Compound Interest & Savings Goal Calculator

Quick answer: what $200 a month becomes

Start with $10,000 and add $200 a month. After 20 years you'll have put in $58,000. At 4% a year, compounded monthly, the balance grows to about $95,581 — $37,581 of it interest. At the FDIC's national average savings rate of 0.37%, the same habit earns just $2,581 in interest. The rate you earn matters almost as much as how much you save.

Compound interest is "interest paid on principal and on accumulated interest" (Investor.gov). Each month's interest is added to the balance, so next month you earn interest on it too. The effect is small at first and larger every year.

The compound interest formula

A = P × (1 + r/n)^(n × t) = future balance

Example: $10,000 at 4% compounded monthly for 10 years ≈ $14,908.38 (the calculator rounds to the cent each month, so it can differ from the pure formula by a few cents). Simple interest at 4% would have added only $4,000.

P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year (12 for monthly) and t the number of years.

APR vs APY

Banks quote savings rates as an annual percentage yield (APY), which already includes compounding. An interest rate of 4% compounded monthly works out to an APY of 4.07%, because each month's interest earns interest for the rest of the year. When you compare accounts, compare APY to APY. If you type an APY into the calculator as the annual rate, the result will be slightly high, since the calculator compounds that rate monthly; the difference is small at savings-account rates.

With monthly deposits

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

Example: $10,000 plus $200 a month at 4% for 10 years = $44,358.28.

The calculator works month by month, like a bank statement: it adds a month's interest (annual rate ÷ 12) to the balance, rounds to the cent, then adds your deposit at the end of the month. If your bank compounds daily, your real balance will be a few cents higher. The calculator can also work backwards — enter a goal, and it shows the monthly amount needed.

Growth at 0.37%, 4% and 7%

$10,000 to start plus $200 a month, compounded monthly. The 4% row is close to what Treasury bills were paying in October 2026 (see rates right now). The 7% row is an illustration of a long-run return sometimes assumed for investments such as stock funds; it isn't a savings-account rate, and investment returns aren't guaranteed and can be negative in any year.

After 20 years, the gap between 0.37% and 4% is $35,000 — money you get for moving the same deposits to a higher-paying account. At the illustrative 7%, interest ($86,573) would be more than everything you put in. Time does a lot of the work: the 4% balance earns $10,358 of interest in the first 10 years and $27,222 in the second 10.

The Rule of 72: how long to double your money

72 ÷ annual interest rate (%) = approximate years to double

Example: 72 ÷ 4.26 (today's I bond rate) = 16.9 years.

It's a quick mental shortcut (Investor.gov). Here's how it compares with the doubling time the calculator finds for a lump sum compounded monthly:

The rule is close for rates from about 3% to 9%. At the national average savings rate of 0.37%, it says doubling would take about 195 years.

Inflation: is your savings really growing?

Consumer prices rose 3.4% over the 12 months to August 2026, the same as in July, with energy up 16.3% and core inflation at 2.4% (BLS). September's figure is due October 14, 2026, so check the latest number. If your savings earn less than inflation, your balance grows but buys less.

A rough real return is your rate minus inflation. At 0.37%, that's about −3.0% a year; at the 6-month Treasury yield of 4.30%, about 0.9%. Assuming prices keep rising 3.4% a year, here's what $10,000 plus $200 a month is worth in today's dollars after 10 years:

At 0.37%, the balance buys $9,074 less than the face value of your deposits. At 4% you end up with $6,826 more buying power than at 0.37%, though still a little below the face value of your deposits: when inflation runs at 3.4%, a rate only slightly higher barely keeps up. Enter an inflation rate in the calculator to see this for your own numbers — the value in today's dollars divides the balance by the cumulative rise in prices.

Where to keep savings: rates right now

These are snapshots; every one of them changes. Shop around, and check whether a rate is a promotion.

Sources: FDIC national rates via FRED (savings, 12-month CD; the FDIC page updates around October 19); Treasury yields via FRED (3-month, 6-month, 1-year); I bonds via TreasuryDirect.

I bonds

I bonds bought from May 1 to October 31, 2026 earn a composite rate of 4.26% for their first six months: a 0.90% fixed rate plus a semiannual inflation rate of 1.67% (TreasuryDirect). The inflation part resets every six months, and the next rate is announced November 1, 2026.

