How to calculate percentage increase or decrease
A change of 20 means more when you started with 40 than when you started with 400. Percentage change compares the difference with the starting value.
What the formula is saying
Find new minus old, divide by old to express the change relative to the starting point, then multiply by 100. A positive answer is an increase; a negative answer is a decrease.
Read the symbols in plain language
- oldValue
- Original valueratio / no unit
- newValue
- New valueratio / no unit
Sort out the units first
The two values must describe the same quantity in the same unit. Changing from 80 to 100 has a difference of 20; 20/80 = 0.25, so the change is 25%.
Let’s solve one together
Read the given values, follow each operation, then check what the result means.
- oldValue · Original value
- 80
- newValue · New value
- 100
Find the actual change
Subtract in this order so decreases remain negative.
(100)-(80) = 20Compare the change with the starting value
Old value is the reference, not the new value.
(20) ÷ (80) = 0.25Convert the ratio into percent
Percent means “per hundred”.
(0.25) × 100 = 25 %
Does this worked answer make sense?
Rebuild the new value: 80 × (1 + 25/100) = 100. Reversing the change from 100 to 80 gives −20%, not −25%.
Now try your own values
Change a value or its unit. The same method will show your calculation, step by step.
Results update only when you calculate. The lesson example above stays unchanged.
Your turn — check your understanding
Use these new values. Work it out first, then check your answer.
- oldValue · Original value
- 200
- newValue · New value
- 150
Find: percentage increase or decrease
A hint, not the answer
Find new minus old, divide by old to express the change relative to the starting point, then multiply by 100. A positive answer is an increase; a negative answer is a decrease.
The two values must describe the same quantity in the same unit. Changing from 80 to 100 has a difference of 20; 20/80 = 0.25, so the change is 25%.
Show the full practice solution
Compare the steps with your work; revealing a solution does not mark the lesson complete.
Find the actual change
Subtract in this order so decreases remain negative.
(150)-(200) = -50Compare the change with the starting value
Old value is the reference, not the new value.
(-50) ÷ (200) = -0.25Convert the ratio into percent
Percent means “per hundred”.
(-0.25) × 100 = -25 %
Avoid the common trap
Dividing by the new value answers a different question. Going from 10% to 15% is 5 percentage points but a 50% relative increase.
When this method applies — and when it does not
This beginner lesson uses a positive original value and a nonnegative new value. A zero baseline makes this relative percentage undefined.
One idea understood. Keep going.
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Sources & further reading
References open in a new tab and explain the underlying principles. The teaching text and examples here are SimpleFlick’s own; the source organisations have not endorsed this calculator.
Lesson updated: · Worked examples checked against the implemented formula; not an independent engineering certification.