Finance
· Reviewed by Ali Abbas

Compound Interest Calculator

The Power of Compound Interest — Explained

Compound interest is interest calculated on both the original principal and the interest already accumulated. Unlike simple interest, which calculates only on the original amount, compound interest grows exponentially — which is why Albert Einstein reportedly called it "the eighth wonder of the world." The difference between simple and compound interest becomes dramatic over long time horizons.

Simple vs Compound Interest — Example

Principal: $10,000. Annual rate: 7%. Time: 20 years.

  • Simple interest: $10,000 × 7% × 20 = $14,000 interest earned. Total = $24,000.
  • Compound interest (annual): $10,000 × (1.07)²⁰ = $38,697. Interest earned = $28,697.
  • Compound interest (monthly): $10,000 × (1 + 0.07/12)²⁴⁰ = $40,064. Interest earned = $30,064.

Monthly compounding adds over $1,300 vs annual compounding over 20 years — purely from the frequency of reinvestment.

Compounding Frequency Comparison Table

FrequencyTimes per Year$10,000 @ 6% after 10 years
Annual1$17,908
Quarterly4$18,061
Monthly12$18,194
Daily365$18,220

The Rule of 72

A quick mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6% per year: 72 ÷ 6 = 12 years to double. At 9%: 72 ÷ 9 = 8 years. At 3%: 72 ÷ 3 = 24 years. This rule is accurate within a few percent for rates between 2% and 20% and is widely used in financial planning as a first-pass estimate.

Compound Interest in Debt

Compound interest works against you in debt. Credit card balances at 20–29% APR compound monthly. A $5,000 credit card balance at 24% APR, with only minimum payments made, can take over 15 years to pay off and cost more than $7,000 in interest — more than the original balance. The same mathematics that builds wealth in savings destroys it in high-interest debt. Paying off high-interest debt is mathematically equivalent to earning that interest rate risk-free.

What Is Compound Interest?

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, compound interest grows exponentially — often called "interest on interest." It is the engine behind long-term investment growth and the reason debt can spiral rapidly.

Compound Interest Formula

A = P × (1 + r/n)^(n×t)

  • A = Final amount
  • P = Principal (initial investment)
  • r = Annual interest rate (decimal)
  • n = Compounding periods per year
  • t = Time in years

Example: £10,000 at 7% compounded monthly over 20 years: A = 10,000 × (1 + 0.07/12)^(240) = £40,388. Total interest earned: £30,388 — three times the original principal.

Simple vs Compound Interest

FeatureSimpleCompound
Calculated onPrincipal onlyPrincipal + interest
GrowthLinearExponential
Long-term resultLowerSignificantly higher

The Rule of 72 — Double Your Money

Years to double ≈ 72 ÷ Annual rate (%)

  • At 6%: 12 years to double
  • At 9%: 8 years to double
  • At 12%: 6 years to double

The Power of Starting Early

£5,000 invested at age 25 at 8%/year grows to £108,623 by age 65. Starting at 35 grows to only £50,313. Ten more years more than doubles the outcome.

Related Calculators

Loan Calculator — see compound interest working against you in debt. Mortgage Calculator — total interest cost over a full mortgage term.

Worked example — savings goal: You invest £5,000 at 6% annual interest compounded monthly. After 10 years: £5,000 × (1 + 0.06/12)^120 = £9,096. After 20 years: £5,000 × (1 + 0.06/12)^240 = £16,551. The power of monthly compounding adds over £500 compared to annual compounding over 20 years — because interest is earning interest on a shorter cycle. Use the calculator to compare different compounding frequencies side by side.

Warren Buffett has described compound interest as the most powerful force in finance — and understanding it is the first step toward building long-term wealth. Whether you are comparing savings accounts, estimating investment returns, or calculating the real cost of credit card debt, the compound interest calculator gives you the numbers you need to decide.

Understanding compound interest is the difference between saving and truly growing your wealth — this calculator visualises the exponential effect of compounding frequency so you can make informed financial decisions with confidence.

Financial Disclaimer: Results are estimates only and should not be relied upon as financial or investment advice. Consult a licensed financial advisor for guidance specific to your situation.

How to Use

  1. 1
    Enter your initial investmentType the amount you are starting with (principal). Leave at 0 if starting from scratch.
  2. 2
    Set interest rate and compoundingEnter the annual interest rate (e.g., 7 for 7%) and choose how often interest compounds: monthly, quarterly, or annually.
  3. 3
    Add monthly contributions and timeEnter any regular monthly deposit amount and the investment period in years.
  4. 4
    View your resultsSee your final balance, total contributions, and total interest earned, plus a year-by-year breakdown table.

Frequently Asked Questions

What is the difference between compound and simple interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on both the principal and the accumulated interest. Over time, compound interest produces dramatically higher returns due to the "interest on interest" effect.
How often does compound interest compound?
Compounding frequency varies by product. Savings accounts typically compound daily or monthly. Most investment accounts compound monthly or quarterly. The more frequently interest compounds, the slightly higher the effective annual return.
What is the Rule of 72?
The Rule of 72 is a quick mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6% interest, your money doubles in roughly 12 years (72 ÷ 6 = 12).
What interest rate should I use?
For long-term stock market investments, the historical average return of the S&P 500 is approximately 7-10% per year after inflation. For savings accounts, check your bank's current APY. For loan calculations, use the stated APR.
How does monthly contribution affect compound growth?
Regular monthly contributions dramatically accelerate compound growth. Each contribution immediately starts earning interest and compounds for the remaining investment period. Even small monthly amounts ($50-$200) can add tens of thousands to your final balance over 20-30 years.
Share