パーセント計算機 (割合・消費税・割引)

Six percentage calculations in one tool

Percentages trip people up not because the math is hard but because the same word covers several different questions. This calculator handles the six most common ones, each with a live, step-by-step formula so you can see how the answer is built rather than just trusting it.

The everyday formulas

• Percentage of a number: X% of Y = (X / 100) * Y. For example, 20% of 80 = 16.
• X is what percent of Y: (X / Y) * 100. For example, 16 is 20% of 80.
• Percentage change: ((new - old) / old) * 100. A rise from 80 to 100 is a 25% increase.
• Percentage difference: compares two values relative to their average, useful when neither is the clear "original".
• Add a percentage: Y + (X% of Y), the basis for adding tax or a markup.
• Subtract a percentage: Y - (X% of Y), the basis for applying a discount.

Percent change vs. percent difference

These two are easy to confuse. Percent change has a clear before and after, so the original value is the denominator (prices, populations, scores over time). Percent difference compares two values where neither is "first", so it divides by their average. Using the wrong one is a frequent source of misleading statistics.

Real-world examples

Working out a 15% discount on a $60 item (subtract a percentage), adding 8% sales tax to a bill (add a percentage), checking that a $45 result is 75% of a 60-point test (X is what percent of Y), or measuring how much a stock moved from open to close (percentage change), all reduce to one of the six modes above. Pick the mode that matches your question and the formula does the rest.