Annuity Calculator: What Your Regular Payments Actually Grow Into
You save a little every month and wonder where it lands. That is the question this annuity calculator answers. Feed it a starting amount, a monthly payment, a rate and a number of years, and it shows you the future value of that steady habit: how much you'll have, how much of it is money you put in, and how much is growth. It can also run the question backwards: give it a target, and it works out the monthly payment that gets you there.
What an annuity is, in plain English
An annuity is just a series of equal payments made at regular intervals. Your monthly retirement contribution is an annuity. So is a savings plan where you put aside the same amount every month. The word sounds like something a bank sells you, and banks do sell products called annuities, but the math underneath is simpler than the sales pitch: you pay in steadily, and growth compounds on top of every payment already made.
Two distinctions matter. The first is timing. An ordinary annuity pays at the end of each period, the way most salaries and bills work. An annuity due pays at the beginning, so every payment gets one extra month of growth. The second is frequency. Monthly payments compound monthly, so January's growth starts earning its own growth in February. Annual payments compound once a year, so growth sits idle between birthdays, which is why the same rate on the same money produces a different result.
Every input, explained
Solve for. Three modes: future value, when you want to know where a plan lands; payment, when you have a target, say $200,000, and want to know what it costs per month; starting principal, when you want to know how much you'd need to put down today, on top of monthly payments, to reach a target.
Starting principal. The lump sum you begin with. It can be zero, or an existing pot you're adding to. This money gets the longest run at compounding because it's there from day one, so even a modest amount punches above its weight.
Payment. The regular amount you add each period, the input you control most directly, and the one most people underestimate. $200 a month sounds small, but over twenty years it's $48,000 of your own money, and growth roughly doubles it at a decent rate.
Annual interest rate. The growth rate, expressed per year but applied month by month when you choose monthly compounding. A 6% annual rate means 0.5% per month, not 6% per month: mixing those up is the most expensive mistake people make with this kind of calculator. Enter a realistic rate.
Years, frequency and timing. Years is how long the payments run. Time does more work than any other single input here, and the milestones below show why. Frequency is how often you pay; timing is whether each payment lands at the start or the end of the period.
A worked example: $10,000 down, $200 a month, 6%, 20 years
- Total paid in: $10,000 starting principal plus 240 monthly payments of $200 ($48,000) = $58,000.
- Monthly rate: 6% a year divided by 12 months = 0.5% per month.
- Growth on the principal: $10,000 at 0.5% a month for 240 months grows to about $33,102.
- Growth on the payments: the 240 payments of $200, each compounding for the months remaining after it lands, grow to about $92,408.
- Future value: $33,102 + $92,408 = $125,510.22.
- Growth (interest earned): $125,510.22 − $58,000 = $67,510.22, more than you put in.
Where the final pot comes from: over half of it, $67,510 out of $125,510, is growth, not contributions. You put in $58,000 and growth added more than that again. That is the argument for starting early: the longer money sits, the more of the result is growth, and growth is the part you don't have to earn twice.
Why time beats size: the milestones
The same plan, checked at three points, shows the pattern. After 5 years: $27,442.51. After 10 years: $50,969.84. After 20 years: $125,510.22. The first five years add about $17,443 on top of your $10,000 start. The next five add about $23,527. The second decade adds about $74,540, more than the entire first decade's growth three times over. Growth accelerates because it compounds on a bigger and bigger balance. Starting early is not a platitude: someone who starts ten years earlier doesn't end up with twice as much. They end up with far more, on the same monthly payment, because their early payments had a decade of extra compounding the late starter can never buy back.
What about timing, ordinary versus due? If every payment lands at the month's start instead of its end, the same plan reaches $125,972.26: only $462.04 more, because each payment gets at most one extra month of growth at 0.5%. Don't lose sleep over timing.
Solving backwards: what does a $200,000 target cost per month?
Suppose you want $200,000 in 20 years at 6%, starting from zero, with no lump sum and just monthly payments. The calculator works backwards and gives you $432.86 a month: the honest price of that target. If that's more than you can afford, the target has to move or the timeline has to stretch. If it's less than you expected, your goal is closer than you thought.
Same payments, different rates
The table below uses the same $10,000 start and $200 a month over 20 years. The no-growth baseline, money sitting under a mattress, is $58,000 at every rate, because that's simply what you paid in. Everything above that line is growth.
| Annual rate | Future value after 20 years | Growth earned |
|---|---|---|
| No growth (0%) | $58,000.00 | $0.00 |
| 4% | $95,580.75 | $37,580.75 |
| 6% | $125,510.22 | $67,510.22 |
| 8% | $167,072.11 | $109,072.11 |
Two percentage points, from 6% to 8%, adds over $41,000 to the result. Two points the other way removes about $30,000. The rate is not a detail you can fudge. It is the biggest lever after time itself. Use a defensible long-run average, not the best year you remember.
Common mistakes
- Forgetting the timing setting. Modelling an annuity due when your plan pays at month-end gives a slightly wrong answer, a different plan than the one you have.
- Mixing an annual rate with monthly payments. Applying 6% every month instead of 0.5% turns your result into a fantasy number.
- Ignoring fees and tax in real annuities. Real products charge fees, and payouts may be taxed. A 1% annual fee turns 6% into roughly 5%, and over twenty years that gap costs tens of thousands.
- Assuming the rate is guaranteed. A fixed annuity guarantees its rate; a variable one doesn't. Run the plan at a lower rate too: if the result stops working, your plan is fragile.
Practical tips
Start early, even if you start small: money paid in during the first five years does more work than money paid in during the last five. Next, raise your payment when your pay rises. Put half of a 5% raise into your monthly payment and your lifestyle still improves while your plan accelerates. Finally, check the real product's fees before you trust any projection: the total annual charge, surrender penalties, and whether the quoted rate is fixed or variable. A product charging 1.5% a year while promising 6% is really a 4.5% product wearing a nicer suit.
Frequently asked questions
What's the difference between an ordinary annuity and an annuity due?
It's just timing: ordinary pays at the end of each period, due pays at the beginning. In the worked example the difference was only $462 over twenty years, because each payment gains at most one extra month of growth.
Is monthly compounding really better than annual compounding?
Yes, slightly, because growth starts earning its own growth sooner. January's growth compounds from February instead of waiting until next January.
Can I use this calculator for a real annuity product I'm considering buying?
Use it to sanity-check the product's illustration. If the salesperson's projection assumes a rate far above what the product has delivered, or ignores fees, this calculator shows you the gap in plain numbers. But a real annuity has features this math doesn't cover, such as surrender charges, income riders and tax treatment, so treat it as a cross-check, not a replacement for reading the contract.
What rate should I enter?
Enter a rate you can defend: the long-run average return of the investment you're modelling, minus fees. Run a second calculation at a lower rate too: a plan that only works at 8% is a hope, not a plan.
What if I can't afford the monthly payment the backwards calculation gives me?
Then change one of the other levers, because the math doesn't negotiate. Extend the timeline: the same $200,000 target over 25 years instead of 20 needs a much smaller monthly payment. Start with a lump sum if you can, since money in from day one works hardest. Or lower the target. A smaller plan you stick to beats a bigger plan you abandon.
Related calculators: Annuity Payout Calculator, Finance Calculator and Bond Calculator.
An annuity is one way to turn savings into income. Payouts, bonds and compound growth are covered on the CalcMega Blog.
General information only, not financial advice. Rates, fees and tax rules vary, so check the terms of any real product before committing money.
Before buying one, read how the SEC's Investor.gov explains annuities, fees and all.