The Good Money Guide Percentage Calculator helps you quickly work out percentage increases, decreases, and differences, without needing to remember any formulas. Whether you’re trying to find what is X% of Y, how much something has gone up or down by a certain percentage, or what percentage one number is of another, this simple tool can save you time and effort.
How to Use the Good Money Guide Percentage Calculator
Below is a full guide on how to use each part of the calculator and how to calculate percentages manually if you want to understand the maths behind it.
1. What is X% of Y?
Use this part of the calculator to find the percentage value of a number.
Example: What is 20% of 150?
Enter 20 in the percentage box and 150 in the number box.
The result will be 30.
This calculation is useful when you want to:
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Find a discount or markup (e.g., 20% off £150 = £30 discount)
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Work out interest, tips, or commission
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Calculate profit margins or tax amounts
Formula:
X% of Y = (X ÷ 100) × Y
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2. X is what percent of Y?
Use this section to find what percentage one number represents of another.
Example: What percent is 30 of 150?
Enter 30 as X and 150 as Y.
The result will be 20%.
This is useful for:
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Checking test scores or completion rates
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Measuring sales performance
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Comparing growth or decline between two values
Formula:
Percentage = (X ÷ Y) × 100
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3. X is Y% of what?
This part tells you the original total when you know a value and the percentage it represents.
Example: 30 is 20% of what?
Enter 30 in X and 20 in Y.
The result will be 150.
You can use this to:
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Reverse engineer a price before tax or discount
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Work out the full amount from a percentage figure
Formula:
Original = X ÷ (Y ÷ 100)
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4. What is X increased or decreased by Y%?
This part of the calculator helps you add or subtract a percentage from a number.
Example: Increase 150 by 20%
Enter 150 as X and 20 as Y, choose “increase,” and the result is 180.
For a decrease, the same numbers would result in 120.
You can use this for:
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Calculating price rises or reductions
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Measuring salary or revenue growth
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Tracking inflation, investment returns, or business performance
Formulas:
Increase: X × (1 + Y ÷ 100)
Decrease: X × (1 − Y ÷ 100)
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How to Calculate Annual Percentage Increase
If you’re tracking changes over time (for example, yearly revenue or savings growth), use this method to calculate the annual percentage increase:
Formula:
Annual % Change = ((New Value − Old Value) ÷ Old Value) × 100
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How to Calculate Percentage Manually
If you want to understand how percentages are calculated:
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Divide the part by the whole.
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Multiply the result by 100.
Example: 30 out of 150 → (30 ÷ 150) × 100 = 20%.
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How to Calculate Percentage Decrease or Reduction
To find how much something has dropped as a percentage:
Formula:
Percentage Decrease = ((Old Value − New Value) ÷ Old Value) × 100
Example: Price dropped from £200 to £150
→ ((200 − 150) ÷ 200) × 100 = 25% decrease
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How to Calculate Percentage Difference
When comparing two values to see the relative difference, use:
Formula:
Percentage Difference = (|A − B| ÷ ((A + B)/2)) × 100
This is useful for comparing performance between months, products, or portfolios.
How to Do Percentages on a Calculator
If you prefer to do it manually instead of using the online calculator:
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Divide the first number by the second.
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Multiply by 100.
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For increases or decreases, add or subtract the percentage.
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Extra Tips
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To calculate percentage in Excel, use formulas like
=A1/B1*100or=(B1-A1)/A1*100for changes. -
To calculate improvement percentage, compare old and new results using the same method.
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To calculate percentage yield or profit percentage, use
(Profit ÷ Cost) × 100.
Summary
The Good Money Guide Percentage Calculator can be used as a:
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Percentage Increase Calculator – to find growth or profit
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Percentage Difference Calculator – to compare two numbers
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Percentage Decrease Calculator – to measure reductions
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Percent Calculator – to work out any percentage question
It’s ideal for quick financial calculations, savings comparisons, or understanding returns — whether you’re figuring out discounts, profits, salary rises, or investment growth.
Use it to save time and ensure your percentage calculations are always accurate.