Per Annum Interest Calculator
Estimate simple or compound annual interest and see how your money can grow over time.
What does “per annum” mean?
Per annum means per year. If a bank says your savings account earns 5% per annum (5% p.a.), that rate is annual. Interest may still be calculated or compounded monthly or daily, but the quoted rate is yearly.
Simple vs compound per annum interest
1) Simple interest
Simple interest is calculated only on the original principal. It does not earn “interest on interest.”
Formula: Interest = Principal × Rate × Time
2) Compound interest
Compound interest grows faster because interest is added back to the balance, then future interest is calculated on this larger amount.
Formula: A = P(1 + r/n)nt
- P = principal (starting amount)
- r = annual rate as decimal (e.g., 6% = 0.06)
- n = number of compounding periods per year
- t = number of years
- A = ending balance
How to use this calculator
- Enter your principal amount.
- Add the annual interest rate.
- Set the number of years.
- Choose simple or compound interest.
- If compound is selected, choose compounding frequency.
- Click Calculate to view total interest, final amount, and a year-by-year estimate.
Example comparison
Suppose you invest $10,000 at 6% p.a. for 10 years:
| Method | Compounding | Approx. Final Amount |
|---|---|---|
| Simple interest | N/A | $16,000 |
| Compound interest | Annually | $17,908 |
| Compound interest | Monthly | $18,194 |
Common mistakes to avoid
- Confusing annual rate with monthly rate.
- Ignoring compounding frequency when comparing financial products.
- Forgetting fees and taxes, which can reduce actual returns.
- Using nominal rate instead of effective annual rate for comparisons.
Quick tips for better long-term growth
Start early
Time is one of the strongest multipliers in compound growth.
Be consistent
Regular saving and investing habits often matter more than trying to time markets perfectly.
Compare effective annual rates
When rates look similar, compounding rules can produce different real outcomes.
Final thought
A per annum interest calculator helps you turn abstract percentages into clear dollar outcomes. Use it before choosing savings accounts, certificates, or investment plans so you can make data-driven decisions with confidence.