Free CalculatorMonthly ContributionsReal vs Nominal2026 Rates

Compound Interest Calculator 2026

See how your savings grow with an initial deposit and monthly contributions. Enter your balance, monthly amount, rate, and years — get your future value and total interest earned. S&P 500 historical avg: ~10% nominal / 7% real. HYSA rates: 4.0–5.0% APY (May 2026).

~10% nominal

S&P 500 97-yr avg

4–5% APY

HYSA rates May 2026

Rule of 72

72 ÷ rate = years to 2×

Start early

10-yr delay costs 50%+

Quick scenarios:

Compound Interest InputsLive preview

Initial deposit

Monthly contribution

Annual interest rate

Years

$10,000 initial + $500/mo · 7% annual · 20 years

Total put in: $130,000

Future value after 20 years

$300,851

$130,000 invested · $170,851 from compounding

$130,000

Total put in

$170,851

Interest earned

2.31×

Growth multiple

You contributed (43%)Compound interest (57%)

Of your final $300,851, 57% came from compound interest — money that grew without you lifting a finger. Every $1 you put in became 2.31 after 20 years.

Growth over 20 years — contributions vs interest

Compound Interest Guide 2026

Validated return rates, the Rule of 72, and what rate to use for your goal

S&P 500 Returns by Time Horizon: What Rate to Use (1928–2026)

Historical data from NYU Stern/Damodaran dataset (updated January 2026) — the most-cited academic source for US equity returns

The most common question when using a compound interest calculator: what return rate should I enter? The answer depends on your time horizon and whether you want nominal (pre-inflation) or real (inflation-adjusted) returns. The S&P 500 has returned approximately 10% nominal / 7% real annually since 1928 with dividends reinvested (NYU Stern Damodaran dataset, updated January 2026). Recent 10-year figures are much higher due to an extended bull market — don't use those for long-term planning.

Time Period	Nominal Return	Real Return	Context
5-year (2021–2025)	13.7%	9.2%	Includes 2022 bear market
10-year (2016–2025)	14.8–15.6%	12.0%	Extended bull market + AI surge
20-year (2006–2025)	10.8–11.0%	8.1%	Includes 2008 financial crisis
30-year (1996–2025)	10.1–10.4%	7.4%	Dot-com + GFC both included
50-year (1976–2025)	11.5–11.7%	7.6–7.8%	Four full market cycles
97-year (1928–2025)	~10.0%	~6.9%	Full Damodaran baseline

What rate to enter in the calculator

7% real — best for retirement/long-term planning (accounts for inflation, conservative). 10% nominal — use to see the dollar amount you'll have (includes inflation). 4–5% — for HYSA / short-term savings goals. Never use the recent 10-year figure of 14.8–15.6% for long-term projections.

Important caveat

The S&P 500 has never delivered a negative total return over any 20-year period in history (PortfolioCalc data). But individual years vary wildly: worst single year −43.8% (1931), best +52.6% (1954). The average only materialises over long holding periods. Some researchers (Vanguard, Damodaran) project only 5–7% nominal for the next decade due to elevated valuations — use conservatively.

Sources: NYU Stern — Historical Returns on Stocks, Bonds and Bills 1928–2025 (Damodaran, updated Jan 2026); TradeThatSwing — S&P 500 Average Returns (through Feb 2026).

The Rule of 72: How Long to Double Your Money at 2026 Rates

Divide 72 by your annual interest rate to estimate years to double — a simple mental check before running the full calculator

The Rule of 72 is the most useful mental shortcut in personal finance: divide 72 by your annual interest rate to estimate how many years it takes to double your money. It illustrates why the FDIC national average savings rate (0.38% as of May 2026) is catastrophic for long-term wealth: at 0.38%, it takes 189 years to double. At a top HYSA (5.0%), it takes 14.4 years. At the S&P 500 nominal average (10%), just 7.2 years.

Annual Rate	Years to Double	What This Is (May 2026)
0.38%	189 yrs	FDIC national avg savings
2.0%	36 yrs	Money market / typical CDs
4.0%	18 yrs	HYSA (broad access, May 2026)
4.5%	16 yrs	Top HYSA tier (May 2026)
5.0%	14.4 yrs	Best HYSA rates (May 2026)
7.0%	10.3 yrs	S&P 500 real return (30-yr avg)
10.0%	7.2 yrs	S&P 500 nominal (97-yr avg)

Traditional savings: 189 years

At 0.38% (FDIC national avg), $10,000 grows to $10,038 after 1 year. After 10 years: $10,388. Barely keeping up with inflation, let alone building wealth. Most big banks (Chase, Wells Fargo) pay 0.01–0.10%.

