How to use this calculator
- Enter what you have now. Enter $0 if you are starting from nothing.
- Enter what you plan to add and how often, each month or each year. Enter $0 to see a single deposit grow on its own.
- Enter the rate and how often it compounds. For a savings account or CD, the APY goes with yearly compounding, and the interest rate goes with the account's own compounding frequency. For an investment, enter the average yearly return you expect and choose yearly.
- Choose how many years the money grows.
- Set how many years later to compare starting your additions. Set it to 0 to turn the comparison off.
Under More options you can add at the end of each period instead of the start, change how much what you add rises each year (2.5% by default), change the inflation rate, and show every figure in today's dollars. The numbers already filled in are an example. Replace them with your own.
How it's calculated
1. Growth each month or year
The balance grows in steps, one for each time you add money: twelve a year for monthly additions, one for yearly. With an annual rate r compounded m times a year, a year's growth factor is (1 + r/m)m, and each step's is that year's factor shared evenly across the steps. Daily compounding uses 365 days in every year. When you add monthly and interest compounds less often, each month's addition grows at the steady monthly rate that produces the same yearly result.
2. Adding, then growing
With additions at the start of each period, each step adds that period's amount and then grows the balance; at the end, it grows first and then adds. What you add rises once a year by the yearly increase you choose. In the example, $10,000 plus twelve monthly additions of $500 at 7% compounded yearly is $16,925 after the first year, of which $925 is growth. After 30 years the balance is $839,006: $273,416 you put in and $565,590 of growth. With what you add held flat at $500, it would be $664,155.
3. Rounding
The balance is carried at full precision from step to step and rounded to the cent only when a figure is shown, then to whole dollars on the page. That is how the textbook future value formulas work, and it is consistent with the worked examples in the federal rules for savings disclosures. Each figure you see that is a difference of two others, such as growth, is the difference of the two figures shown, so they add up.
4. The rate as an APY, and doubling
The annual percentage yield is (1 + r/m)m − 1, rounded to two decimals: 7.00% compounded monthly is an APY of 7.23%. A single deposit doubles in ln 2 ÷ ln(1 + APY) years, 10.2 years at 7% compounded yearly. The Rule of 72 estimates it as 72 divided by the rate: 10.3 years.
5. Today's dollars
A balance at the end of a year is divided by (1 + inflation) raised to the number of years, so it is in dollars with today's purchasing power. Each year's additions are divided the same way for the start of that year, and growth in today's dollars is what is left. In the example, at 2.5% inflation, $839,006 in 30 years is $399,990 in today's dollars, and the $273,416 you put in is $190,000.
6. Starting later
The comparison keeps your starting amount invested and starts your additions later, ending at the same time. From the year they start, the later plan adds what the plan adds in those years, so the only difference is the years it skips. In the example, starting 5 years later ends at $635,428 instead of $839,006: $203,578 less, of which $31,538 is the additions skipped and the rest is the growth they would have earned. The catch-up amount is the smallest whole-dollar amount that, added from the later start and rising by the same yearly increase, ends at least as high: $772 a month.
Assumptions and limits
- One rate for the whole period. Investments rise and fall, and an average return is not the same as a steady one: when you are adding money, a bad decade near the end costs more than one near the start, because more money is invested then. Savings and CD rates change too.
- No taxes, fees or withdrawals. Growth is before income tax, fund expenses and advisory fees. In a taxable account, interest and dividends are taxed each year, which slows the growth shown; in a 401(k) or IRA the tax comes later or not at all.
- When you add monthly and interest compounds less often, or daily, each addition grows at the same steady monthly rate that produces the annual percentage yield. An account that pays simple interest on a new deposit until the next compounding date pays slightly more: with yearly compounding, about $2 a year on $500 a month at 7%; with monthly compounding, nothing.
- A year of 365 days and twelve equal months. Daily compounding uses 365 days in every year.
- Waiting means the additions wait. The comparison keeps your starting amount invested in both plans and starts the additions later, ending at the same time.
- Balances are in the dollars of each future year unless you choose today's dollars, which divides by inflation at the rate you enter.
- The Rule of 72 uses the rate as you entered it; the exact doubling time uses the annual percentage yield. Both are for a single deposit.
- It does not cover taxes, fees, withdrawals, returns that vary from year to year, horizons in months or days, additions weekly or every two weeks, continuous compounding, negative rates, or the time or rate needed to reach a goal.
Input field details
What to enter in each box, where to find the number, and how this calculator treats it. When two calculators give different answers from the same numbers, it's usually because they handle one of these differently.
