How to Calculate Percentage Increase and Decrease

The formula

Percentage change measures how much a value increased or decreased compared with where it started.

Percentage change = ((new value - old value) / old value) x 100

Use the Percentage Calculator when you want the result without doing the formula by hand.

Percentage increase example

Suppose a subscription price rises from 20 to 25.

((25 - 20) / 20) x 100 = 25%

The price increased by 25%. The difference is 5, but the percentage is based on the original value of 20.

Percentage decrease example

Suppose a product drops from 80 to 60.

((60 - 80) / 80) x 100 = -25%

The result is negative, so the value decreased by 25%.

Why reversing the values changes the result

Going from 80 to 60 is a 25% decrease. Going from 60 back to 80 is not a 25% increase. It is a 33.33% increase because the new baseline is 60.

This is why phrases like “prices returned to normal” can be misleading. Always check the starting value.

Practical uses

Percentage change is useful for sales, traffic, grades, expenses, revenue, weight, time, and performance metrics. Marketers use it to compare conversion rates. Students use it for grade changes. Shoppers use it for price changes.

For sale prices, the Discount Calculator may be faster because it directly calculates savings and final price.

Step-by-step method

Use the same process every time:

1. Write down the original value.
2. Write down the new value.
3. Subtract original from new.
4. Divide the difference by the original value.
5. Multiply by 100.
6. Add a minus sign only if the result is negative.

The original value is the anchor. If you choose the wrong anchor, the percentage will answer a different question.

Canadian tax example

Percentage math appears constantly in tax estimates. If a taxable item in Alberta costs CAD 200 and GST is 5%, the tax is CAD 10. If the same item is discounted by 15% before tax, the discount is CAD 30 and the pre-tax sale price is CAD 170.

For Canadian sales-tax context, read Canadian GST, HST, PST and QST Explained. For direct calculations, use the GST/HST Calculator or Percentage Calculator.

Business metric example

Suppose monthly revenue increases from CAD 12,000 to CAD 15,000.

((15,000 - 12,000) / 12,000) x 100 = 25%

Revenue increased by 25%. If revenue later drops from CAD 15,000 back to CAD 12,000, the decrease is 20%, not 25%, because the baseline changed.

This same logic applies to website traffic, ad spend, conversion rates, loan payments, and savings goals. Percentages are only meaningful when the baseline is clear.

Common mistakes

The first mistake is dividing by the new value instead of the original value. That changes the question.

The second mistake is assuming equal-looking increases and decreases cancel each other out. A 50% decrease followed by a 50% increase does not return to the original value.

The third mistake is mixing percentage points and percent change. If an interest rate moves from 4% to 5%, that is a 1 percentage-point increase. Relative to 4%, it is a 25% increase.

Related tools and guides

Use the Percentage Calculator, Discount Calculator, and GST/HST Calculator for quick checks.

Related reading: Loan Calculator Guide, Canadian GST, HST, PST and QST Explained, and Compound Interest Explained.

Conclusion

Percentage change is simple once you remember the baseline: compare the difference to the original value, then multiply by 100.