Increase or Decrease by Percentage

The calculator answers questions like "What is 500 increased by 15%?" or "What is 500 decreased by 15%?" You choose whether to add or subtract a percentage, then enter the number and the percent. The result updates as you type.

Step-by-Step Instructions

1. Choose Increase or Decrease: Use the dropdown at the start of the form. Select Increase when you want to add a percentage (e.g. add tax, apply markup, or give a raise). Select Decrease when you want to subtract a percentage (e.g. apply a discount or reduce a value).

2. Enter the starting value: In the first number field (before the word by), enter the number you want to change. This might be a price, a salary, a quantity, or any other value. The placeholder suggests values like 500—use whatever your starting number is.

3. Enter the percentage: In the second number field (after the word by, with the % sign shown next to it), enter the percentage. Type the number only; you don't need to include the % symbol. For example, for 15% enter 15, for 8.25% enter 8.25.

4. Read the result: The result appears below the inputs. It’s shown as "It's" followed by the new value. That value is your starting number after it has been increased or decreased by the percentage you entered.

Modes and Options

• Increase: The tool multiplies your starting value by (1 + percentage ÷ 100). Use this for tax, markup, raises, growth, or any situation where you're adding a percentage.
• Decrease: The tool multiplies your starting value by (1 − percentage ÷ 100). Use this for discounts, reductions, or any situation where you're subtracting a percentage.

You can switch between Increase and Decrease at any time. The result updates automatically.

What You Can Enter

• Decimals: Both fields accept decimals (e.g. 99.99 or 8.25).
• Negative numbers: Supported in both fields. For example, increasing by -10% is the same as decreasing by 10%. Decreasing by -10% is the same as increasing by 10%.
• Large numbers: The calculator works with numbers of any size.

Number formatting in the result (e.g., thousands separators, decimal symbols) follows your browser's locale settings. Enter numbers in your usual way; the tool interprets them correctly.

This calculator is useful whenever you need to "add X%" or "subtract X%" from a number. You already know the percentage; you want to know the new value.

Everyday Situations

Sale prices: A store has "30% off." Select Decrease, enter the original price in the first field and 30 in the second field. The result is the price you pay.

Prices with tax: You’re buying something for 75 and sales tax is 8%. Select Increase, enter 75 and 8. The result is the total including tax.

Tips: To add a tip to a bill, use Increase with the bill amount in the first field and your tip percentage (e.g., 15, 18, or 20) in the second field.

Salary raises: You earn $50,000 and get a 5% raise. Select Increase, enter 50000 and 5. The result is your new salary.

Budget cuts or savings: "Reduce spending by 10%." Use Decrease with your current spending and 10 to see the new amount.

Work and Business

Markup: You buy at 60 and sell at a 40% markup. Use Increase with 60 and 40 to get the selling price.

Margin vs. markup: This tool applies a percent to a cost (markup-style). For margin (percent of selling price), you’d need a different setup, but for "add X% to cost" this tool fits.

Discounts and promotions: "20% off," "extra 10% off sale items," or "prices reduced by 15%." Use Decrease with the relevant price and percentage.

Budgets and forecasts: "Increase the marketing budget by 12%." Use Increase with the current budget and 12.

Why It’s Handy

You don’t have to remember formulas or worry about which number goes where. You choose Increase or Decrease, enter two numbers, and get the answer. It’s quick for shopping, invoices, planning, and everyday math.

A few mix-ups can lead to wrong results or confusion. Watch out for these:

Wrong Mode (Increase vs. Decrease)

Increase adds the percentage; Decrease subtracts it. Using the wrong one flips the result.

• For "30% off" or "reduce by 30%," use Decrease.
• For "add 8% tax" or "raise by 5%," use Increase.

Double-check the dropdown before trusting the result.

Confusing This With "Percent Change"

This tool does not compute "from X to Y, what’s the percent change?" It answers "X increased or decreased by Y% — what’s the new value?"

• This tool: You have a value and a percent. You get the new value. Example: 100 increased by 20% → 120.
• Percent change: You have two values. You get the percent change. Example: from 100 to 120 → +20%.

If you have "before" and "after" and want the percent, use a percent-change calculator instead.

Entering the Percent Incorrectly

Enter the percentage as a plain number (e.g. 15 for 15%, 8.25 for 8.25%). Don’t enter 0.15 for 15%—the tool expects 15. The second field is labeled with %, so only the numeric part goes in.

Reversing the Two Numbers

The first field is the starting value; the second is the percentage. Swapping them gives wrong results. For "500 increased by 15%," 500 is first, 15 is second.

