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MONEY GUIDE

How to Calculate Money Market Interest — Step by Step

The example is a $10,000 deposit at 4% APY compounded monthly, with $200 a month for 5 years — future value $25,469.76, interest $3,469.76, effective yield 4.07%. Calculating money market interest is two multiplications and an addition, and this guide walks through it step by step.

The formula, in one paragraph

A money market account pays interest on the balance, and the interest is added to the balance each month, so the next month earns interest on the interest. The formula has two parts: the deposit grown by monthly compounding, and the monthly contributions grown the same way. Add the two parts for the future value, and subtract what you put in for the interest.

Step 1: Start with the deposit

Start with the money you put in on day one. The example in this guide uses a $10,000 deposit, $200 added each month, a 4 percent APY, and 5 years. Keep the deposit and the contributions apart until the last step.

Step 2: Find the monthly rate

The APY is a nominal rate compounded monthly, so the monthly rate is the APY divided by 12. At 4 percent the monthly rate is 4 divided by 12, which is 0.3333 percent a month, or 0.003333 as a decimal. Five years is 60 months, so the compounding runs 60 times.

Step 3: Grow the deposit

Grow the deposit by the monthly rate for the number of months. Multiply the deposit by 1 plus the monthly rate, raised to the power of the months. On a $10,000 deposit at 4 percent for 5 years, that is $10,000 times 1.003333 to the power of 60, which is $12,209.97.

Step 4: Grow the monthly contributions

Grow the monthly contributions the same way: the monthly amount times the growth factor minus 1, divided by the monthly rate. On $200 a month at 4 percent for 5 years, that is $200 times 1.003333 to the power of 60 minus 1, divided by 0.003333, which is $13,259.80.

Step 5: Add them up and split out the interest

Add the two parts to get the future value: $12,209.97 plus $13,259.80 is $25,469.76. The contributions are $10,000 plus 60 times $200, which is $22,000.00. The interest is $25,469.76 minus $22,000.00, which is $3,469.76.

APY versus effective yield: the difference that matters

The APY and the effective yield are not the same number. The APY is the nominal rate, compounded monthly. The effective yield is what you actually earn in a year: 1 plus the monthly rate, raised to the power of 12, minus 1. At 4 percent APY the effective yield is 4.07 percent, exact 4.0742 percent. At 2 percent APY it is 2.02 percent, and at 5.5 percent APY it is 5.64 percent. When you compare accounts, compare the effective yield, not the APY.

Working backwards from $50,000

The formula also runs backwards. To reach $50,000 in 5 years with the same $200 a month, the APY would need to be 21.37 percent: far above any money market rate. At 4 percent APY it would take 11.32 years. The deposit needed is $30,090.34. And the monthly contribution needed is $569.99. If $50,000 in 5 years is the goal, the deposit or the monthly contribution has to do the work, not the rate.

When the numbers are small

The formula still works at the edges. With a $5,000 deposit the future value is $19,364.78, and with a $25,000 deposit it is $43,784.71. At 2 percent APY it is $23,660.26, at 5.5 percent it is $26,933.20, and at $500 a month it is $45,359.46. After 1 year it is $12,851.91, and after 10 years it is $44,358.29. With a $0 deposit the $200 a month alone grows to $13,259.80. With no monthly contribution the $10,000 alone grows to $12,209.97. With 0 years it is the deposit itself, $10,000.00, and at 0 percent APY it is exactly what you put in, $22,000.00, with $0.00 of interest.

What NOT to do

Do not confuse the APY with the effective yield: the APY is the nominal rate, and the effective yield is a little higher. Do not count the contributions as interest: $22,000.00 of the $25,469.76 is money you put in, and only $3,469.76 is interest. Do not assume the rate stays the same: money market rates move.

Monthly rate = APY ÷ 12; future value = deposit × (1 + monthly rate)months + monthly contribution × ((1 + monthly rate)months − 1) ÷ monthly rate; interest = future value − contributions; effective yield = (1 + monthly rate)12 − 1

That box is the whole formula.

Worked example: the $10,000 deposit

Step 1: Monthly rate = 4% ÷ 12 = 0.3333%.
Step 2: $10,000 grown 60 months = $12,209.97.
Step 3: $200 a month grown 60 months = $13,259.80.
Step 4: $12,209.97 + $13,259.80 = $25,469.76.
Step 5: $25,469.76 − $22,000.00 = $3,469.76.

Answer: future value $25,469.76, contributions $22,000.00, interest $3,469.76, effective yield 4.07%, final-month interest $83.95.

Cite this guideBright Side Kit. “How to Calculate Money Market Interest — Step by Step.” Bright Side Kit. "How to Calculate Money Market Interest — Step by Step." https://brightoolbox.com/guides/how-to-calculate-money-market-interest/ Accessed October 25, 2026.
Planning estimate, not professional advice: this is a savings estimate, not financial advice. Check the APY, fees, and terms with your bank before you rely on them.

Frequently asked questions

How do I calculate money market interest?

Grow the deposit and the monthly contributions separately, then add them up. On a $10,000 deposit at 4% APY compounded monthly, with $200 added each month for 5 years, the future value is $25,469.76. The contributions are $22,000.00, so the interest is $25,469.76 minus $22,000.00, which is $3,469.76. The effective annual yield is 4.07%, and the final month earns $83.95 of interest.

What is the difference between APY and effective yield?

The APY is a nominal rate compounded monthly, so the effective yield is a little higher. A 4% APY pays a monthly rate of 4% divided by 12, which is 0.3333% a month, and over a year that compounds to an effective yield of 4.07% (exact 4.0742%). At 2% APY the effective yield is 2.02%, and at 5.5% APY it is 5.64%. The difference is small at these rates, but it grows with the rate.

How long does it take to reach $50,000?

11.32 years at 4% APY with $200 a month. The exact answer is 11.3185 years. With a bigger deposit the wait shortens: a $30,090.34 deposit reaches $50,000.00 in 5 years at the same 4% APY and $200 a month. Or raise the monthly contribution to $569.99 a month, which also reaches $50,000.00 in 5 years.

What if I add nothing each month?

The $10,000 alone grows to $12,209.97 in 5 years. With no monthly contribution the interest is $2,209.97, against $3,469.76 with $200 a month. The contributions are the bigger lever here: $200 a month for 5 years is $12,000.00 of the $22,000.00 put in. At $500 a month the future value is $45,359.46, with $5,359.46 of interest on $40,000.00 put in.

What APY would I need to reach $50,000 in 5 years?

21.37% — far above any money market rate. The exact APY is 21.3731%. That is the point: a money market account is for safe, liquid savings, not for doubling money in 5 years. At a realistic 5.5% APY the same $10,000 and $200 a month reaches only $26,933.20 in 5 years.

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