Everyday calculation guides

How to calculate a percentage

To find a percentage of a value, divide the percentage by 100 and multiply by the value.

Example: 15% of 200 is (15 ÷ 100) × 200 = 30.

Understand compound interest

Compound interest means returns are added to the balance, so later returns can be earned on both the original amount and earlier growth.

Results are estimates: actual returns vary, and taxes, fees, inflation, and contribution timing can change outcomes.

Count days between dates

Date differences usually count elapsed days and exclude the starting date. If you need to count both the first and last date, add one day to the elapsed-day result.

Use a calculator result wisely

Check that your units match, review the numbers you entered, and remember that estimates such as loans, body-mass index, and investment growth do not replace professional financial or medical advice.

How to calculate a discount and final price

Convert the discount percentage to a decimal, multiply it by the original price to find the savings, then subtract the savings from the original price.

Example: a 20% discount on 50 saves 10, so the final price is 40 before any taxes or extra fees.

Create and check a QR code

A QR code stores text that a compatible camera or scanner can read. Paste the exact URL or text you want to share, generate the code, download the PNG, and scan it yourself before printing or sharing it.

Use a clear, high-contrast image and leave a quiet margin around the code.

Use a currency converter carefully

Enter an amount, choose the source and destination currencies, and convert while connected to the internet. The result is calculated from the rate returned by the external rate provider and the update time is shown when available.

Exchange rates may differ from the rate offered by a bank or exchange office because of fees, spreads, and update timing.

Estimate a loan payment

A fixed-rate loan estimate uses the principal, annual interest rate, and number of monthly payments. It assumes a constant rate and regular payments; real offers may include fees, insurance, variable rates, or different payment timing.

Compare the estimated monthly payment and total paid across different terms, but check a lender's official repayment schedule before committing.

Convert units without mixing systems

Before converting, confirm that the starting and target units measure the same quantity. For example, convert centimeters to inches for length, kilograms to pounds for mass, and Celsius to Fahrenheit for temperature.

For work involving engineering, medicine, or safety, independently verify conversions and rounding requirements.

Calculate age and date differences

Age is based on the difference between a birth date and today, while a date-difference calculation counts elapsed calendar days. The usual elapsed-day count excludes the starting date; for inclusive counting, include both endpoints and add one day when appropriate.

Time zones and daylight-saving changes can matter when working with exact timestamps.

Find the percentage change between two values

To measure how much a value increased or decreased, subtract the original value from the new value, divide the difference by the original value, then multiply by 100.

Example: a price rises from 80 to 100. The change is 20, and 20 ÷ 80 × 100 = 25%, so the increase is 25%.

Calculate an average correctly

The arithmetic average gives each listed value equal weight. Add all values and divide by how many values you entered.

Example: the average of 12, 15, and 18 is 15.

Understand profit margin versus markup

Profit is revenue minus cost. Profit margin compares profit with revenue, while markup compares profit with cost.

Example: cost 60 and selling price 100 gives profit 40, margin 40%, and markup about 66.7%.

Convert Celsius and Fahrenheit

Celsius and Fahrenheit use different scales and different zero points, so conversion is not a simple multiplication.

Example: 20°C equals 68°F.

Understand decimal and binary file sizes

File-size labels may use decimal units or binary units. In decimal units, 1 KB is 1,000 bytes; in binary units, 1 KiB is 1,024 bytes.

Example: a 5 MB decimal file is 5,000,000 bytes.

What a BMI result can and cannot tell you

Body mass index (BMI) is calculated from weight and height: kilograms divided by height in metres squared. It is a broad screening measure, not a diagnosis.

Example: 70 kg at 1.75 m gives a BMI of about 22.9. Interpretation depends on age and other factors.

Estimate a fixed-rate loan payment

A standard monthly-payment estimate uses the loan principal, monthly interest rate, and number of monthly payments.

Compare monthly payments and total repayment for different terms, then check the lender's official offer.