Percentage Calculator

How to Use the Percentage Calculator

This percentage calculator provides three different calculation modes to handle all common percentage problems. Select a tab to switch between modes and enter your values. Results update in real-time as you type, with no need to click a calculate button. Each mode has its own clear button to reset the inputs. All calculations are performed entirely in your browser.

Three Calculation Modes

The first mode answers "What is X% of Y?" - enter a percentage and a value to find the result. The second mode answers "X is what percent of Y?" - enter two values to find what percentage the first is of the second. The third mode calculates the percentage change between two values, showing whether it is an increase or decrease with color-coded output (green for increase, red for decrease).

Practical Applications

Percentage calculations are used extensively in everyday life and business. Common use cases include calculating discounts and sales tax, determining tip amounts, computing profit margins and markup percentages, analyzing investment returns, converting between fractions and percentages, and comparing data changes over time. The percentage change mode is particularly useful for analyzing financial performance, price changes, and statistical variations between time periods.

パーセント計算ツールの使い方

3つのタブで「YのX%はいくつ?」「XはYの何%?」「XからYへの増減率は?」を切り替えて計算できます。 入力と同時に結果が表示され、全ての計算はブラウザ内で完結します。 割引価格の計算、達成率の確認、前年比の算出などにご利用ください。

よくある落とし穴と対処 / Common Pitfalls

計算式とコード例 / Formulas & Code

// YのX%        → Y × (X / 100)
// XはYの何%     → (X / Y) × 100
// 増減率        → ((新 − 旧) / 旧) × 100

// JavaScript: 浮動小数点の丸め誤差に注意
const pct = (x, y) => Math.round((x / y) * 1000) / 10;  // 小数1桁

-- SQL: 構成比の算出
SELECT category, COUNT(*) * 100.0 / SUM(COUNT(*)) OVER () AS pct
FROM products GROUP BY category;

計算例 / Examples

8,500円の30%引き → 8,500 × 0.7 = 5,950円
売上 120万 → 150万の増減率 → (150−120)/120 = +25%
税込 9,900円の税抜価格(10%) → 9,900 ÷ 1.1 = 9,000円
達成率: 目標80件・実績68件 → 68/80 = 85%

Why browser-only?

The numbers you punch into a percentage calculator are often unannounced figures: sales results, salary changes, margins, or budget drafts. This calculator runs entirely in your browser — values are never transmitted or stored anywhere, and the page keeps working offline. Check the DevTools Network tab while you calculate: zero requests.

計算する数字は未公開の売上・予算・利益率であることも多いもの。 本ツールは計算がブラウザ内で完結し、入力値が外部に送信されることはありません。