Bright Side Kit

MONEY GUIDE

How to Calculate an Annuity Payment — Step by Step

The example is $500 a month at 7% for 30 years — future value $609,985.50, $180,000.00 paid in, $429,985.50 of interest, $4,729.21 a month for 20 years, or $2,033.28 a month on the 4% rule. Calculating an annuity is six steps, and this guide walks through them.

The formula, in one paragraph

Calculating an annuity is six steps. A monthly contribution grows by compounding: every month you put money in, and every month the whole balance earns the monthly return. The monthly rate is the annual return divided by 12, and the number of months is the years times 12. The future value is the monthly contribution times the growth factor — see the formula in the box below — plus any lump sum, grown at the same rate. The total paid in is the contributions plus the lump sum, and the interest is the future value minus the total paid in.

Step 1: Find the monthly rate and the months

Start with the return and the time. The example uses 7% a year for 30 years. The monthly rate is 7% ÷ 12 = 0.5833% a month (0.005833), and the months are 30 × 12 = 360.

Step 2: Work out the growth factor

The growth factor turns one dollar a month into a pot. It is one plus the monthly rate to the power of the months, minus one, divided by the monthly rate: ((1.005833)360 − 1) ÷ 0.005833 = 1,219.97. Every dollar a month becomes $1,219.97 after 30 years at 7%.

Step 3: Multiply by the monthly contribution

Multiply the factor by what you put in each month: $500 × 1,219.97 = $609,985.50. The future value scales in direct proportion to the contribution: $250 a month gives $304,992.75, exactly half, and $1,000 a month gives $1,219,971.00, exactly double.

Step 4: Work out what you paid in and what growth added

The total paid in is the monthly contribution times the months: $500 × 360 = $180,000.00. The interest earned is the future value minus the total paid in: $609,985.50 − $180,000.00 = $429,985.50. Growth supplies $429,985.50 of the $609,985.50 — more than twice what you put in.

Step 5: Work out the monthly payout

The pot pays out over 20 years at the same 7%: $609,985.50 spread over 240 months gives $4,729.21 a month, or $1,135,010.66 over the 20 years — the pot keeps earning while you draw it down. On the 4% rule the same pot gives $2,033.28 a month, or $24,399.42 a year, and the pot should last far longer than 20 years. The 20-year payout spends the pot down to zero; the 4% rule is meant to last.

Step 6: Check the answer backwards

The formula also runs backwards. To reach $500,000 in 30 years at 7% you need $409.85 a month (forward: $500,000.00). To reach $1,000,000 you need $819.69 a month, and to reach $250,000 you need $204.92 a month. At $500 a month it takes 27.53 years to reach $500,000 (330.41 months; after 331 months the pot is $502,001.25), and 36.38 years to reach $1,000,000. The return needed for $500,000 at $500 a month over 30 years is 5.9764% a year, and for $1,000,000 it is 9.4222% a year.

When the return is lower, or the time is shorter

At 5% the same $500 a month for 30 years grows to only $416,129.32, with $236,129.32 of interest — two points off the return costs $193,856.18. At 9% it grows to $915,371.74, with $735,371.74 of interest. Over 10 years the pot is only $86,542.40, with $26,542.40 of interest: ten years is too short for compounding to do much. Over 40 years it is $1,312,406.70, with $1,072,406.70 of interest — more than four times what you put in. Starting ten years earlier beats doubling the contribution.

What a lump sum adds

A lump sum you start with grows on its own: $10,000 at 7% for 30 years becomes $81,164.97, and with the $500 a month on top the pot is $691,150.47. With no contributions at all, the $10,000 lump alone is the whole pot: $81,164.97, of which $71,164.97 is growth. With 1 year of $500 a month the future value is $6,196.29, of which $6,000.00 is paid in and $196.29 is interest.

