How to Add or Subtract a Percentage from a Number
This calculator helps you increase or decrease a number by a specific percentage. It's perfect for calculating prices after discounts, determining totals with tax, adjusting quantities, or finding the original price before a percentage was added (like removing VAT/GST).
Understanding Percentage Adjustments
Adding a percentage increases a value by that proportion of itself. Subtracting a percentage decreases it. This is different from percentage change calculations-here you're applying a known percentage to get a new value, rather than finding what percentage changed.
The Formulas
To add a percentage: Result = Original × (1 + Percentage/100). To subtract a percentage: Result = Original × (1 - Percentage/100). To find the original before a percentage was added: Original = Total ÷ (1 + Percentage/100).
Example:
Add 20% to 150: 150 × 1.20 = 180. Subtract 20% from 150: 150 × 0.80 = 120. Find original if 180 includes 20%: 180 ÷ 1.20 = 150.
Common Use Cases
Real-world applications for this calculator
Calculating Sale Prices
Find the price after a discount. A $80 item with 25% off: $80 × 0.75 = $60 final price.
Adding Sales Tax
Calculate total with tax included. A $50 item with 8% tax: $50 × 1.08 = $54 total.
Removing VAT/GST
Find the pre-tax price. If an item costs $110 including 10% GST: $110 ÷ 1.10 = $100 original price.
Adjusting Recipes
Scale ingredient quantities. To increase a recipe by 50%: if it calls for 200g flour, use 200 × 1.50 = 300g.
Tips
- To add X%, multiply by (1 + X/100). To subtract X%, multiply by (1 - X/100).
- A 10% discount means you pay 90% of the original price.
- To reverse a percentage addition, divide by (1 + rate/100), don't subtract the percentage.
- Stacking discounts: apply each one sequentially. 20% off then 10% off is not the same as 30% off.
Frequently Asked Questions
How do I add a percentage to a number?
Multiply the number by (1 + percentage/100). For example, to add 15% to 200: 200 × 1.15 = 230. Alternatively, calculate the percentage amount (200 × 0.15 = 30) and add it: 200 + 30 = 230.
How do I subtract a percentage from a number?
Multiply the number by (1 - percentage/100). For example, to subtract 20% from 150: 150 × 0.80 = 120. Alternatively, calculate 20% of 150 (= 30) and subtract: 150 - 30 = 120.
How do I find the original price before tax was added?
Divide the total by (1 + tax rate/100). For example, if an item costs $220 including 10% tax: $220 ÷ 1.10 = $200 original price.
What is $100 plus 10% tax?
$100 plus 10% tax equals $110. Calculate as $100 × 1.10 = $110, or $100 + ($100 × 0.10) = $100 + $10 = $110.
Why doesn't subtracting 10% then adding 10% give the original number?
Because each operation uses a different base. If you start with 100 and subtract 10%, you get 90. Adding 10% to 90 gives 99 (not 100), because 10% of 90 is 9, not 10. The percentages apply to different values.