HYSA: 14–18 years

At 4–5% (top HYSA rates, May 2026): $10,000 doubles in 14–18 years. FDIC insured, liquid, much better than traditional savings. Best for emergency funds and goals under 5 years. Vio Bank 4.03%, Varo up to 5.00% (conditions apply).

S&P 500: 7–10 years

At 7–10% (S&P 500): $10,000 doubles in 7.2–10.3 years. Over 30 years at 10%: $10,000 → $174,000 without any contributions. This is why long-term stock market investing dramatically outpaces savings accounts for wealth building.

Sources: FDIC national average savings rate (0.38%, May 2026); NerdWallet — Best HYSA rates May 2026; Motley Fool — HYSA rates May 27, 2026.

Which Rate to Use for Your Goal: HYSA vs Investing vs Retirement (2026)

The right interest rate depends entirely on your time horizon and account type — using the wrong rate leads to wildly inaccurate projections

Emergency fund (1–3 years)

Use: 4.0–5.0%High-yield savings account (HYSA)

FDIC insured, liquid. Vio Bank 4.03%, Varo up to 5.00% (conditions apply). National avg only 0.38% — don't leave money in a traditional savings account.

Short-term savings (3–7 years)

Use: 4.5–5.5%CDs, I-bonds, money market

Lock in a rate with CDs if you don't need liquidity. I-bonds are inflation-indexed (check TreasuryDirect for current rate). Better certainty than equities over short horizons.

Long-term investing (10–20+ years)

Use: 7.0% real / 10% nominalIndex funds (S&P 500, total market)

Use 7% real or 10% nominal for projections. The S&P 500 has delivered 10.0% nominal over 97 years and 10.4% over 30 years. Never use the recent 10-year figure (14.8%) for planning — it includes an abnormally strong bull market.

Retirement (30+ years)

Use: 6–7% real401(k), IRA, Roth IRA

Most financial planners use 6–7% after inflation for retirement projections. 2026 401(k) limit: $23,500 (under 50), $31,000 (50+ with catch-up). Roth IRA: $7,000 / $8,000 catch-up.

The cost of starting late — quantified

Contributing $500/month from age 25 at 7% real return = ~$1,195,000 at 65. Starting at age 35 instead (same $500/month): ~$567,000. The 10-year delay costs $628,000 — more than the $60,000 in contributions those 10 years would have added. Compounding rewards early starters exponentially. Use the calculator to compare starting now vs. in 5 years.

Sources: NYU Stern Damodaran returns data 1928–2025; IRS 2026 401(k) and IRA contribution limits; NerdWallet HYSA rates May 2026.

How we calculate compound interest

Step-by-step breakdown of the future value shown in the calculator above. Last reviewed 2026-06-22.

Rates and returns vary. For inflation or tax adjustments, try our investment calculator or retirement calculator.

The future value above comes from your initial deposit, monthly contributions, annual interest rate, and time horizon—not a third-party feed. We compound your starting balance and periodic contributions at a monthly rate derived from your annual rate. Below are the formulas, the order we follow, and worked examples you can check by hand.

Formulas

Line	Formula
Monthly rate	r = annual rate % ÷ 100 ÷ 12
Total periods	n = years × 12
Lump sum growth	FV₁ = initial deposit × (1 + r)^n
Contribution growth (ordinary annuity)	FV₂ = monthly payment × [((1 + r)^n − 1) / r]
Future value	FV₁ + FV₂
Total contributions	Initial deposit + (monthly contribution × 12 × years)
Interest earned	Future value − total contributions

Order of operations

Convert annual rate to monthly compounding

r = annual % ÷ 12; n = years × 12 months

The calculator compounds monthly (12 times per year). Your stated annual rate is divided into equal monthly periods.

Grow the initial deposit

Compound lump sum for n months at rate r

Your starting balance earns interest each month for the full horizon. This is the standard future value of a present lump sum.

Grow monthly contributions

End-of-month payments into an ordinary annuity

Each monthly contribution is assumed to be invested at month-end and compound until the end of the projection. At 0% interest, contributions simply sum with no growth.

Sum components and interest

Future value = lump sum FV + annuity FV; interest = total − principal contributed

The headline number is the sum of both growth paths. We also show how much came from the initial deposit vs. contributions, and total interest earned.

Build the yearly chart

Re-run the same math at each year boundary

Each chart point uses identical inputs with a shorter time horizon, so you can see balance, contributions, and interest accumulate year by year.