Assuming 20% Up Then 20% Down = Same Value

Percent changes apply to the current value each time. Increase 100 by 20% → 120. Decrease 120 by 20% → 96, not 100. Don’t expect symmetrically opposite percents to cancel out.

Definition

Increasing a number by X% means finding a new value that is the original plus X% of the original. You end up with (100 + X)% of the original.

Decreasing a number by X% means finding a new value that is the original minus X% of the original. You end up with (100 − X)% of the original.

So "500 increased by 15%" means 500 + (15% of 500) = 500 + 75 = 575. "500 decreased by 15%" means 500 − (15% of 500) = 425.

How It Fits With Percentages Generally

A percentage is a way of expressing a number as a fraction of 100. "15%" means 15⁄100, or 0.15. When we "add 15% to 500," we add 15% of 500, i.e. 0.15 × 500 = 75, so 500 + 75 = 575.

This idea is used everywhere: tax, tips, discounts, raises, markup, growth rates, and so on. Reputable educational and government sources routinely describe percentages this way. For example, the Khan Academy covers percent increase and decrease in a similar manner.

Formula for Increase

When you increase a value by Y%:

New value = Original × (1 + Y ÷ 100)

Or written as: New value = Original × (1 + Y/100)

Example: 500 increased by 15%:

• New value = 500 × (1 + 15 ÷ 100) = 500 × 1.15 = 575.

Formula for Decrease

When you decrease a value by Y%:

New value = Original × (1 − Y ÷ 100)

Or written as: New value = Original × (1 - Y/100)

Example: 500 decreased by 15%:

• New value = 500 × (1 − 15 ÷ 100) = 500 × 0.85 = 425.

What Each Variable Is

• Original: The number you start with. In the tool, this is the first number field (before by).
• Y: The percentage you add or subtract. In the tool, this is the second number field (after by). It’s always "as a number" (e.g. 15 for 15%).
• New value: The result after the increase or decrease. The tool shows this as "It's" followed by the value.

Why the Formula Works

• Increase: (1 + Y∕100) is the multiplier for "100% + Y%." So you’re taking 100% + Y% of the original, which is exactly "add Y%."
• Decrease: (1 − Y∕100) is the multiplier for "100% − Y%." So you’re taking the original minus Y% of it.

Other Useful Forms

• Increase: New = Original + Original × (Y∕100). Same as above, just written as "original plus Y% of original."
• Decrease: New = Original − Original × (Y∕100). Same as "original minus Y% of original."

For this calculator, the multiplicative form (Original × (1 ± Y∕100)) is what’s used.

Below are step-by-step examples. The tool uses your locale for formatting; we use plain numbers in the text.

Example 1: Sale Price After a Discount

Scenario: A coat is 80. It’s 25% off. What do you pay?

1. Select Decrease.
2. First field (before by): 80.
3. Second field (after by): 25.
4. Result: 60.

You pay $60. (80 × 0.75 = 60.)

Example 2: Price Including Tax

Scenario: An item costs 42 before tax. Tax is 7.5%. What’s the total?

1. Select Increase.
2. First field: 42.
3. Second field: 7.5.
4. Result: 45.15.

Total is $45.15. (42 × 1.075 = 45.15.)

Example 3: Salary After a Raise

Scenario: Your salary is $55,000. You get a 4% raise. What’s your new salary?

1. Select Increase.
2. First field: 55000.
3. Second field: 4.
4. Result: 57200.

New salary: $57,200. (55,000 × 1.04 = 57,200.)

Example 4: Markup

Scenario: You buy at 24 and sell at a 50% markup. What’s the selling price?

1. Select Increase.
2. First field: 24.
3. Second field: 50.
4. Result: 36.

Selling price: $36. (24 × 1.5 = 36.)

Example 5: Reducing a Budget

Scenario: The budget is $10,000. You need to reduce it by 12%. What’s the new budget?

1. Select Decrease.
2. First field: 10000.
3. Second field: 12.
4. Result: 8800.

New budget: $8,800. (10,000 × 0.88 = 8,800.)

These tables give typical results for "increase by X%" or "decrease by X%" applied to a base of 100. Use them as a quick check. Your locale may format numbers differently.

Increase by Percentage (base 100)

Decrease by Percentage (base 100)

Common Discounts on 100

Common Markups on 100

For other bases, scale accordingly. For example, 200 increased by 10% = 220; 50 decreased by 20% = 40.

Tax Rates on 100 (common rates)

Use Increase with the pre-tax price and the tax rate to get the total you pay.

Example: scaling the tables

If your base is 200 instead of 100, double the table results (e.g. 200 + 10% = 220, 200 − 20% = 160). For 50, halve them (50 + 10% = 55, 50 − 20% = 40). The percentages stay the same; only the base changes.