Starting amount
What you have now in the account or investment you are projecting.
Where to find itYour latest statement or online balance.
How this tool treats itIt is invested from day one and grows for every year you enter, in the plan and in the comparison with starting later.
Allowed range$0 to $10,000,000
You add, and how often
The amount you put in each month or each year, and which. Enter $0 to see a single deposit grow on its own.
Where to find itYour pay stub, your plan’s statement or the automatic transfer you have set up with your bank.
How this tool treats itEach month or each year in the first year, raised once a year at the start of each year by the yearly increase under More options (2.5% unless you change it). Every addition within a year is the same amount.
Allowed range$0 to $1,000,000
Annual interest rate or return
The yearly rate your bank quotes, or the average yearly return you expect on an investment, before inflation.
Where to find itFor a savings account or CD, the account disclosure, which states both the interest rate and the annual percentage yield (APY) to two decimals.
How this tool treats itIf you have an APY or an investment’s average annual return, choose Yearly under Compounds: those figures already include compounding. If you have a bank’s interest rate, choose the bank’s compounding. The same rate is used every year. Returns are an assumption, not a forecast. Over the long run the S&P 500 returned about 10.2% a year nominally (January 1926 to September 2023, computed from Robert Shiller's series, dividends reinvested, before taxes, fees and any advisor cost), while consumer prices rose about 3.0% a year (1926 to 2024, computed from the CPI-U). Neither is what any single decade delivered, and a real portfolio is not an index: you cannot invest directly in one. Every figure this tool produces is a hypothetical illustration of the assumptions you entered.
Allowed range0% to 20%, in steps of 0.01%
Not includedReturns that vary from year to year, and negative rates.
Compounds
How often interest is added to the balance: yearly, twice a year, quarterly, monthly or daily.
Where to find itThe account disclosure states how often interest is compounded and credited.
How this tool treats itA year of compounding at rate r, m times a year, multiplies the balance by (1 + r/m) raised to the power m. Daily uses 365 days in every year. When you add monthly and interest compounds less often, or daily, each addition grows at the steady monthly rate that produces the same annual percentage yield; an account that pays simple interest on a new deposit until the next compounding date pays slightly more.
Not includedContinuous compounding, and a 366-day leap year.
Years
How many years from now you want to see the balance.
How this tool treats itWhole years. The table shows the balance at the end of every year.
Allowed range1 to 60
Not includedHorizons in months or days.
Compare starting later by
How many years later you might start adding money, to see what the delay costs. Enter 0 to turn the comparison off.
How this tool treats itYour starting amount stays invested in both plans; only the additions wait. The later plan ends at the same time and, from the year it starts, adds what the plan adds in those years, so the difference is the years it skips and the growth they would have earned. The amount that catches up is the smallest whole-dollar amount to add in the later plan’s first year, rising at the same yearly increase, that ends with at least the same balance.
Allowed range0 to 40, and less than the years you entered (a wait that long is set to one year less)
Not includedA comparison when you add nothing: there is nothing to delay.
When you add it
Whether each addition goes in at the start or at the end of its month or year.
How this tool treats itAt the start, each addition earns a full period of growth before the next one; at the end, it earns none in the period it is made. Adding at the start always ends with at least as much.
Yearly increase in what you add
How much more you add each year than the year before. Enter 0% to keep it flat.
How this tool treats itApplied once a year, at the start of the year. 2.5% unless you change it, the same as the inflation default, so what you add keeps its buying power.
Allowed range0% to 10%
Inflation
How fast you expect prices to rise each year.
How this tool treats itUsed only for the today’s-dollars figures: it never changes the plan or what you add. 2.5% unless you change it.
Allowed range0% to 10%
Show in today’s dollars
Turn on to show every figure in what it would buy today.
How this tool treats itA balance at the end of a year is divided by one plus inflation, raised to the number of years from now. Each year’s additions are divided the same way for the year they are made, and growth is what remains. The amount that catches up is never converted, because it is an amount you would type.
Questions
How does compound interest work?
Does daily compounding make much difference?
Is my rate an APY or an interest rate, and which do I enter?
How accurate is the Rule of 72?
What does waiting 5 years cost?
Should I count inflation?
Why might your balance end up lower than this?
Should I add money monthly or once a year?
Version history
- October 18, 2026 (version 1.0.0): First version: growth of a starting amount and regular additions at any of five compounding frequencies, the cost of starting later and the amount that catches up, today's dollars, the APY and doubling time.