How Does Compound Interest Work? Formula, Examples + Calculator

The most powerful force in personal finance has nothing to do with how much you earn. It has everything to do with time, and compound interest is how time turns small amounts of money into large ones.

Here is what compound interest actually is: it is interest earned on interest. Your money earns a return, that return gets added to your balance, and then your larger balance earns a return. Each cycle, the number gets bigger. Over years and decades, that process becomes extraordinary.

The best time to understand compound interest is before you have much money, because the earlier you put it to work, the more compounding does for you. In this guide, you will learn what compound interest is, how the formula works, how it compares to simple interest, what the Rule of 72 is, and why starting in your teens puts you years ahead of most people. You will also see how compound interest works against you in debt and what to do about it.

The key concepts covered here are: the definition of compound interest, the compound interest formula (A = P(1 + r/n)^nt), simple vs. compound interest comparison, the Rule of 72, real examples with current 2026 rates, and where Canadian students can put compound interest to work today.

What Is Compound Interest?

Compound interest is interest calculated on both your original deposit and all the interest you have already earned. Each time interest is added to your balance, it becomes part of the new principal, and then earns interest itself. This is what separates it from simple interest, which is calculated only on your original deposit, every single time.

Here is the same $1,000 at 5% annual interest shown both ways over 10 years:

Simple interest: You earn $50 per year every year, calculated on the original $1,000. After 10 years: $1,000 + $500 = $1,500

Compound interest (annually): Year 1 earns $50 on $1,000. Year 2 earns $52.50 on $1,050. Year 3 earns $55.13 on $1,102.50. Each year’s interest is slightly bigger because you are earning on a growing balance. After 10 years: $1,628.89

That extra $128.89 looks modest at 10 years. At 30 years, the same calculation gives you $4,321.94 with compound interest versus $2,500 with simple interest, a difference of $1,822 from the exact same starting point.

Simple vs. Compound Interest: The Numbers Side by Side

Compound interest consistently outperforms simple interest, and the gap grows dramatically the longer the time period. Starting with $1,000 at 5% annual interest:

Nothing changed except the type of interest. Same principal, same rate, same time. The difference is whether your earned interest sits idle or gets put back to work.

The Compound Interest Formula

The standard formula for compound interest is: A = P(1 + r/n)^(nt)

Breaking it down:

• A = the final amount you end up with
• P = your principal (the starting amount)
• r = the annual interest rate expressed as a decimal (5% = 0.05)
• n = how many times interest compounds per year (1 = annually, 12 = monthly, 365 = daily)
• t = time in years

Worked example: You deposit $1,000 into a savings account paying 3% annual interest, compounded monthly, for 5 years.

A = 1,000 × (1 + 0.03/12)^(12×5)
A = 1,000 × (1.0025)^60
A = 1,000 × 1.1616
A = $1,161.62

You started with $1,000, did nothing, and ended up with $1,161.62, which is $161.62 in interest with no effort on your part.

One important detail: the more frequently interest compounds, the faster your money grows. Monthly compounding produces slightly more than annual compounding at the same rate, because interest is being added and earning on itself 12 times a year instead of once. Daily compounding produces a little more still.

If you want to run your own scenarios, the NerdWallet compound interest calculator lets you input any principal, rate, term, and compounding frequency.

The Rule of 72

The Rule of 72 is a quick mental shortcut for estimating how long it takes money to double at a given interest rate: divide 72 by the interest rate to get the approximate number of years.

72 ÷ interest rate = approximate years to double

With current 2026 rates in Canada:

• EQ Bank HISA (2.75%): 72 ÷ 2.75 = about 26 years to double
• Best 1-year GIC (~3.6%): 72 ÷ 3.6 = about 20 years to double
• Balanced investment fund (~6%): 72 ÷ 6 = 12 years to double

The Rule of 72 also works in reverse, and this is where it gets sobering. At a typical Canadian credit card rate of 20%, your debt doubles in 72 ÷ 20 = 3.6 years if you are not paying it down. That is the same math, pointed directly at you.

Why Starting Early Makes Such a Difference

Starting to invest at 18 rather than 38, with the same $100 per month, results in approximately four times more money by age 65. Here is what that actually looks like.

Two people investing the same $100 per month at a 6% average annual return, according to the Bank of Canada’s interest rate context:

Alex starts at 18. They invest $100/month until age 65, 47 years of contributions. Total invested: $56,400. Final balance: approximately $313,000.

Jordan starts at 38. They invest $100/month until age 65, 27 years of contributions. Total invested: $32,400. Final balance: approximately $81,000.

Alex invested $24,000 more than Jordan. But ended up with $232,000 more. The difference is not discipline or sacrifice. It is the 20 extra years compound interest had to work.

