Calculation guides
The question answered first, then the formula, a worked example, and the calculator that does the work for you.
How to calculate a mortgage payment
A repayment mortgage payment is the amount borrowed multiplied by the monthly interest rate, divided by one minus the discount factor for the whole term. Everything you need is the loan amount, the annual rate and the number of years.
How to calculate a tip
A tip is the bill multiplied by the percentage you want to leave. To split it, add the tip to the bill and divide by the number of people.
How to calculate your net worth
Your net worth is the total value of everything you own minus the total of everything you owe. It is a snapshot on one date, not an income figure.
How to calculate BMI
Body mass index is weight in kilograms divided by height in metres squared. In imperial units it is weight in pounds divided by height in inches squared, multiplied by 703.
How to calculate a percentage
A percentage of a number is the number times the percentage divided by 100. To find what percent one number is of another, divide the part by the whole and multiply by 100.
How to calculate your age
Your age is the number of whole years since your birth date, plus the months and days left over. Count the years first, then the months, then the days.
How to calculate hours between two times
The hours between two times are the end time minus the start time, with any unpaid break taken off. If the end time is earlier than the start, add 24 hours because the shift crosses midnight.
How to calculate compound interest
Compound interest adds each period's interest to the balance so later interest is earned on it too. The balance is the starting amount multiplied by one plus the periodic rate, raised to the number of periods.
How to calculate VAT
To add VAT, multiply the net price by one plus the rate. To take VAT out of a gross price, divide by one plus the rate — never just subtract the percentage.
What is the difference between margin and markup?
Margin is profit as a share of the selling price; markup is the same profit as a share of the cost. The same money gives a smaller margin percentage than markup percentage.
How to calculate a loan payment
A fixed-rate loan payment is the amount borrowed times the monthly rate, divided by one minus the discount factor for the number of payments. The total interest is all the payments added up minus the amount borrowed.
How to calculate your hourly rate from a salary
Your hourly rate is your annual salary divided by the hours you actually work in a year. Those hours are your weekly hours multiplied by the number of paid weeks.