Monthly Compound Interest Calculator

Divide the yearly rate by 12.

• 6% a year ÷ 12 = 0.5% a month
• 5% a year ÷ 12 = 0.4167% a month

To find one month's interest, multiply your balance by that monthly rate. On $10,000 at 0.4167%, you earn about $41.67 the first month. Next month you earn interest on $10,041.67, so the amount grows a little each time.

Use the Rule of 72. Divide 72 by your interest rate to get the rough number of years.

• 3% rate: 72 ÷ 3 = 24 years
• 5% rate: 72 ÷ 5 = about 14.4 years
• 8% rate: 72 ÷ 8 = 9 years

With monthly compounding it happens a bit faster. At 5% compounded monthly, money doubles in about 13.9 years.

About $511.62, giving you $10,511.62 at the end of the year.

Simple interest would pay only $500. The extra $11.62 comes from earning interest on your interest each month. Over 10 years, that same $10,000 grows to about $16,470.

APR is the plain yearly rate before compounding is counted. APY (also called effective annual yield) shows what you really earn after compounding.

A 5% APR compounded monthly equals an APY of about 5.116%. When you compare savings accounts or CDs, look at APY. Two banks can post the same rate but pay different amounts if they compound at different speeds.

Most U.S. banks and credit unions compound daily and pay the interest once a month. Some compound monthly or quarterly instead.

Check the account's disclosure. The difference is small: on $10,000 at 5%, daily compounding earns about $1 more per year than monthly. The rate itself matters far more than the compounding schedule.

Yes, in a regular savings account, CD, or money market account. Interest counts as ordinary income and is taxed the year you earn it, even if you leave the money in the account. Banks send a 1099-INT form if you earn $10 or more.

Interest inside a 401(k), traditional IRA, or Roth IRA is not taxed each year, so it compounds faster.

A lump sum invested today usually earns more, because every dollar starts compounding right away.

But most people do not have a big lump sum. Monthly deposits work well because they keep adding fuel to the balance. Doing both is strongest: put in what you have now, then add every month.

It depends on your rate and how many years you save. At a 7% yearly return:

• 40 years: about $380 a month
• 30 years: about $820 a month
• 20 years: about $1,920 a month
• 10 years: about $5,780 a month

Waiting 10 years more than doubles what you must save each month.

Yes. Most credit cards compound interest daily on any balance you carry past the due date. Unpaid interest gets added to what you owe, and then you pay interest on that too.

At 24% APR, a $5,000 balance grows by roughly $100 in one month if you pay nothing. Paying the full statement balance each month avoids interest entirely.

Because the last years of compounding add the most dollars. Saving $250 a month at 7%:

• Start at age 25, stop at 65: about $657,000
• Start at age 35, stop at 65: about $306,000

The early saver put in only $30,000 more but ended with more than double. Time, not the deposit size, does most of the work.

The balance keeps growing, just slower. Interest still gets added every month to whatever is in the account.

For example, $50,000 left alone at 5% compounded monthly becomes about $82,350 in 10 years with zero new deposits. Taking money out is what hurts, since every dollar you pull is a dollar that stops earning.

Yes, and the difference is large. Saving $300 a month for 30 years at 7% gives about $366,000. Raising that deposit by just 3% a year gives about $540,000.

A 3% yearly bump is often smaller than a normal raise, so your take-home pay barely changes while your final balance jumps.