Compound vs Simple Interest: Which Grows Faster?
Last updated: June 2025 · 7 min read
If you've ever compared savings accounts or loan terms, you've encountered two types of interest: simple and compound. The difference between them might seem academic at first, but over time it determines whether your savings grow steadily or exponentially. Understanding this distinction is fundamental to making informed financial decisions.
Simple Interest Explained
Simple interest is calculated only on the original principal — the amount you initially deposit or borrow. The formula is straightforward:
Interest = P × r × t
P = principal, r = annual rate, t = years
With £10,000 at 5% simple interest for 10 years, you earn exactly £500 per year, every year. After 10 years, your total interest is £5,000 and your balance is £15,000. The interest amount never changes because it's always calculated on the original £10,000.
Simple interest is rare in modern banking but still appears in some personal loans, car finance agreements, and certain government bonds. It's also used as a baseline for understanding how compound interest differs.
Compound Interest Explained
Compound interest calculates interest on the principal plus all accumulated interest. Each period, the base amount grows, so the interest earned also grows. As explained in our complete guide to compound interest, this creates an exponential growth curve rather than a straight line.
Using the same £10,000 at 5% compound interest for 10 years (compounded annually): your first year earns £500, but by year ten you're earning £776. The total interest is £6,288.95 — more than £1,288 extra compared to simple interest. That gap only widens with time.
Side-by-Side Comparison
| Year | Simple Interest | Compound Interest |
|---|---|---|
| 1 | £10,500 | £10,500 |
| 5 | £12,500 | £12,763 |
| 10 | £15,000 | £16,289 |
| 20 | £20,000 | £26,533 |
| 30 | £25,000 | £43,219 |
After 30 years, compound interest produces £43,219 versus £25,000 with simple interest — a difference of over £18,000 from the same initial deposit and rate. The compound balance is 73% higher. This demonstrates why Einstein's famous (if possibly apocryphal) quote about compound interest being the most powerful force in the universe resonates so deeply with investors.
When Does It Matter Most?
Long time horizons: The gap between simple and compound interest widens dramatically over time. For short periods (under 3 years), the difference is modest. Over decades, it's transformative. This is why compound interest is particularly relevant for retirement planning.
Higher interest rates: At 2%, the difference between simple and compound interest after 20 years is about £2,400 on £10,000. At 7%, it's over £18,700. Higher rates amplify the compounding effect significantly.
More frequent compounding: Daily compounding earns slightly more than annual compounding, though the difference is smaller than many expect. Our compounding frequency guide breaks this down with exact figures.
The Borrower's Perspective
For borrowers, the picture flips. Compound interest on debt means you're paying interest on interest — the balance grows faster, and total repayment costs increase. Credit cards, mortgages, and student loans all typically charge compound interest. Understanding this helps explain why minimum payments on credit cards barely reduce the balance and why overpaying your mortgage can save thousands.
Practical Takeaways
- Always check whether quoted rates are simple (flat) or compound (AER). In the UK, savings rates are typically quoted as AER, which includes compounding.
- For savings and investments, compound interest is your ally. Start early and let time do the heavy lifting.
- For debt, compound interest works against you. Prioritise paying off high-interest compound debt quickly.
- Use the Rule of 72 for quick mental estimates of compound doubling time.
Frequently Asked Questions
Do any savings accounts use simple interest?
Very few modern savings accounts use pure simple interest. Most UK banks compound interest daily or monthly. If you see an AER (Annual Equivalent Rate), that already accounts for compounding.
Is simple interest ever better for savers?
No. For the same rate and period, compound interest always produces a higher return than simple interest. The only exception is if you withdraw interest as it's earned rather than letting it accumulate, which effectively turns compound interest into simple interest.
How do I convert between simple and compound rates?
To find the equivalent compound rate for a simple rate, use: AER = (1 + r/n)^n - 1, where r is the simple rate and n is compounding periods per year. Our calculator handles this conversion automatically.
Compare compound vs simple growth with real numbers
Try the Compound Interest Calculator →This guide is for informational purposes only and does not constitute professional financial advice.