Per-annum interest is the yearly interest amount, found by multiplying the annual rate by the principal and adjusting for the exact time and compounding.
“Per annum” means “per year.” You’ll see it on loan offers, credit cards, fixed deposits, and investment notes. The tricky part is that the same headline rate can land you in different places depending on compounding, fees, and how time is counted. This page walks you through the steps so you can check a quote or compare offers.
What Per Annum Interest Means In Real Money
A per-annum rate is an annual rate. If a bank says 12% per annum, it’s telling you the rate used for a one-year period. Your interest charge (or interest earned) depends on four inputs:
- Principal: the starting balance (loan amount or deposit).
- Annual rate: the stated percentage per year.
- Time: how long the balance is outstanding.
- Compounding: whether interest is added to the balance during the year.
Get those four right and the rest is plain arithmetic.
How To Calculate Per Annum Interest For Any Loan Or Deposit
Start by identifying which rate you’ve been given. Many offers show a stated annual rate, while the cost you feel can be closer to a disclosure rate that rolls in certain fees. If you want a clear definition of how those two ideas differ, the CFPB’s APR vs. interest rate explainer is a solid baseline.
Step 1: Convert The Rate To A Decimal
Turn the per-annum percentage into a decimal by dividing by 100.
- 8% per annum → 0.08
- 12.5% per annum → 0.125
Step 2: Decide Whether It’s Simple Or Compound
Simple interest calculates interest only on the principal. Compound interest adds interest to the balance on a schedule (monthly, quarterly, daily), so later interest is calculated on a larger balance.
If you’re unsure, check the terms for words like “compounded monthly” or “interest credited quarterly.” When a product uses compounding, you need one extra input: the number of compounding periods per year.
Step 3: Use The Right Formula
Simple Interest (Straight-Line)
Use this when the balance does not get interest added to it during the period:
Interest = Principal × Annual Rate × Time (in years)
Example: A ৳50,000 deposit at 10% per annum for 9 months (0.75 years):
Interest = 50,000 × 0.10 × 0.75 = ৳3,750
Compound Interest (Interest On Interest)
Use this when interest is added on a schedule. The common formula is:
Ending Balance = Principal × (1 + r/n)^(n×t)
Where r is the annual rate as a decimal, n is compounding periods per year, and t is time in years. Interest earned (or charged) is ending balance minus principal.
Example: ৳50,000 at 10% per annum, compounded monthly, for 9 months:
- r = 0.10
- n = 12
- t = 9/12 = 0.75
Ending balance = 50,000 × (1 + 0.10/12)^(12×0.75)
Step 4: Match The Time Count Used In The Contract
Time seems simple until a contract counts days in a specific way. Some products use actual days (365 or 366), while others use a 360-day convention. That choice shifts the daily rate.
If the contract says “per annum, calculated on a 360-day year,” the daily rate is r/360. If it says “actual/365,” the daily rate is r/365. Many bond and money-market deals use these rules; the ISDA’s 2021 Definitions page points you to the standard language used for day-count methods.
Common Scenarios And The Inputs You Need
Before you reach for a calculator, pin down the exact situation. The table below shows what to gather so you don’t plug the wrong numbers into the right formula.
| Situation | What The Rate Usually Means | Inputs To Collect |
|---|---|---|
| Personal loan with fixed monthly payments | Stated annual rate used to build an amortization schedule | Principal, rate, term, payment frequency, fee list |
| Credit card balance | APR split into a daily periodic rate for accrual | APR, statement dates, balance method used by the card |
| Fixed deposit with monthly crediting | Annual rate compounded and credited on the stated schedule | Principal, annual rate, compounding frequency, term |
| Simple interest note | Interest charged only on the original principal | Principal, annual rate, start/end dates, day-count basis |
| Short-term loan quote with fees | Often shown as an annualized disclosure figure | Cash received, fees, repayment date, total repaid |
| Late payment interest in a contract | Annual rate applied daily to the overdue amount | Overdue balance, annual rate, overdue days, day-count rule |
| Savings account with daily compounding | Annual rate compounded daily, credited monthly or daily | Rate, compounding basis, crediting schedule, balance changes |
| Bond or sukuk coupon calculation | Annual coupon with a formal day-count convention | Coupon rate, face value, coupon dates, day-count method |
Simple Interest Calculations You Can Do On A Phone
Simple interest is the easiest way to check a quote. If a lender quotes a yearly rate and you borrow for part of a year, you can estimate the interest charge in seconds.
Month-Based Shortcut
If the terms work in months and there’s no compounding during the term, treat time as months/12.
- Borrow ৳200,000 at 15% per annum for 4 months
- Time = 4/12
- Interest = 200,000 × 0.15 × (4/12) = ৳10,000
Day-Based Shortcut
When the contract charges interest daily, convert the annual rate into a daily rate, then multiply by days.
Daily interest = Principal × (Annual Rate / Day Count)
Example: ৳80,000 overdue for 19 days at 12% per annum on an actual/365 basis:
Interest = 80,000 × 0.12 × (19/365)
This method is also a clean way to verify statements that show daily accrual.
