Enter an original value and a new value to find the percentage increase or decrease. Use the same units for both values, such as shillings for both prices or learners for both class sizes.
The percentage change formula
Percentage change = (new value − original value) ÷ original value × 100. A positive answer means an increase; a negative answer means a decrease. The original value is the base, so do not divide by the new value.
Worked example: a school club grows
A club had 80 members and now has 100. The increase is 100 − 80 = 20 members. Divide 20 by the original 80, then multiply by 100: 20 ÷ 80 × 100 = 25%. The club grew by 25%.
If membership falls from 100 to 80, the decrease is 20 ÷ 100 × 100 = 20%. Reversing the numbers does not give the same percentage, because the original value changes. Try both examples above and compare the working.
What this tool does not calculate
A change between two percentages can also be measured in percentage points. Moving from 40% to 50% is a rise of 10 percentage points, or a relative increase of 25%. Those are different descriptions. This tool calculates relative percentage change, not an examination grade or a grading band.
Input limits and rounding
The original value must be greater than zero; the new value may be zero but not negative. Each input must be finite and no larger than 1 trillion. Results are displayed to at most 12 significant digits, so recurring decimals are rounded. A zero original value has no defined percentage change and is rejected.
This is a free, independent learning aid for Kenyan families and teachers. It does not produce an examination grade or placement prediction. The calculator works on this device: it does not send or save the values you type.
Continue practising
Use our decimals and percentages revision guide for the basics. The fraction simplifier helps with equivalent fractions, and the Grade 6 Mathematics papers let you practise. Return to all free tools.
Method source
See the percentage increase and decrease method in OpenStax, Prealgebra 2e, section 6.2, published by OpenStax at Rice University. Our club example and explanation are original. Source checked 7 November 2026.
Frequently Asked Questions
How do I calculate percentage increase?
Subtract the original value from the new value, divide by the original value, and multiply by 100. A positive result is a percentage increase.
What does a negative answer mean?
It means the new value is smaller than the original. For example, a change of -20% is a 20% decrease.
Can I start from zero?
No. The formula divides by the original value, so percentage change from zero is undefined. State the change in amount instead.
Does the calculator save my values?
No. Calculation happens in your browser, without sending or storing the values you enter.