Worked example

$10,000 start · $500/mo · 7.0% annual rate · 20 years

Lump sum: $10,000 × (1 + 0.6%/mo)^240 mo = $40,387

Contributions: $500/mo ordinary annuity → $260,463

$40,387 + $260,463 = $300,851 future value

Interest earned: $300,851 − $130,000 contributed = $170,851

Line item	Amount
Initial deposit	$10,000
Monthly contribution	$500
Annual rate	7.0%
Years	20
Initial deposit grew to	$40,387
Contributions grew to	$260,463
Future value	$300,851
Total contributed	$130,000
Interest earned	$170,851

Lump sum only: Lump sum only: $10,000 invested, no monthly contributions → $40,387.

Higher savings rate: Double contributions: $1,000/mo instead of $500/mo → $561,314 (vs $300,851 at $500/mo).

Zero return: 0% rate: future value equals deposits plus all contributions → $130,000.

Constants we use

Parameter	What we use	Source
Default initial deposit	$10,000	Pre-fill on page load
Default monthly contribution	$500	Pre-fill on page load
Default annual rate	7.0%	Rate presets include 4.5%, 7%, 10%
Default horizon	20 years	Clamped to 1–50 years in the UI
Compounding frequency	Monthly (12× per year)	Fixed in this calculator
Contribution timing	End of month	Ordinary annuity convention

What we do not model on this page

We use a constant interest rate and level monthly contributions—we do not model inflation, taxes, account fees, market volatility, variable contribution schedules, or intra-month contribution timing. Compounding is monthly only on this page (not daily or quarterly). Negative rates are treated as 0% growth on contributions. Results are illustrative, not investment advice.

Official references

Frequently Asked Questions

How compound interest works, what rate to use, and the Rule of 72 explained

Your initial balance earns interest each month. Each monthly contribution also earns interest from the month it's added. The key: interest earned in month 1 becomes part of the balance that earns interest in month 2. This compounding effect accelerates growth over time — the longer the period, the more dramatic the effect. Formula: FV = P(1 + r)^n + C × [((1 + r)^n − 1) / r], where r = monthly rate, n = months, P = principal, C = monthly contribution.

For a high-yield savings account (HYSA): use 4.0–5.0% APY based on May 2026 rates (Vio Bank 4.03%, Varo up to 5.00% with conditions). For long-term stock market investing: use 7% (inflation-adjusted real return over 30 years) or 10% (nominal, 97-year average) per NYU Stern/Damodaran data. Don't use the recent 10-year average of 14.8% for planning — it overstates typical long-run expectations. For conservative retirement planning, most advisors use 6–7% real.

The Rule of 72 estimates how long it takes to double your money: divide 72 by your annual interest rate. At 4% APY (top HYSA), money doubles in 18 years. At 7% (S&P 500 real return), it doubles in ~10.3 years. At 10% (S&P 500 nominal), it doubles in 7.2 years. At the FDIC national average of 0.38%, it takes 189 years. This is why high-yield accounts and long-term investing matter so much.

Simple interest is calculated only on the principal: $1,000 at 7% for 10 years = $700 in interest = $1,700 total. Compound interest calculates on principal + prior interest: $1,000 at 7% compounded annually for 10 years = $1,967 — $267 more. The difference widens dramatically over time: at 30 years, simple interest gives $3,100 while compound gives $7,612. Most savings accounts, bonds, and investments use compound interest.

At 7% annual return (S&P 500 real return, 30-year average): $500/month for 30 years = $180,000 contributed but grows to ~$567,000. At 10% (nominal): the same $500/month grows to ~$1,130,000. Starting 10 years earlier makes a massive difference: $500/month for 40 years at 7% = ~$1,195,000 vs $567,000 for 30 years. This illustrates why starting early is the single most important factor in building wealth.

More frequent compounding produces slightly more growth. $10,000 at 5% for 20 years: annually = $26,533; monthly = $27,126; daily = $27,182. The difference between monthly and daily compounding is minimal (~$56 on $10,000 over 20 years). Monthly compounding is the standard for most savings accounts and investments, and what this calculator uses. Annual compounding is simplest and gives a slightly conservative estimate.

Nominal return is what you see stated (e.g., 10% for the S&P 500). Real return adjusts for inflation (~7% for S&P 500). If your goal is future spending power, use real returns (7%). If you want to see the dollar amount your account will show, use nominal (10%). For retirement planning: use real returns to understand what your money will actually buy. Example: $1M in 30 years at 3% inflation is worth about $412,000 in today's dollars.

Compound interest is the core engine of retirement savings. Contributing $500/month to a 401(k) from age 25 to 65 (40 years) at 7% real return = ~$1,195,000. Starting at 35 instead (30 years): ~$567,000. The 10-year delay costs over $600,000 — more than the actual contributions over those 10 years. The 2026 401(k) contribution limit is $23,500 (under 50) or $31,000 (50+ with catch-up). Maximize tax-advantaged accounts first.

Sources & References