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Direct answer: Compound interest pays interest on your earlier interest. Saving $10,000 plus $200 a month for 10 years at 4.2% APY, the best standard savings rate we track, grows to about $44,729: $34,000 you put in and $10,729 interest. At the FDIC's 0.37% national average it's about $34,821.
By the numbers: FDIC national average savings rate: 0.37% (Sept. 21, 2026). At that rate, money takes about 195 years to double by the Rule of 72. Source: FDIC: National Rates and Rate Caps
“Interest paid on principal and on accumulated interest.” — Investor.gov (SEC) glossary: Compound Interest
Updated 2026-10-17
Balance = principal × (1 + r/n)^(n × years), where r is the yearly rate as a decimal and n is how many times a year interest compounds. For example, $730 at 5% compounded yearly for 5 years is 730 × 1.05^5 = $931.69, the SEC's Investor.gov example.
Divide 72 by the interest rate to estimate how many years it takes money to double. Investor.gov's example: at 9%, about 8 years. At 4.2% it's about 17.1 years (exact math: 16.8); at the FDIC's 0.37% savings average it's about 195 years.
The interest rate is the yearly rate before compounding. APY (annual percentage yield) includes compounding over a year, using the formula in Regulation DD, Appendix A. A 4.00% rate compounded daily is a 4.08% APY; compounded monthly, 4.07%. When a bank advertises a savings rate, Regulation DD says it must be stated as an APY.
Not much at savings-account rates. $10,000 at a 4.00% rate for 10 years ends at about $14,918 compounded daily, $14,908 monthly and $14,802 yearly. The rate itself matters far more than how often it compounds.
At 4.2% APY with no deposits, about $15,090. At the FDIC's 0.37% national savings average, about $10,376. Savings rates are variable, so real results change when banks change rates.
Yes. Interest on savings and CDs is taxable in the year it's credited to your account, even if you leave it there, and banks report it on Form 1099-INT. This calculator doesn't subtract taxes.
As of Oct. 17, 2026
Balance = principal × (1 + rate ÷ n)n × years, plus each deposit growing from the month you add it. The rate you earn matters much more than how often it compounds.
The math:
Sources: Investor.gov calculator · Investor.gov Rule of 72 · Reg DD Appendix A
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Same inputs as the SEC's Investor.gov calculator. The rate starts at the best standard savings APY we track (4.2% at EverBank). Change anything. Nothing you type leaves your phone.
| Rate | 1 year | 5 years | 10 years | Years to double |
|---|---|---|---|---|
| Best standard savings APY we track (EverBank, 4.2%) | $10,420 | $12,284 | $15,090 | 16.8 |
| FDIC national average savings (0.37%) | $10,037 | $10,186 | $10,376 | 187.7 |
Savings APYs are variable and can change any time; these figures assume today's rate holds. Best tracked APY from the bank's own page (bank page read 2026-10-12); FDIC average as of Sept. 21, 2026. Sources: FDIC national rates.
$10,000 at a 4.00% nominal rate for 10 years, no deposits:
| Compounding | APY | After 10 years |
|---|---|---|
| Annually | 4.000% | $14,802.44 |
| Semiannually | 4.040% | $14,859.47 |
| Quarterly | 4.060% | $14,888.64 |
| Monthly | 4.074% | $14,908.33 |
| Daily | 4.081% | $14,917.92 |
APY = (1 + rate ÷ n)n − 1. Regulation DD makes banks state savings yields as an APY, which already includes compounding, so compare APYs, not compounding schedules. Sources: Reg DD Appendix A · Investor.gov calculator.
Investor.gov: divide 72 by the interest rate to estimate how many years it takes money to double. At 9%, that's about 8 years (72 ÷ 9). It's an estimate; the exact answer is ln 2 ÷ ln(1 + rate). Sources: Investor.gov Rule of 72.
Sources: Reg DD Appendix A.
Arithmetic only. VaultRubric doesn't forecast rates or returns. Savings rates are variable; investments can lose money and have no set rate.
Balance = principal × (1 + r/n)^(n × years), where r is the yearly rate as a decimal and n is how many times a year interest compounds. For example, $730 at 5% compounded yearly for 5 years is 730 × 1.05^5 = $931.69, the SEC's Investor.gov example.
Divide 72 by the interest rate to estimate how many years it takes money to double. Investor.gov's example: at 9%, about 8 years. At 4.2% it's about 17.1 years (exact math: 16.8); at the FDIC's 0.37% savings average it's about 195 years.
The interest rate is the yearly rate before compounding. APY (annual percentage yield) includes compounding over a year, using the formula in Regulation DD, Appendix A. A 4.00% rate compounded daily is a 4.08% APY; compounded monthly, 4.07%. When a bank advertises a savings rate, Regulation DD says it must be stated as an APY.
Not much at savings-account rates. $10,000 at a 4.00% rate for 10 years ends at about $14,918 compounded daily, $14,908 monthly and $14,802 yearly. The rate itself matters far more than how often it compounds.
At 4.2% APY with no deposits, about $15,090. At the FDIC's 0.37% national savings average, about $10,376. Savings rates are variable, so real results change when banks change rates.
Yes. Interest on savings and CDs is taxable in the year it's credited to your account, even if you leave it there, and banks report it on Form 1099-INT. This calculator doesn't subtract taxes.
Estimates, not financial advice. Taxes, insurance, PMI and fees vary by lender, county and city; confirm with your lender, dealer and state agency before acting.