Compound Interest: Turning A Per-Annum Rate Into The Rate That Hits Your Balance
Compounding can make two offers with the same headline rate land in different places. To compare them, translate the annual rate into the periodic rate used by the product, then compute the effective annual rate (the one-year growth the compounding schedule creates).
Find The Periodic Rate
If interest compounds monthly, the periodic rate is r/12. If it compounds daily, it’s r/365 (or r/360 when the contract says so).
Compute The Effective Annual Rate
Effective annual rate = (1 + r/n)^n − 1
This converts a stated annual rate into the one-year growth rate created by compounding. If you want a public, regulator-run calculator you can use for quick checks, Investor.gov’s compound interest calculator is a straightforward option.
| Compounding Schedule | n (Periods/Year) | How To Compute One-Year Growth |
|---|---|---|
| Yearly | 1 | (1 + r)^1 − 1 |
| Quarterly | 4 | (1 + r/4)^4 − 1 |
| Monthly | 12 | (1 + r/12)^12 − 1 |
| Weekly | 52 | (1 + r/52)^52 − 1 |
| Daily (Actual/365) | 365 | (1 + r/365)^365 − 1 |
| Daily (360-Day Convention) | 360 | (1 + r/360)^360 − 1 |
Loan Payments: Per Annum Interest Inside Monthly Installments
Many loans don’t charge interest as a one-time figure. They spread it across payments. Each payment pays that period’s interest, then reduces principal. The rate you see is still an annual rate, but the schedule works per month (or per week).
Turn The Annual Rate Into A Payment-Period Rate
For monthly payments, a common starting point is monthly rate = r/12. Some lenders use a day-based approach between payment dates, yet the r/12 method is still a useful check.
Compute The Interest Part Of A Single Payment
For a given month, interest is roughly:
Monthly interest ≈ Current balance × (r/12)
Example: Balance ৳300,000, rate 14% per annum:
Monthly interest ≈ 300,000 × (0.14/12) = ৳3,500
If your payment is ৳9,000, then about ৳3,500 goes to interest and the rest reduces the balance.
APR, Stated Rate, And Fees: Getting A Fair Comparison
Two offers can show the same per-annum rate and still cost different amounts once fees enter the picture. Some fees are charged up front, some are added to the loan, and some show up monthly. When a lender provides an APR, it’s meant to roll certain costs into a single annualized number so you can compare offers on the same footing.
When you have only a stated rate, a practical comparison method is to calculate your total paid (or total earned) over the same time horizon for each option, using the fee schedule from the terms sheet. If you’re comparing growth on savings, it can help to double-check your math with a second tool; ASIC MoneySmart’s compound interest calculator is one public reference point on a regulator site.
Common Mistakes That Change The Answer
Interest math is simple. The mistakes happen before the math starts. These checks keep you from being off by a wide margin.
- Mixing up percent and decimal. 12% is 0.12, not 12.
- Using months as years without dividing by 12. Four months is 4/12.
- Ignoring compounding. A monthly-compounded product won’t match a simple-interest estimate.
- Ignoring day-count rules. r/360 and r/365 won’t match over many days.
- Comparing rates without fees. A lower rate with high fees can still cost more.
- Forgetting balance changes. Deposits, withdrawals, and partial prepayments change accrual.
Build Your Own Per-Annum Interest Checker In A Spreadsheet
If you want repeatable answers, a spreadsheet beats a one-off calculator. You can build a clean checker with three blocks: inputs, calculations, and outputs.
Inputs Block
- Principal
- Annual rate (%)
- Start date and end date
- Day-count basis (365 or 360)
- Compounding periods per year (1, 4, 12, 365…)
Calculations Block
For simple interest over days:
= Principal * (Rate/100) * (Days/DayCount)
For compound growth when the balance stays constant:
= Principal * (1 + (Rate/100)/n)^(n*t) – Principal
Outputs Block
Show the interest amount and the ending balance.
Mini Checklist Before You Trust Any Quote
Run this checklist each time you see “per annum” on a quote:
- Is the rate a stated rate, or does it include fees (APR)?
- Is interest simple, or is it compounded?
- What is the compounding schedule (monthly, daily, quarterly)?
- How is time counted (actual/365, actual/360, 30/360)?
- Does the balance change during the period?
Once those are settled, the calculation is direct and you can check the numbers with a phone, a spreadsheet, or a calculator.
References & Sources
- Consumer Financial Protection Bureau (CFPB).“Loan interest rate vs. APR.”Defines APR and explains how it differs from a stated interest rate.
- International Swaps and Derivatives Association (ISDA).“2021 ISDA Interest Rate Derivatives Definitions.”Gateway to standard language that includes day-count conventions used in interest calculations.
- U.S. Securities and Exchange Commission (SEC) Investor.gov.“Compound Interest Calculator.”Public calculator that demonstrates how compounding changes one-year and multi-year growth.
- Australian Securities and Investments Commission (ASIC) MoneySmart.“Compound Interest Calculator.”Calculator and plain-language notes on compounding and growth.