Add a percentage to a number
Add X% to Y
What this calculator does
It increases a base amount Y by X percent of Y and shows the new total. That matches how many taxes, markups, and tips work: the extra amount is a percentage of the subtotal, then added on.
It is not the same as “what is X% of Y?” alone—that gives only the add-on. Here you get the full amount after the increase.
Formula
Y + (Y × (X ÷ 100))
Factored form: Y × (1 + X ÷ 100). Both are equivalent; use whichever is easier to reason about or type into a spreadsheet.
Worked example
Add 12% to £349.99: 349.99 × 0.12 = 41.9988; 349.99 + 41.9988 = £391.9888 (round for real-world pricing as your rules require).
Common questions
- How do you add a percentage to a number?
Take X% of Y and add it to Y: Y + (Y × X ÷ 100), same as Y × (1 + X ÷ 100).
- Can X be negative?
If you enter a negative percent, you reduce Y. For typical “percent off” wording, Subtract a percentage may be clearer.
- How do I find only the tax amount, not the total?
Use What is X% of Y? with your subtotal as Y—that returns the add-on only.
- What is 20% added to 100?
120. 20% of 100 is 20; 100 + 20 = 120.
- Is this the same as a markup?
Often yes, when markup is “add X% of this base.” Confirm whether your policy uses cost, list price, or another base—this tool always uses the Y you enter.
- Can I apply two increases in a row?
This form applies one percent to one base. For stacked increases, use the output as the new Y in a second step or use a spreadsheet.