Compare compound vs simple interest on the same principal.
Compound
—
Simple
—
Compound advantage
—
Your breakdown
Updates live as you type| After | Simple balance | Compound balance | Gap |
|---|
Two formulas that start identical and end miles apart
Simple interest pays you only on your original principal. Compound interest pays you interest on your interest, so the balance feeds itself. For the first year or two the gap is almost invisible, which is exactly why people underestimate it. The divergence is not linear. It widens faster every year because each period compounds on a bigger base. This calculator runs both formulas on the same principal and rate so you can see the dollar value of that feedback loop instead of arguing about it in the abstract.
The math behind the two boxes is short. Simple interest is principal times one plus rate times years, written A = P(1 + rt). Compound interest is A = P(1 + r/n)nt, where n is how many times a year the interest is added back. The compounding frequency dropdown changes n directly. Daily compounding uses 365, monthly uses 12, quarterly uses 4, and annual uses 1. Notice that frequency matters far less than time. Going from annual to daily on a 6% rate adds only a fraction of a percent to your effective yield, while adding ten years can double the balance.
$10,000 at 6% for 20 years, compounded monthly
Take the default inputs. You deposit $10,000 once, earn a 6% annual rate, leave it for 20 years, and let it compound monthly. Simple interest gives you 6% of $10,000, or $600, every year for 20 years. That is $12,000 of interest on top of your principal, for a final balance of $22,000. Compound interest, adding 0.5% each month and rolling it forward, grows the same deposit to $33,102. The compound advantage is $11,102, which is more than the original principal. The table walks the compound side at five-year checkpoints so you can see the curve bend upward.
A practical read on rate type
The everyday lesson is to favor compounding when you are saving and dread it when you are borrowing. A high-yield savings account or a brokerage that reinvests dividends works in your favor. A credit card that compounds daily works against you with the same ruthless geometry. One tip I give clients: when a bank quotes you a rate, ask whether it is the nominal rate or the annual percentage yield. The annual percentage yield already bakes in the compounding frequency, so it is the honest number for comparing accounts. Truth in Savings rules, Regulation DD, require depository institutions to disclose the yield precisely for this reason.
Who this tool is for
Use it when you want to settle a side-by-side question fast: how much is compounding actually worth on this specific deposit over this specific horizon. It assumes a single lump sum with no further contributions, so it is cleaner for a one-time deposit, a bond, or a certificate of deposit than for a monthly savings habit. If you are adding money every month, reach for a contributions-based growth tool instead. A common mistake is comparing two products at different compounding frequencies and blaming the gap on the frequency. Run both here and you will usually find the rate and the time horizon, not the frequency, drove the result.
Does compounding more often always beat a higher rate?
No. A flat 6.1% compounded annually beats 6.0% compounded daily. Frequency adds only a sliver to the effective yield, so a meaningfully higher rate wins even at lower frequency. Convert both to annual percentage yield and compare those, which is the apples-to-apples figure.
How long until compounding overtakes simple interest by a meaningful margin?
On the default 6% example the gap is under $500 at five years but balloons past $11,000 by year twenty. As a rough rule, the divergence becomes hard to ignore once the time horizon is long enough that the money roughly doubles, which at 6% is about twelve years by the rule of 72. Short horizons barely separate the two; long horizons are where compounding earns its reputation.