Simple vs. Compound Interest Calculator

What separates simple interest from compound interest

Simple interest only ever looks at your original deposit. Whatever that starting number is, the interest check stays the same size every single period, no matter how long the money sits. Compound interest looks at a moving target instead: your original deposit plus every dollar of interest you have already collected. Because that base keeps growing, the interest itself keeps growing too.

The difference is small at first and large later. Put $50,000 at 5% for 20 years and simple interest hands you a flat $50,000 in interest, for a $100,000 total. Let that same $50,000 compound annually at 5% and it finishes near $132,665, about $82,665 in interest. Roughly $32,665 of that gap exists purely because interest was allowed to earn interest. That mechanic is behind most CDs, savings accounts and fixed annuities, which is exactly why the calculator on this page compares the two head to head.

How to use the simple vs. compound interest calculator

• Enter your starting amount. This is the lump sum you are putting in on day one, for example $50,000.
• Add a recurring contribution and how often you make it. If you plan to keep adding money monthly, quarterly or annually, enter that amount and frequency so both projections include it.
• Set the rate you expect to earn. Use a realistic annual percentage, or a round hypothetical number if you are just exploring the concept.
• Choose your time horizon in years. Longer horizons are where compounding does its most visible work.
• Pick a compounding frequency. Annual, semiannual, quarterly, monthly or daily options are available; more frequent compounding nudges the ending balance up a little at the same stated rate.
• Choose the timing of your contributions. Deposits made at the start of a period have one more period to earn interest than deposits made at the end, so the calculator lets you pick which one matches your plan.

The results section shows a simple-interest ending balance next to a compound-interest ending balance, the dollar size of the compounding advantage, and a year-by-year table so you can watch exactly when the two paths pull apart.

The timing choice is worth a second look because it is easy to skip past. A schedule where money goes in at the start of each period is sometimes called an annuity due, and a schedule where money goes in at the end of each period is an ordinary annuity. The names come from how retirement math has classified payment streams for a long time, and the same idea applies whether you are contributing to a savings plan or receiving income payments later.

Why compounding pulls ahead the longer you wait

Under simple interest, the math never changes. At 5% on $50,000, the interest check is $2,500 in year one and $2,500 in year twenty, because it is always calculated against that same original $50,000. Compound interest keeps recalculating against a bigger number every period, so the dollar amount of interest keeps climbing. By year twenty in the example above, a 5% compound balance near $132,000 is earning well over $6,000 that year alone, more than double what simple interest ever pays in a single year on the identical starting deposit.

That is also why time in the market, or time in a contract, tends to matter more than chasing an extra point of rate. In the early years, a simple-interest account and a compound-interest account paying the same rate look almost identical. Stretch the timeline out and the lines separate quickly, which is the entire argument for starting sooner rather than waiting to find a slightly higher number.

Take a 55-year-old named Carla with $60,000 she will not touch for 15 years. She plugs in $60,000, a 5% rate, 15 years, no added contributions, and annual compounding. The tool shows a simple-interest ending balance of $105,000 against a compound-interest ending balance near $124,736, a gap of roughly $19,736 for choosing a product that compounds instead of one that does not, without changing the rate or the deposit at all.

Curious what more frequent compounding would add, Carla switches the setting to monthly. Her compound total climbs to about $126,830, an extra $2,094 over annual compounding at the identical rate. Watching the year-by-year table, she notices the simple and compound lines track closely through her first several years, then widen sharply as she nears her 15-year mark, since each additional year of compounding is now working off a larger and larger balance. Her conclusion: for money parked this long, a simple-interest product would have to advertise a meaningfully higher rate just to catch up to a compounding one, and even then it would need to hold that higher rate for the entire term to actually close the gap.

Carla also tests what happens if she keeps adding money along the way. Setting a $200 monthly contribution on top of her $60,000 starting balance, with deposits landing at the start of each month, pushes her 15-year compound total meaningfully higher again, since every added dollar gets its own head start on earning interest. Switching that same contribution to land at the end of each month instead trims the total slightly, because each deposit then has one fewer period to grow. It is a small effect next to the simple-versus-compound gap, but it explains why the calculator asks about timing at all.

Compound growth inside a fixed annuity

Fixed annuities, including multi-year guaranteed annuities, are a common way retirement savers capture compound growth on a guaranteed basis. A MYGA locks a fixed rate for a set number of years, and the interest compounds inside the contract each year it is held. Because the growth is tax-deferred, there is no yearly tax bill chipping away at the balance the way there often is in a taxable brokerage or bank account, so the full amount keeps compounding rather than shrinking a little each April.

That tax-deferred compounding is one of the structural reasons a guaranteed annuity rate can out-earn a taxable account paying the same stated percentage. In a regular savings or brokerage account, interest is typically reportable and taxable the year it is earned, which quietly shrinks the balance that gets to keep compounding. Inside a fixed annuity, that yearly tax bite is deferred, so the entire pile, deposit plus every year's interest, keeps working until you actually take money out.

If you want to see how a specific rate would grow your own deposit, the MYGA calculator and fixed annuity calculator run that math for you, and the CD vs annuity calculator lines up the after-tax comparison directly against a bank CD. Rates on any of these products change often, so treat the numbers you enter here as planning inputs rather than a live quote, and confirm current numbers with a licensed strategist before you commit money to a term. For a plain-language overview of how annuities are structured and regulated, the SEC's investor education site is a good starting point.