What NOT to do

Do not judge by the contribution alone: $250 a month still grows to $304,992.75 over 30 years, and starting small beats waiting for the perfect amount. Do not assume the return: 7% is a long-run average, not a promise for any single 30-year stretch, and a 1% annual fee cuts the pot by far more than 1%. Do not forget inflation: $609,985.50 in 30 years buys less than $609,985.50 today. And do not treat the payout as a salary: the 20-year payout of $4,729.21 a month spends the pot down to zero, while the 4% rule of $2,033.28 a month is meant to last.

Monthly rate i = annual return ÷ 12; months n = years × 12; growth factor = ((1+i)n − 1) ÷ i; future value = monthly contribution × growth factor + lump sum × (1+i)n; total paid in = contribution × n + lump sum; interest earned = future value − total paid in; monthly payout over 20 years = future value × i ÷ (1 − (1+i)−240); 4% rule = future value × 4% a year

That box is the whole formula.

Worked example: $500 a month at 7% for 30 years

Step 1: monthly rate 0.5833%, 360 months.
Step 2: growth factor 1,219.97.
Step 3: $500 × 1,219.97 = $609,985.50.
Step 4: $180,000.00 paid in, $429,985.50 of interest.
Step 5: $4,729.21 a month for 20 years, or $2,033.28 a month on the 4% rule.
Step 6: $409.85 a month reaches $500,000; 27.53 years at $500 a month reaches $500,000.

Answer: $609,985.50 in 30 years — $180,000.00 paid in, $429,985.50 of growth, $4,729.21 a month for 20 years.

Rounding convention: figures are computed from unrounded values, then rounded for display — so two rounded split parts can differ by a cent from their rounded sum.

Cite this guideBright Side Kit. “How to Calculate an Annuity Payment — Step by Step.” Method: monthly rate i = annual return ÷ 12; months n = years × 12; growth factor = ((1+i)n − 1) ÷ i; future value = monthly contribution × growth factor + lump sum × (1+i)n; total paid in = contribution × n + lump sum; interest = future value − paid in; monthly payout over 20 years = future value × i ÷ (1 − (1+i)−240); 4% rule = future value × 4% a year. Example: $500/mo / 7% / 30 yr → $609,985.50; $180,000.00 paid in; $429,985.50 interest; $4,729.21/mo for 20 years; $2,033.28/mo on the 4% rule. Reverses: $409.85/mo for $500,000; 27.53 yr to $500,000; 5.9764% for $500,000. Bright Side Kit. "How to Calculate an Annuity Payment — Step by Step" https://brightoolbox.com/guides/how-to-calculate-annuity/ Accessed October 25, 2026.
Planning estimate, not professional advice: check the return, the fees, and the tax treatment with your provider before you rely on them.

Frequently asked questions

How do I calculate an annuity?

Six steps: monthly rate, growth factor, multiply, paid in, payout, check backwards. On $500 a month at 7% for 30 years the monthly rate is 0.5833%, the growth factor is 1,219.97, and the future value is $609,985.50 — $180,000.00 paid in and $429,985.50 of interest.

What is the annuity formula?

Future value = monthly contribution times the growth factor, plus the lump sum grown at the same rate. The monthly rate is the annual return divided by 12, the months are the years times 12, and the growth factor is one plus the monthly rate to the power of the months, minus one, divided by the monthly rate.

How much will $500 a month be worth in 30 years?

$609,985.50 at 7%. You pay in $180,000.00 and growth supplies $429,985.50. That pot pays $4,729.21 a month for 20 years, or $2,033.28 a month on the 4% rule.

How much do I need to save to reach $500,000?

$409.85 a month at 7% over 30 years. Forward-checked: $409.85 a month grows to exactly $500,000.00. At $500 a month it takes 27.53 years to reach $500,000, and the return needed is 5.9764% a year.

Is it better to save more or start earlier?

Starting earlier usually wins. $500 a month for 40 years grows to $1,312,406.70, while $1,000 a month for 30 years grows to $1,219,971.00. The last ten years alone add $702,421.20 to the pot.

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