The compound interest formula

The amount of interest you earn (rather than the final balance) is simply A − P. Everything in compound interest comes back to these five variables, so once you can identify each one, you can work out any scenario.

Compound vs simple interest

The difference is what the interest is calculated on. Simple interest is always calculated on the original principal only: $10,000 at 6% earns $600 every year, forever. Compound interest is calculated on the principal plus all the interest already earned, so each year you earn interest on a slightly bigger balance.

Over a year or two the gap is small. Over decades it is enormous. Take $10,000 at 6%: after 5 years, simple interest gives $13,000 while annual compounding gives about $13,382 — a modest $382 difference. After 30 years, simple interest still gives just $28,000, but compounding gives about $57,435 — more than double. That widening gap is exactly why compounding is the engine behind long-term investing and early retirement.

Where do you actually earn compound interest?

Compound interest is not just a textbook formula; it is how most savings and investment products grow:

• High-interest savings accounts (HISA): interest is usually calculated daily and paid monthly, so it compounds while your money stays fully accessible.
• GICs and term deposits: a fixed rate compounds over the term, though some shorter GICs pay simple interest, so check the terms before you assume.
• Investments: in a diversified portfolio, compounding comes from reinvested dividends and growth rather than a fixed rate, so the return varies from year to year.

If you are deciding where to keep your cash, our GIC vs HISA guide and current savings rates compare the options.

How to calculate compound interest step by step

Take $10,000 invested at 6% per year, compounded monthly, for 10 years.

1. Write down the variables: P = 10,000, r = 0.06, n = 12 (monthly), t = 10.
2. Divide the rate by n: 0.06 / 12 = 0.005 per month.
3. Add 1: 1 + 0.005 = 1.005.
4. Find the exponent (n × t): 12 × 10 = 120 periods.
5. Raise and multiply: 1.005¹²⁰ = 1.8194, then 10,000 × 1.8194 = $18,194.

So your $10,000 becomes about $18,194, meaning you earned roughly $8,194 in interest. With simple interest you would have earned only $6,000 ($600 a year × 10), so compounding added about $2,194 with no extra effort.

Why compounding frequency matters

The more often interest compounds, the more you earn, because interest starts earning its own interest sooner. Here is the same $10,000 at 6% for 10 years at different frequencies:

Notice the jump from annual to monthly is bigger than from monthly to daily — there are diminishing returns to compounding more often. What matters far more than frequency is the interest rate and, above all, time.

Adding regular contributions

Most people do not invest once and stop; they add money every month. When you make regular contributions, each one starts compounding from the day you add it. The future value of those contributions uses a related formula, but the idea is the same: every dollar you add buys more time in the market.

For example, adding $200 a month for 10 years at 6% compounded monthly grows to about $32,776. You contributed $24,000 of that, and compounding added roughly $8,776. Combine a starting lump sum with monthly contributions and the totals grow quickly — which is the math our compound interest calculator runs for you automatically.

The Rule of 72: a shortcut

When you want a quick estimate without the full formula, use the Rule of 72: divide 72 by your annual interest rate to estimate how many years it takes your money to double. At 6%, that is 72 / 6 = 12 years to double; at 8%, about 9 years. You can also flip it: to double your money within a target number of years, divide 72 by that number to find the rate you need — to double in 8 years, you need roughly a 9% return. It is an approximation, but it is close enough for mental math and shows why a slightly higher rate shortens the doubling time so much.

Compounding and early retirement

Compound interest is the reason someone who starts investing in their twenties can end up with far more than someone who starts in their forties, even if the later starter contributes more in total. Time is the most powerful variable in the formula because it sits in the exponent.

Consider two savers. Ava invests $5,000 a year from age 25 to 35 — just ten years — then stops and never adds another dollar. Ben invests the same $5,000 a year from 35 to 65, a full thirty years. At a 7% return, Ava reaches 65 with about $526,000 from $50,000 of contributions, while Ben has about $472,000 from $150,000 of contributions. Ava contributed a third as much yet finishes ahead, purely because her money had an extra decade to compound. That is the single most important lesson in saving: starting early usually beats contributing more.

This is the core idea behind financial independence: invest enough early, let compounding do the heavy lifting, and your portfolio can eventually grow to fund retirement on its own. You can model exactly that with our FIRE calculator and Coast FIRE calculator, or browse all of our free financial calculators. For a deeper primer on how compounding builds wealth, the US Securities and Exchange Commission also offers a clear compound interest explainer.

One honest caveat: real-world returns

The fixed-rate examples above are clean illustrations, not predictions. Real returns are not smooth: investment returns vary year to year and can be negative, fees quietly reduce your effective rate, and inflation eats into the purchasing power of the final number. Taxes can apply too, unless the money grows inside a registered account such as a TFSA or RRSP (or a 401(k) or IRA in the United States). None of this changes the formula — it just means you should treat any single-rate projection as a guide, not a guarantee.

Frequently asked questions

Last updated: June 2026