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calcthai.com · Percentage Calculator
General & conversion
Percentage Calculator
Five calculations in one: X% of a number, the price after a discount, A as a share of B, percentage change, and the original price before a discount.
Discount levels compared
| Discount | Amount off | You pay |
|---|---|---|
| 5% | 75 | 1,425 |
| 10% | 150 | 1,350 |
| 15% | 225 | 1,275 |
| 20% | 300 | 1,200 |
| 25% | 375 | 1,125 |
| 30% | 450 | 1,050 |
| 40% | 600 | 900 |
| 50% | 750 | 750 |
| 70% | 1,050 | 450 |
Ask about this page
The AI assistant answers only questions about the calculator on this page.
Percentages look simple, but several cases trip people up consistently — particularly working back to a full price from a discounted one, and percentage change, where the answer depends entirely on which number you divide by. This tool covers the five cases that come up most often. Pick the mode that matches your question and fill in the numbers.
How it works
X% of a number is the basic case: multiply the number by the percentage over one hundred. 20% of 1,500 is 1,500 × 0.20 = 300.
Price after a discount. Subtract the discount from the full price, or multiply by what remains. A 20% discount means paying 80%, so 1,500 × 0.80 = 1,200.
Working back to the full price is where most errors happen. Suppose a sign says 20% off, now 1,200. The instinct is to add 20% to 1,200, giving 1,440 — and that is wrong. The correct answer is 1,200 ÷ 0.80 = 1,500.
The reason is that the 20% was calculated on the full price, not on the discounted one. It is exactly the same principle as stripping VAT out of a tax-inclusive total.
A as a percentage of B. Divide A by B and multiply by a hundred. Scoring 30 out of 120 is 30 ÷ 120 × 100 = 25%.
Percentage change. Divide the difference by the absolute value of the old value |A| and multiply by a hundred. The critical point is to always divide by the absolute value of the old value, never the new one. A price rising from 100 to 150 is an increase of 50 ÷ 100 = 50%. But the same price falling back from 150 to 100 is a decrease of 50 ÷ 150 = about 33.3%, not 50%.
This is why a stock that falls 50% has to rise 100% to break even: the base the calculation runs on has changed.
Stacked discounts. A sign reading 50% off then a further 20% off is not 70% off — it is 60% off, because the second discount applies to the already-reduced price. Multiply what remains at each step: 0.50 × 0.80 = 0.40, so you pay 40% of the original.
Frequently asked questions
An item is 20% off at 1,200. What was the original price?
1,500. Divide 1,200 by 0.80. The common error is adding 20% to 1,200, which gives 1,440 and is wrong — the 20% was taken off the original 1,500, not off the 1,200 you see now.
What is 50% off plus another 20% off?
60% off, not 70%. The second discount applies to the already-reduced price. Multiply what you pay at each step: 0.50 × 0.80 = 0.40, so you pay 40% of the original, meaning a 60% total discount.
If a stock falls 50%, how much must it rise to break even?
100%, because the base has changed. Buy at 100, fall 50% to 50. Getting back to 100 means adding 50 to a base of 50, which is a 100% rise. This is why large losses are harder to recover than they appear.
Do a 50% rise and a 50% fall cancel out?
No. 100 rising 50% becomes 150, and 150 falling 50% becomes 75, not 100. The second calculation runs on a base of 150 rather than 100.
Sources
Rates last verified 25 Aug 2026