aren't)
Compound growth is back loaded: in the first years your contributions do almost all the work, and the returns look insultingly small next to what you put in. That is normal, not a sign of a bad investment, the curve bends upward late, which is why starting ten years earlier beats saving twice as much ten years later. The calculator above shows this plainly: compare the "total contributions" and "interest earned" lines at year 10 versus year 30, and watch interest quietly take over. Time in the market does the heavy lifting; the rate just decides how heavy.
Use projections as what ifs, not promises
The number at the end is only as honest as the rate you typed. Markets don't deliver a smooth 8% every year, they lurch, so treat the projection as a scenario, not a forecast: run it at 5%, 7% and 9% and plan around the middle, not the top. Enter returns after inflation if you want purchasing power rather than a headline figure, and remember the figure is before tax. The most useful question this calculator answers isn't "how rich will I be" but "what changes if I add $100 more a month", contribution size is the lever you actually control.
How to use the Compound Interest Calculator
- Enter the lump sum you’re starting with, or 0 if you’re starting from nothing.
- Add what you can contribute each month and the annual return you expect.
- Pick how often interest compounds, monthly is the usual assumption for savings accounts.
- Set the years, press Calculate, and see the future value, your contributions and the interest.
Frequently asked questions
Is it better to compound monthly or annually?
More frequent compounding always wins, though the difference is modest. At 7% over 10 years, $10,000 becomes about $19,672 compounded annually and $20,096 compounded monthly, and the gap widens with higher rates and longer time.
Does this include tax or inflation?
No, the figures are before tax and inflation. If you want purchasing power instead of the headline number, enter a real return: your expected rate minus inflation, often around 2 to 3%.
What is the rule of 72?
A handy shortcut: divide 72 by your annual rate to estimate how many years it takes to double your money. At 7% that’s about 10.3 years, which matches what this compound interest calculator shows for a lump sum.
How much do I need to save monthly to reach $100,000?
It depends on your rate and timeline. Play with the monthly contribution until the projected value hits your goal, at 7% over 10 years you’d need roughly $578 a month starting from zero.
What is the difference between compound and simple interest?
Simple interest pays only on your original amount; compound interest pays on the original plus all previous interest. Put $1,000 at 5% for 10 years: simple interest gets you to $1,500, but annual compounding grows it to about $1,629, and the gap keeps widening.
Can compound interest work against me?
Yes, it’s the same maths running in reverse on debt. A $5,000 credit card balance at 24% APR compounding monthly piles on roughly $100 a month in interest alone if you only make minimum payments. That’s why expensive debt snowballs so brutally.
What is a realistic annual return to assume for investing?
For US stock market planning, 7% a year after inflation is the classic long run figure (about 10% before inflation). In the UK, many planners use 5% real returns for a global equity portfolio. And never build a plan on 12%+ returns, no matter what a social media guru promises.
What is APY and how does it relate to compound interest?
APY (annual percentage yield) is the real yearly growth once compounding is folded in. A 5% nominal rate compounded monthly works out at about 5.116% APY, which is why comparing APYs, not headline rates, shows you which savings account actually pays more.
How much difference do monthly contributions make?
A huge one: $10,000 at 7% for 20 years grows to about $40,387 on its own, but adding $200 a month takes it to roughly $144,573. Consistent contributions often build more wealth than the starting balance does.
How do I calculate compound interest?
Use A = P(1 + r/n)^(nt), where P is principal, r is the annual rate, n is compounding periods per year, and t is years. $10,000 at 8% compounded monthly for 20 years grows to about $49,268, more than triple the simple interest result of $26,000. Starting early beats higher contributions later.