Compound Interest Calculator 2026
By Sarah Davis and Axis Intelligence Research · Last updated: October 3, 2026 · Rates as of September 21, 2026 (FDIC) · Inflation as of August 2026 (BLS)
Compound Interest Calculator Axis Intelligence Research · rates as of Sep 21, 2026 · CPI as of Aug 2026
Real-world adjustments
Quick answer: Compound interest grows a balance with the formula A = P(1 + r/n)^(nt), where interest is added to the principal and then earns interest itself. At the FDIC national savings rate of 0.37% (September 21, 2026), $10,000 compounded monthly becomes $10,376.87 in 10 years — but Axis Intelligence Research estimates it is worth about $7,428 in today’s dollars if inflation holds at August 2026’s 3.4%.
What Does This Compound Interest Calculator Do?
It projects how a deposit grows with interest compounded daily, monthly, quarterly, semiannually, annually or continuously, with optional weekly, biweekly, monthly, quarterly or yearly contributions. Unlike most compound interest calculators, it also subtracts tax on interest, converts the ending balance into today’s dollars, solves for the time or deposit needed to hit a savings goal, and compares two rates on one chart. Every default comes from a dated federal source, and every result can be shared by link or downloaded as a CSV.
How Is Compound Interest Calculated?
The core formula, for a single deposit:
A = P × (1 + r/n)^(n × t)
P is the starting principal, r the annual nominal rate as a decimal, n the number of compounding periods per year, and t the number of years. For continuous compounding the limit is A = P × e^(r × t).
Show the math with the FDIC’s average 12-month CD rate (1.73% on September 21, 2026), compounded monthly for 10 years: $10,000 × (1 + 0.0173/12)^120 = $11,887.18. The calculator returns the same figure to the cent.
Compound interest with monthly contributions
Regular deposits add a second term, the future value of an annuity:
FV = PMT × [((1 + i)^N − 1) / i], where i is the rate per deposit period and N the number of deposits. If you deposit at the start of each period instead of the end, multiply by (1 + i), because every deposit earns one extra period of interest.
Example: $100 a month for 10 years at 5%, monthly compounding, end-of-month deposits → $15,528.23. Switch to start-of-month deposits → $15,592.93. Same money in, $64.70 more out, purely from timing.
Daily vs monthly compound interest: does frequency matter?
Less than banks’ marketing implies. On $10,000 at 5% for 10 years, daily compounding yields $16,486.65 and continuous compounding $16,487.21 — a gap of 56 cents. The rate you earn moves your outcome; the compounding frequency mostly moves the decimals. That’s why the calculator shows the APY (effective annual yield), which folds frequency into a single comparable number: APY = (1 + r/n)^n − 1.
When deposits and compounding don’t line up
If you deposit weekly but interest compounds monthly, the calculator converts the nominal rate to an equivalent rate per deposit period: (1 + r/n)^(n/m) − 1, where m is deposits per year. Balances are then simulated period by period, so contribution step-ups and annual tax deductions apply exactly when they would.
How Much Will My Savings Grow at Today’s Average Rates?
Axis Intelligence Research ran four scenarios through the calculator using only the FDIC national rates published September 21, 2026, monthly compounding, no tax, and an inflation input of 3.4% (the BLS CPI-U 12-month change to August 2026), held constant as an assumption.
| Scenario (10 years) | Rate | Ending balance | In today’s dollars* | Real Growth Multiple* | Source of rate |
|---|---|---|---|---|---|
| $10,000, no deposits | 0.37% savings | $10,376.87 | $7,427.81 | 0.74× | FDIC national rate |
| $10,000, no deposits | 1.73% 12-mo CD | $11,887.18 | $8,508.90 | 0.85× | FDIC national rate |
| $10,000 + $200/month | 0.37% savings | $34,822.56 | $24,926.16 | 0.73× | FDIC national rate |
| $10,000 + $200/month | 1.73% 12-mo CD | $38,067.71 | $27,249.05 | 0.80× | FDIC national rate |
*Axis Intelligence Research estimates; assumes inflation stays at 3.4% for all 10 years and the rate stays fixed. Neither assumption is likely to hold exactly — change both in the tool.
The fee here isn’t printed on any statement. It’s the spread between what the average account pays and what prices do. At 0.37% against 3.4% inflation, that spread runs about three points a year, and over a decade it takes roughly a quarter of the deposit’s purchasing power with it. The balance line goes up the whole time, which is precisely why nobody notices.
