Interest Calculator
Model compound interest on a starting balance plus ongoing monthly or annual contributions. Choose your compounding frequency and contribution timing, then layer in a tax rate on interest and an inflation rate to see what your balance is really worth.
Principal vs. contributions vs. interest
How your rate compares
💰 Find a better place to keep this money
Compare savings ratesHow contributions change compound growth
Compound interest pays interest on interest, so the earlier money arrives, the longer it has to compound. A lump sum grows on its own curve, but regular contributions add a second, steady stream of new principal that each start compounding from the moment they land. Beginning-of-period contributions get one extra compounding cycle compared to end-of-period ones — a small edge per period that adds up over a long horizon. Compounding frequency matters far less than people assume: the jump from annual to monthly compounding is real but modest, while the rate itself and the size and consistency of your contributions do almost all of the work.
How it’s calculated
Each compounding period, contributions are added (at the start or end of the period, per your setting) and interest is charged on the balance at rate ÷ compounding frequency. If a tax rate is set, that period’s interest is reduced by the tax before it’s added back to the balance, so tax drags on future compounding too, not just the year it’s owed. Buying power divides the ending balance by (1 + inflation rate)years, restating the result in today’s dollars.
Results update as you type and are estimates for education, not investment or tax advice — actual returns, taxes, and inflation will vary.
Accumulation schedule
Shows year-by-year contributions, interest, and ending balance at your selected compounding frequency and tax rate.
Worked example
A $10,000 initial investment plus $300 a month (end of period) at 6% compounded monthly for 15 years grows to $111,786.55 — $54,000 of that is contributions, and $47,786.55 is interest. Add a 24% tax rate on interest and 3% inflation, and the after-tax ending balance falls to $97,099.19, worth about $62,324.28 in today’s buying power.
Common mistakes
- Ignoring taxes on interest — ordinary interest income is taxed as it’s earned, which quietly slows compounding every single year.
- Forgetting that end-of-period contributions earn one fewer compounding cycle than beginning-of-period ones, so the timing toggle isn’t cosmetic.
- Comparing a nominal ending balance across two long time horizons without adjusting for inflation, which overstates how much richer the longer horizon really leaves you.
Where it is used
- Projecting a savings or brokerage balance that combines a starting deposit with ongoing monthly or annual contributions.
- Comparing compounding frequencies or contribution timing before committing to an account.
- Estimating the real, inflation-adjusted value of a long-term savings plan.
Frequently asked questions
Should contributions be at the beginning or end of the period?
Beginning-of-period deposits earn one extra compounding period of interest each time, so they produce a slightly higher ending balance. Most payroll-deducted savings and retirement contributions land close to the start of the period, so “beginning” is a reasonable default if you are not sure.
How much does compounding frequency actually matter?
Less than most people expect once you get past annual compounding. Moving from annual to monthly compounding on a typical savings rate adds a small fraction of a percent to your effective yield; moving from monthly to daily adds far less than that. The rate itself matters far more than the frequency.
Why is my after-tax total so much lower?
Ordinary interest income (savings, CDs, most bonds) is taxed every year at your marginal rate, which quietly compounds against you — you lose growth on the money paid in taxes, not just the tax itself. Tax-advantaged accounts avoid this drag, which is why the same rate produces a noticeably larger balance inside an IRA or 401(k) than in a plain taxable account.
What does the inflation adjustment actually show me?
It restates your ending balance in today’s purchasing power. A big nominal number can still buy less than you expect if inflation ran ahead of your rate of return — the “buying power” figure is the number that matters for real financial planning, not the sticker total.
How does my rate compare to a savings account today?
The FDIC’s national average savings APY was about 0.38% as of June 2026, while top nationally available high-yield savings accounts pay roughly 4%. If your rate assumption is near the national average, moving that same cash to a top online savings account or CD could multiply your interest without adding risk.