Keep it insured

FDIC deposit insurance covers $250,000 per depositor, per FDIC-insured bank, for each account ownership category (FDIC). If your savings exceed that at one bank, spreading them across ownership categories or across banks can keep more of it covered.

Taxes on interest

Interest is taxable income in the year it becomes available to you. Banks send a Form 1099-INT if you earn $10 or more, but you must report all taxable interest even if you don't receive one (IRS Topic 403). For example, $20,000 at 4% earns about $815 in a year, and that's added to your taxable income. The calculator shows growth before tax; your after-tax growth will be somewhat lower.

Setting a savings goal

Working backwards is often more useful than projecting forward. Say you have $5,000 and want $25,000 in five years for a down payment. At 4%, the calculator says you'd need to save $285.00 a month.

• Many people start with an emergency fund. The emergency fund calculator sets a target from your essential expenses.
• Then high-interest debt. A credit card charging far more than your savings earn almost always costs more than the savings bring in. The debt payoff calculator shows what extra payments save.
• Find the monthly amount. The budget calculator shows how much of your take-home pay can go to savings, and the paycheck calculator shows that take-home pay.
• Retirement. The 401(k) paycheck calculator shows how a pre-tax contribution affects each paycheck.
• A home. Use the house affordability calculator to set a down-payment goal, or the rent affordability calculator to plan move-in costs.

Common questions

What is the compound interest formula?

A = P(1 + r/n)^(nt), where P is the starting amount, r the annual rate, n the compounding periods per year and t the number of years. With regular deposits, add PMT × [((1 + r/n)^(nt) − 1) ÷ (r/n)].

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

At 4% compounded monthly, about $14,908 with no further deposits. Adding $200 a month brings it to about $44,358.

What is the average savings account rate?

The FDIC national average was 0.37% for savings and 1.73% for a 12-month CD in September 2026.

What is the I bond rate right now?

4.26% for I bonds bought May 1 to October 31, 2026 (0.90% fixed plus inflation). The next rate is announced November 1, 2026.

What is the Rule of 72?

Divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 4%, that's about 18 years; with monthly compounding the calculator finds 17 years, 5 months.

How much does FDIC insurance cover?

$250,000 per depositor, per insured bank, for each account ownership category.

Is interest on savings taxable?

Yes, in the year it's credited to you. You'll get a Form 1099-INT if you earn $10 or more, but you must report interest even without one.

Sources

1. Compound interest (glossary) — Investor.gov, U.S. Securities and Exchange Commission. Accessed October 13, 2026.
2. Rule of 72 (glossary) — Investor.gov, U.S. Securities and Exchange Commission. Accessed October 13, 2026.
3. National rates and rate caps — Federal Deposit Insurance Corporation. Accessed October 13, 2026. Updated monthly.
4. National deposit rates: savings (SNDR) — Federal Reserve Bank of St. Louis (FRED), FDIC data. Accessed October 13, 2026.
5. National deposit rates: 12-month CD (NDR12MCD) — Federal Reserve Bank of St. Louis (FRED), FDIC data. Accessed October 13, 2026.
6. I bonds interest rates — TreasuryDirect. Accessed October 13, 2026.
7. I bond and EE bond rates announced May 1, 2026 — TreasuryDirect. Accessed October 13, 2026.
8. 3-month Treasury bill secondary market rate (DTB3) — Federal Reserve Bank of St. Louis (FRED). Accessed October 13, 2026.
9. 6-month Treasury constant maturity (DGS6MO) — Federal Reserve Bank of St. Louis (FRED). Accessed October 13, 2026.
10. 1-year Treasury constant maturity (DGS1) — Federal Reserve Bank of St. Louis (FRED). Accessed October 13, 2026.
11. Consumer Price Index news release — Bureau of Labor Statistics. Accessed October 13, 2026. August 2026 data, released September 11, 2026.
12. Understanding deposit insurance — Federal Deposit Insurance Corporation. Accessed October 13, 2026.
13. Topic no. 403, Interest received — Internal Revenue Service. Accessed October 13, 2026.