How do you calculate the percentage of a number, a percentage ratio, and a percentage increase/decrease? We explain the formulas with simple examples and show how they're used in e-commerce for discount, VAT, and profit calculations. Try it right away with the free tool inside the article.
Percentages are among the mathematical concepts we use most often in daily life, and especially in e-commerce. When setting a discount rate, adding VAT, measuring profit margin, or asking "how much did sales grow this month?", we always turn to the same basic calculation. Even so, percentage calculation easily causes confusion when the formula isn't firmly understood, and it leads to costly pricing mistakes.
In this guide, we explain the three basic types of percentage calculation with simple examples: finding the percentage of a number, calculating what percentage one value is of another, and finding the percentage increase/decrease between two values. If you prefer, you can see the result instantly with the free tool below, without dealing with formulas.
Calculate right away: Choose the calculation type, enter two values, and get the result in seconds.
What Is a Percentage?
A percentage is the ratio that shows, when we divide a whole into 100 equal parts, how many of those parts we're talking about. The "%" sign means "per hundred"; for example, 20% refers to 20 parts of a whole when it's divided into a hundred, and it equals 0.20 as a decimal. This simple logic forms the basis of all percentage calculations: when factoring in a percentage, we actually divide it by 100 and relate it to the relevant number.
Percentage Calculation Formulas
Nearly all of the percentage calculations we encounter in practice are based on one of three formulas. Let's look at each with an example.
1. The Percentage of a Number
This is the most commonly used calculation: finding how much a certain percentage of a number comes to. The formula is:
B% of A = (A × B) ÷ 100
For example, if we want to find 15% of a product priced at 200 TL: (200 × 15) ÷ 100 = 30 TL. This could be a 30 TL discount applied to the product or a 30 TL additional line item added to it. The tool's "Percentage of" tab does exactly this calculation and simultaneously shows the values for "when the percentage is added" (230 TL) and "when the percentage is removed" (170 TL).
2. What Percentage Is One Number of Another?
The second common calculation is finding the ratio of a part to the whole as a percentage:
Ratio = (Part ÷ Whole) × 100
Say 30 out of 120 visitors made a purchase; your conversion rate is (30 ÷ 120) × 100 = 25%. With the same formula, you can also answer questions like "what is the ratio of a 150 TL cost to a 600 TL price?" The tool's "Ratio" tab does this calculation and also shows the remaining ratio (75% in the example).
3. Percentage Increase and Decrease
The third calculation measures the change between two values as a percentage:
Change = ((Final Value − Initial Value) ÷ Initial Value) × 100
If a product's price rose from 80 TL to 100 TL, the increase is (20 ÷ 80) × 100 = 25%. Conversely, if it dropped from 100 TL to 80 TL, the change is (−20 ÷ 100) × 100 = a 20% decrease. The critical point here is to always divide by the initial value; mixing up the divisor is one of the most common mistakes. The tool's "Increase/Decrease" tab calculates the result automatically and distinguishes decreases in red.
Step-by-Step Example: Finding the Discounted Price
The most concrete use of percentage calculation in e-commerce is discounts. Suppose you're going to apply a 30% discount to a product priced at 850 TL:
- Find the discount amount: (850 × 30) ÷ 100 = 255 TL.
- Find the discounted price: 850 − 255 = 595 TL.
- Check it: 595 is (595 ÷ 850) × 100 = 70% of 850; in other words, the 30% discount has been applied correctly.
By the same logic, you can also add a percentage to a price: to add 18% to 595 TL, you get (595 × 18) ÷ 100 = 107.10 TL, and the price rises to 702.10 TL. If you're working with a single rate, you can do this calculation with the tool above; to include/exclude different VAT rates such as 1%, 10%, or 20%, you can do it in seconds with our VAT Calculation tool.
Where Does Percentage Calculation Help You in E-Commerce?
Percentages come up in nearly every metric of online sales. The most critical use cases are:
- Discounts and campaigns: To set the right discount rate and see how much the campaign affects profit.
- VAT calculation: To add tax to a price or to separate the base amount from a VAT-inclusive total.
- Profit margin: To measure the ratio of net profit to the sale price. For a more comprehensive cost and target margin calculation, you can use the Profit Margin Calculation tool.
- Conversion rate: To measure what percentage of visitors made a purchase; the basis for interpreting ad and page performance.
- Growth analysis: To compare month-over-month growth in sales, turnover, or traffic as a percentage.
Tip: Always calculate percentages against the correct "whole." Discounts are calculated on the sale price; profit margin on the sale price; markup, on the other hand, on the cost. Mixing up the whole completely changes the result.
Common Mistakes When Calculating Percentages
A few common mistakes will lead you to the wrong result even if you know the right formula:
- Adding consecutive discounts together: A 10% discount on top of a 20% discount doesn't equal 30%. Discounts are applied in sequence; the net effect is 28%. Instead of adding the rates on top of each other, calculate them step by step.
- Confusing percentage points with percentages: If a rate rose from 20% to 25%, that's a "5 percentage point" increase; however, the relative increase is 25%. The two are different things.
- Dividing by the wrong divisor: For increase/decrease, always divide by the initial (old) value. Dividing by the final value gives a different result than the percentage change.
- Incorrectly separating VAT as a percentage: To find the base amount from a VAT-inclusive price, you need to divide the amount by (1 + rate); simply subtracting the rate is wrong.
Alis Digital Is by Your Side for Accurate Pricing
Percentage calculation may seem simple, but when discount, VAT, commission, and profit margin come together in e-commerce, pricing quickly becomes complex. A poorly structured discount or an overlooked commission can cause you to lose money even while generating turnover. At Alis Digital, we're by your side throughout the entire process, from store setup to pricing strategy, and from ad management to conversion optimization. Contact us for a free consultation and let's turn the numbers in your favor.
