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Compound interest calculator: how much could your savings grow?

Enter a starting amount, a monthly deposit, a rate and a time span to see how much of the result is your money and how much is interest earning interest.

By the Bitfolio Explainer Desk · Checked against primary sources · 5 min read

Education only, not investment, tax or legal advice. Crypto-assets are high-risk — risk disclosure.

Stacks of coins rising in height from front to back, like steps — Photo: “Vintage Coin Steps” by ccPixs.com, CC BY 2.0, via flickr.com · Edited: duotone, cropped.

The short answer

Compound interest is interest earned on your deposits and on interest already added. With $1,000 to start, $200 a month and an assumed 6% a year compounded monthly, this calculator projects $95,718 after 20 years, of which $46,718 is interest.

Key takeaways

Compound interest is paid on your money and on interest already credited, so growth speeds up over time.
Time and the rate matter far more than compounding frequency: annual versus daily changes the example by about 2%.
The result assumes a fixed rate. Real rates and returns change, so treat it as a scenario, not a forecast.
Fees, taxes and inflation are not deducted, and a yearly fee compounds against you just as interest compounds for you.

How do you use the compound interest calculator?

The calculator models a simple habit: a lump sum once, then the same deposit every month. Its fields mirror the SEC's compound interest calculator on Investor.gov², so you can cross-check a result there.

Five inputs

1
Starting amount
Money already set aside. Use 0 if you are starting from nothing.
2
Monthly contribution
What you add each month, assumed to arrive at the end of the month.
3
Annual interest rate
The yearly rate before fees and tax. For investments, use a cautious assumption, not last year's return.
4
Years
Whole years. Compare 10 and 20 years to see how much time does.
5
Compounding
How often interest is credited: annually, quarterly, monthly or daily, as your account terms state.

What formula does the calculator use?

Investor.gov defines compound interest as interest paid on the principal and on interest that has already accumulated¹. The calculator applies that one month at a time:

Monthly growth factor: g = (1 + r ÷ m)^(m ÷ 12), where r is the annual rate as a decimal and m the compounding periods per year (1, 4, 12 or 365).
Each month: balance becomes balance × g + C, where C is the monthly contribution.
Repeat for years × 12 months, starting from the starting amount P.
Interest earned = final balance − (P + C × 12 × years).

Turning every compounding choice into an equivalent monthly factor keeps deposits and interest on one calendar. With no deposits, the result equals the textbook formula P × (1 + r ÷ m)^(m × years).

Figure · One month inside the calculator

01Balancestart of the month
02Add interestbalance × growth factor
03Add depositmonthly contribution
04New balancecarried to next month

↻ then back to step 01

Interest earned this month becomes part of the balance that earns interest next month.

Worked example: what does $200 a month at 6% become?

Worked example

$1,000 plus $200 a month for 20 years (illustrative)

Inputs: $1,000 start, $200 a month, 6% a year, 20 years, monthly compounding.

You pay in $1,000 + $200 × 240 months = $49,000.00. The projected balance is $95,718.38, so interest earned is $46,718.38, about 49% of the total. The 6% rate is an assumption, not a forecast.

Year-by-year balance for the example (illustrative, calculator formula)

Year	Paid in	Balance	Interest so far
0	$1,000.00	$1,000.00	$0.00
1	$3,400.00	$3,528.79	$128.79
5	$13,000.00	$15,302.86	$2,302.86
10	$25,000.00	$34,595.27	$9,595.27
15	$37,000.00	$60,617.84	$23,617.84
20	$49,000.00	$95,718.38	$46,718.38

Interest adds $2,303 in the first five years but $23,101 in the last five, with identical deposits. On its own, the $1,000 starting amount would reach $3,310.20.

Does compounding frequency or the rate matter more?

Keeping every other input the same, compounding frequency barely moves the result:

Same example, different compounding (illustrative)

Compounding	Final balance	vs annual
Annually	$93,894.86	$0.00
Quarterly	$95,373.79	$1,478.93
Monthly	$95,718.38	$1,823.52
Daily	$95,887.21	$1,992.35

The rate is a far bigger lever:

Same example, different annual rates (illustrative)

Rate	Final balance	Interest
4%	$75,577.51	$26,577.51
5%	$84,919.37	$35,919.37
6%	$95,718.38	$46,718.38
8%	$122,730.89	$73,730.89

Costs work the same way in reverse. In an SEC bulletin, $100,000 growing 4% a year for 20 years ends near $208,000 with a 0.25% annual fee but near $179,000 with a 1% fee⁴. Here, a fee cutting the net rate from 6% to 5% would leave $84,919.37 instead of $95,718.38.

What does the calculator leave out?

Changing rates. Savings rates move and investment returns vary. Investor.gov's calculator adds a range around the rate for this reason²; you can run a low and a high rate here.
Fees. Subtract account or fund fees from the rate before entering it.
Taxes. Interest and gains may be taxed yearly or on withdrawal, depending on the account and country.
Inflation. Results are in future dollars; our inflation calculator converts them to today's money.
Irregular deposits and withdrawals. Missed, larger or withdrawn amounts change the outcome.

Risk warning

A projection, not a promise

Only a fixed rate on an insured deposit is set in advance, and only for its term. Investments can lose value. Compare scenarios rather than planning around one number.

What mistakes do people make with compound interest calculators?

Common beginner mistakes

Using a past return as the future rate
One strong year says little about the next twenty. Run a cautious rate too.
Mixing APR and APY
An APY already includes compounding; adding monthly compounding counts it twice. See our APR to APY converter.
Forgetting fees
A 1% yearly fee looks small but compounds over decades.
Reading the total as spending power
Dollars 20 years from now buy less than dollars today.

Frequently asked questions

What is the difference between simple and compound interest?

Simple interest is paid only on the original amount; compound interest is also paid on interest already credited¹. Our compound interest explainer goes deeper.

Is daily compounding much better than monthly?

Only slightly. In the example, daily compounding ends $168.83 higher than monthly after 20 years.

Can I use this for crypto staking or lending yields?

You can enter a yield, but crypto rewards are usually variable, often paid in a volatile token and exposed to platform risk. The calculator assumes a stable dollar rate.

Why does my bank's figure differ?

Banks may credit interest on a daily balance, pay on a different schedule or quote an APY. Match the rate type and compounding setting to your account terms.

The bottom line

Compound interest rewards time above all: in the example, interest supplies almost half of the balance after 20 years. Use the calculator to compare rates, deposits and time spans, remembering it ignores fees, taxes, inflation and changing returns. Our guide to interest rates explains why rates move.

Sources

Compound Interest (glossary) — Investor.gov, U.S. Securities and Exchange Commission Primary source
Compound Interest Calculator — Investor.gov, U.S. Securities and Exchange Commission Primary source
12 CFR Part 1030 (Regulation DD), Appendix A — Annual Percentage Yield Calculation — Consumer Financial Protection Bureau, via eCFR Primary source
How Fees and Expenses Affect Your Investment Portfolio (Investor Bulletin) — Investor.gov, U.S. Securities and Exchange Commission, 2025 Primary source

How we checked this page: every figure above links to the numbered source it came from. Spotted an error? Tell the desk — see our editorial policy.