ToolPetal
Percentage calculator
| Step | Value |
|---|
Client-side only. Plain arithmetic, not tax, loan, or investment advice. Rounding shown to a few decimals; your bank or POS may round differently.
Pick the mode that matches the sentence you mean
Percent questions sound similar and are easy to mix up. This page keeps five jobs separate so you do not paste the wrong answer into a spreadsheet.
| Mode | Sentence | Core formula |
|---|---|---|
| X% of Y | What is 15% of 200? | Y × (X / 100) |
| X is what % of Y | 45 is what % of 180? | (X / Y) × 100 |
| % change A to B | 80 rose to 100. What % change? | ((B − A) / A) × 100 |
| Increase / decrease | Raise 250 by 10%, or cut it by 10% | N × (1 ± X/100) |
| Tip / discount | Bill 48.50 with 18% tip, or 20% off a price | Same shape as increase / decrease |
Tip and discount use the same multiply-by-(1 ± p/100) shape as increase / decrease. They exist as a separate mode so the labels read like a receipt instead of a generic adjuster.
Worked table: same story, three wrong answers people mix
Take a price that moves from 80 to 100. People often ask three different questions and expect one number.
| Question | Correct mode | Result |
|---|---|---|
| What is 25% of 80? | X% of Y | 20 |
| 20 is what % of 80? | X is what % of Y | 25% |
| 80 to 100: what % change? | % change A to B | +25% |
The absolute difference is 20 either way. The percent of and percent change answers match here only because 20 / 80 = 0.25. If the new value were 60 (a drop of 20), percent of old would still be 25% of 80, but percentage change would be −25%. Direction matters.
Second worked case for tip mode: original 48.50, tip 18%. Tip amount = 48.50 × 0.18 = 8.73. New total = 57.23. A 18% discount on the same sticker yields new price 39.77 and savings 8.73.
Where percent math quietly fails
Division by zero: "X is what % of 0" and "% change from 0" are undefined. The tool refuses those inputs instead of inventing infinity.
Stacked percents are not additive. A 10% increase followed by a 10% decrease does not return to the start: 100 → 110 → 99. Two successive 10% discounts are not 20% off; they are 19% off the original.
This page does not amortize loans, split CGST/SGST, or compound over years. Use the related finance tools for those jobs. Rounding here is display-side (typically a few decimals). POS systems and payroll may banker's-round or truncate.
FAQ
What percentage modes does this calculator support?
Five modes: what is X% of Y; X is what % of Y; percentage change from A to B; increase or decrease a number by X%; and tip or discount (original price plus or minus a percent).
How is percentage change calculated?
Percentage change = ((new − old) / old) × 100. Positive means an increase; negative means a decrease. Division by zero (old = 0) is rejected because the change is undefined.
Is tip mode the same as discount mode?
Same math shape, opposite intent. Tip adds percent to the original (new = original × (1 + p/100)). Discount subtracts percent (new = original × (1 − p/100)). Both show the absolute dollar or rupee difference.
Are my numbers uploaded?
No. Inputs and results stay in this browser tab. Nothing is sent to ToolPetal.
Does this replace a loan EMI or GST calculator?
No. This page is plain percentage arithmetic. For loan schedules use the EMI calculator; for India GST add or remove use the GST calculator; for compounding growth use compound interest or SIP.