Add or Subtract a Percentage Calculator

Use this calculator to add a percentage to a number or subtract a percentage from it. Enter the original amount and a known rate to see the final amount and how much was added or removed. For example, adding 25% to 80 gives 100.

What this calculator does

This tool answers a forward calculation: “What is this amount after a percentage increase or decrease?” It shows the percentage amount, the original amount, the new amount and the multiplier used.

That differs from finding the rate between two known amounts. For that question, use the existing Percentage Increase Calculator. To recover a missing starting amount from a final amount and rate, use Reverse Percentage.

How to add or subtract a percentage

1. Choose Add percentage or Subtract percentage.
2. Enter the starting amount.
3. Enter the rate as a percentage number, such as 25 for 25%.
4. Read the final amount and the percentage amount in the breakdown.

Keep the units consistent. A starting value of 80 dollars produces amounts in dollars; a starting mass of 80 grams produces amounts in grams. The calculator does not convert units or currencies.

Formula and multiplier

Let A be the original amount and r be the rate:

• Percentage amount = A × r ÷ 100.
• Add: final amount = A × (1 + r ÷ 100).
• Subtract: final amount = A × (1 − r ÷ 100).

The multiplier describes the proportion retained after the change. Adding 25% uses 1.25; subtracting 20% uses 0.8. See the York St John University percentages factsheet for forward increase/decrease methods. Percent means a quantity measured per hundred, as explained in Math Is Fun's percentage introduction.

Example: add 25% to 80

A quantity starts at 80 and increases by 25%:

• Percentage amount = 80 × 25 ÷ 100 = 20.
• Final amount = 80 + 20 = 100.
• Multiplier = 1.25, so 80 × 1.25 = 100.

If these are prices, $80 becomes $100 before any additional charges. The calculator makes no assumption about why the increase occurs.

Example: subtract 20% from 100

Start with 100 and remove 20%:

• Percentage amount = 100 × 20 ÷ 100 = 20.
• Final amount = 100 − 20 = 80.
• Multiplier = 0.8.

Notice that a 25% rise from 80 followed by a 20% reduction from 100 returns to 80. Equal percentage increases and decreases do not normally cancel: adding 20% and then subtracting 20% applies the second rate to a different base.

How to use the result

The main result is the new total, not the amount of the increase or reduction. Look at Percentage amount when you need the change alone. Use original inputs for subsequent steps and apply each later percentage to the appropriate new base.

For discounts, the Discount Calculator provides a shopping-specific starting point. For taxes, use a suitable regional calculator rather than treating a supplied rate as a confirmed legal obligation.

Assumptions and limitations

The original amount must be nonnegative and at most 10^15. Addition accepts rates from 0% to 10^6%; subtraction accepts 0% through 100% so the final amount stays nonnegative. A 100% subtraction leaves zero. Negative amounts and rates are outside this tool's scope.

This is one arithmetic change, without compounding periods, tax policy or accounting rules. Amounts use the same unit throughout; there is no currency selector or cent-rounding model. Results normally show up to two decimal places; very small or large values use scientific notation. Multipliers show up to eight decimals. Numerical precision errors are reported rather than silently losing a nonzero change.

How we explain formulas, assumptions and sources