What Is the Real Growth Multiple (RGM)?
The Real Growth Multiple is an Axis Intelligence Research metric (methodology v1.0): your ending balance converted into today’s dollars, divided by everything you deposited.
RGM = [Ending balance ÷ (1 + inflation)^years] ÷ Total deposits
Above 1.00×, your money bought more at the end than it did going in. Below 1.00×, you saved diligently and still lost ground. A nominal balance can’t tell you that; a statement showing “+$376.87 interest” never will.
What is the crossover year?
The calculator also reports the crossover year: the first year in which the interest earned exceeds the new money you deposited. Before it, your savings habit does most of the work. After it, the balance does. At the FDIC’s 0.37% average, $10,000 plus $200 a month doesn’t reach crossover within any realistic horizon, because a year’s interest stays far below $2,400 of deposits. Push the rate up in the tool and watch the crossover arrive — it is the most direct way to see what a rate difference is actually worth.
How Long Does It Take to Double Your Money?
The shortcut is the Rule of 72: divide 72 by the rate in percent. The calculator skips the shortcut and shows the exact answer, ln(2) ÷ ln(1 + APY), after tax. At the average 12-month CD rate of 1.73% (APY 1.744% with monthly compounding), doubling takes 40.1 years. At the 0.37% savings average, 187.4 years.
Why It Matters
Most compound interest calculators stop at a nominal number that looks like progress. A saver deciding where to park an emergency fund needs the after-tax, after-inflation figure, because that is what the money will buy. Federal data published September 21, 2026 put the average savings account at 0.37% and the average 12-month CD at 1.73%, while August’s CPI-U ran at 3.4%. In that environment, the rate you choose matters far more than how often it compounds — and this tool is built to make that gap visible in one screen.
Limitations
- Projections, not promises. Rates on savings accounts are variable; the calculator holds your input rate fixed for the whole period.
- Inflation is an input, not a forecast. The 3.4% default is the latest observed 12-month CPI-U change, not a prediction.
- Tax is simplified. It applies one flat rate to each year’s interest and deducts it from the balance at year-end. It does not model brackets, state tax, tax-advantaged accounts or capital gains.
- No fees, no withdrawals, no market risk. For investments with variable returns, a single fixed rate understates the range of outcomes.
- Rounding. Banks may round interest daily; results can differ from a statement by a few cents.
Estimates only — not financial advice.
Data Freshness
Rate presets: FDIC National Rates and Rate Caps, published September 21, 2026 (next release October 19, 2026). Inflation default: BLS CPI-U, 12 months ending August 2026, released September 11, 2026 (September data due October 14, 2026). Review trigger: each monthly FDIC release and each monthly CPI release; constants are updated and this page’s date changes only when a source value changes.
Frequently Asked Questions
What is the formula for compound interest?
A = P(1 + r/n)^(nt), where P is principal, r the annual rate as a decimal, n compounding periods per year and t years. Continuous compounding uses A = Pe^(rt).
What is the difference between APR and APY?
APR is the nominal annual rate before compounding; APY includes the effect of compounding within the year. A 1.73% rate compounded monthly equals an APY of about 1.744%.
Is daily compounding better than monthly?
Slightly. On $10,000 at 5% over 10 years, the difference between daily and continuous compounding is under a dollar; between monthly and daily it is also small. The rate itself matters far more.
What is the average savings account interest rate right now?
The FDIC national rate for savings accounts was 0.37% as of September 21, 2026, and 1.73% for 12-month CDs.
Does compound interest beat inflation?
Only if your after-tax rate exceeds the inflation rate. With CPI-U at 3.4% for the 12 months to August 2026, the average savings and CD rates both fell short. The calculator flags this automatically.
How do I calculate compound interest in Excel?
Use =FV(rate/n, nyears, -contribution, -principal). For a single deposit, =P(1+r/n)^(n*t).
Related Resources
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Cite this tool: Axis Intelligence Research. (2026). Compound Interest Calculator (methodology v1.0). https://axis-intelligence.com/compound-interest-calculator/
Sources
- Federal Deposit Insurance Corporation, National Rates and Rate Caps, September 21, 2026 (series SNDR and NDR12MCD via FRED).
- U.S. Bureau of Labor Statistics, Consumer Price Index — August 2026, released September 11, 2026.