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Understanding Percentage Calculations
Sales tax on a receipt, a test score, a pay rise, a discount at checkout: each one is a percentage question. Almost all of them come down to one formula and two variations of it, covered below with a worked example each.
What Is a Percentage?
A percentage expresses a number as a fraction of 100. If something is 20%, it equals 20/100, or 20 hundredths. That makes a percentage a number per hundred: 25% is 25 per 100. Percentages are often used for a part of a whole, but they are not capped there: 150% growth is more than the original, and a -5% change is a drop.
The Basic Percentage Formula
The basic formula for calculating a percentage is:
Percentage = (Part / Whole) x 100
Here is what each piece means:
-
Part: the portion you want to express as a percentage. -
Whole: the total amount the part is taken from. -
x 100: multiplying the fraction by100turns it into a percentage.
For example, if you scored 80 points on a test out of a possible 100 total points, your percentage would be:
(Part / Whole) x 100
(80 / 100) x 100
0.8 x 100
= 80%
Calculating Percentages of Quantities
Often you need the amount a percentage stands for, not the percentage itself: the sales tax in dollars, the size of a discount, the profit on a sale.
To get it, write the percentage as a decimal (move the decimal point two places left) and multiply it by the original quantity.
For example, if an item costs $100 and sales tax is 9%, the calculation is:
9% as a decimal is 0.09
0.09 x $100
= $9
So the sales tax on a $100 item at a 9% tax rate is $9.
Percentages come up in many areas: use our discount calculator for shopping deals, the tip calculator for restaurant bills, or the profit margin calculator for business pricing.
Free Percentage Calculator Tools
Find percentage that a specific value represents out of the total value
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For example to calculate how much is 20% of 10:
(20 / 100) x 10
0.2 x 10
= 2
Find what percent is a value of another
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For example to calculate what percent is 20 of 50:
20 / (50 / 100)
20 / 0.5
= 40%
Find the sum of a value and a percentage
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For example to calculate what value is 200 + 10%:
((10 / 100) * 200) + 200
(0.1 * 200) + 200
20 + 200
= 220
Frequently Asked Questions
How do I calculate a percentage of a number?
Divide the percentage by 100 to convert it to a decimal, then multiply by the number. For example, 25% of 200: divide 25 by 100 to get 0.25, then multiply 0.25 by 200 to get 50.
How do I find what percentage one number is of another?
Divide the part by the whole, then multiply by 100. For example, to find what percentage 30 is of 120: (30 / 120) x 100 = 25%.
How do I add a percentage to a number?
Multiply the number by (1 + percentage/100). For example, 200 + 10%: 200 x 1.10 = 220. Or calculate the percentage amount first (200 x 0.10 = 20) and add it to the original (200 + 20 = 220).
Percentage Calculations in Notes Calculator
In Notes Calculator you skip the formulas: each of the three tools above is one line in a note, written the way you would say it.
| Note | Result |
|---|---|
| // What is 20% of 10? | |
| 20% of 10 | 2 |
| // What percent is 20 of 50? | |
| 20 / 50 as % | 40% |
| // What is 200 plus 10%? | |
| 200 + 10% | 220 |
The app reads many more percent forms, such as 10% off 200, 180 is what % off 200 and stacked discounts with previous. The percentage syntax guide walks through each one. To see where a notepad beats a grid for everyday math, read notepad calculator vs spreadsheet. The docs show more of these forms in the percentages reference.
For risk-based position sizing, R-multiples on trade sheets, and running P/L that reuses the same stored-percent idioms, our calculator for investors and traders walks through how the notepad turns an account-risk-per-trade percentage into a share count, compares reward to risk, and totals a week of P/L.
Percentages on real invoices usually mean VAT and discounts stacked on one subtotal: the freelance invoice calculator workflow shows that math end to end.