This is why the phrase “start early” exists in every personal finance conversation. It is not motivation. It is math. A part-time job at 16 or 17, even a modest one, creates the opportunity to start building a balance that time will multiply. Our guide on the 15 best part-time jobs for teens covers which ones pay well enough to actually put something aside each month.

How Compound Interest Works Against You in Debt

Everything that makes compound interest valuable in savings works just as powerfully against you when you are carrying debt.

Canadian credit cards typically charge 19% to 22% interest on unpaid balances, compounded daily. If you carry a $1,000 balance at 20% and only make minimum payments, you will pay back well over $1,300 total, which is more than $300 in interest on a $1,000 purchase. And if you stop making payments entirely, the Rule of 72 tells you that $1,000 becomes $2,000 in about 3.6 years.

Student loans also compound. Under Canada’s federal student loan system, interest on Canada Student Loans was eliminated in 2023, but provincial loans and private lines of credit still carry interest. Understanding how compounding affects a loan balance is important before signing for anything. Our article on how student loans work in Canada covers this in detail.

The practical rule: pay your credit card balance in full every month. You benefit from compound interest in your savings account. You do not want it working against you in your wallet.

Where to Put Compound Interest to Work as a Canadian Student

The best accounts for Canadian students to use compound interest in 2026 are a TFSA, a high-interest savings account (HISA), a GIC, and an FHSA. Here is how each one works.

TFSA (Tax-Free Savings Account): Available to Canadians aged 18 and up (19 in BC, New Brunswick, Newfoundland, Nova Scotia, and the territories). The 2026 annual contribution limit is $7,000, and all growth inside a TFSA is completely tax-free. That means your compound interest grows without the Canada Revenue Agency taking a portion of the gains each year. Our guide on how to use a TFSA explains everything you need to get started.

HISA (High-Interest Savings Account): Major banks like TD and RBC offer savings account rates as low as 0.01% to 0.55% in 2026. Online banks and credit unions do significantly better. EQ Bank offers 2.75% with direct deposit, and some credit unions reach 3.6% on promotional savings rates. Switching where your money sits can meaningfully affect how fast it grows.

GIC (Guaranteed Investment Certificate): 1-year GIC rates from institutions like Achieva Financial are reaching 3.60% in 2026. You lock your money in for a set term in exchange for a guaranteed return. If you have money you will not need for 12 months or more, a GIC often outperforms a standard savings account.

FHSA (First Home Savings Account): If you are planning to buy a home eventually, the FHSA offers a rare double benefit. The 2026 annual limit is $8,000, with a $40,000 lifetime cap. Contributions are tax-deductible (like an RRSP), and withdrawals for a first home are tax-free (like a TFSA). Compound interest inside an FHSA grows with both advantages working together. Details are on the Canada.ca FHSA page.

Once you understand where to put money, building the habit of actually saving it is the other half. Our guide on how to save money in high school covers practical strategies that work on a student income. And if you are interested in going further with investing, investing tips for teens is a good next step after you understand the compounding basics.

Updated: May 2026 | This article is for educational purposes and does not constitute financial or investment advice.

Frequently Asked Questions (FAQ)

What is compound interest in simple terms?

Compound interest is interest that earns interest. When your savings account earns interest, that interest gets added to your balance. Your larger balance then earns more interest, and the cycle repeats. Over time, this snowball effect grows your money significantly faster than simple interest, which only ever calculates on your original deposit.

What is the compound interest formula?

The formula is A = P(1 + r/n)^(nt). A is your final amount, P is your starting principal, r is the annual interest rate as a decimal, n is how many times per year interest compounds, and t is the number of years. Example: $1,000 at 3% compounded monthly for 5 years gives A = 1,000 × (1 + 0.03/12)^60 = $1,161.62.

How is compound interest different from simple interest?

Simple interest is calculated only on your original deposit, every year. Compound interest is calculated on your deposit plus all interest you have already earned. On $1,000 at 5% annual interest for 30 years: simple interest produces $2,500 total. Compound interest produces $4,322, a difference of $1,822 from the same starting point and rate.

When should a teenager start using compound interest?

As soon as you have any income to save. In Canada, you can open a TFSA at 18 (19 in some provinces) and start growing money tax-free. Even $25 or $50 a month invested in your late teens compounds significantly over decades. Waiting until 38 to start investing the same amount as someone who started at 18 results in roughly four times less money by age 65. Time is the ingredient that makes compounding work.

Last updated: May 2026

Robert Puharich is the founder of TeenLearner, where he helps teens build real-world skills in money, AI, and life. With over 20 years in education and a Master of Education (M.Ed.) from UBC, he created TeenLearner to teach practical skills such as budgeting, career readiness, decision-making, and the wise use of technology. Robert is also a published author and business founder.