By Claire Gillespie | Updated August 30, 2022
Mathematics often requires you to increase a number by a percentage. The process is simple: convert the percentage to a decimal, compute the increase, then add it to the original value. This same method can be inverted to decrease a number.
The most efficient way is to shift the decimal point two places left. For example, 30 % becomes 0.30, 50 % becomes 0.50. If you use a calculator with a % button, press 50 % after entering the number. Remember that 100 % equals 1, so 25 % is 0.25, 75 % is 0.75, etc.
To raise 10 by 50 %, multiply 10 × 0.50 = 5. On a calculator, 10 × 50 % = gives the same result.
Adding the increase (5) to the original (10) gives 15. Thus, 10 increased by 50 % equals 15. To decrease, subtract the increase instead: 10 – 5 = 5.
Sometimes you know the sale price and the discount percentage, and you need the original price. For instance, a bed now costs $280 after a 30 % reduction. The sale price represents 70 % (100 % – 30 %) of the original. Divide 280 by 0.70 to recover the original price: 280 ÷ 0.70 = 400. Therefore, the bed’s original price was $400.
By following these clear, step‑by‑step instructions, you can confidently calculate percentage increases and decreases in any context.
