Percentage Increase/Decrease Calculator

Percentage change is one of those calculations that sounds simple until you are staring at a spreadsheet trying to decide whether to divide by the original, the average, or the absolute value. The Percentage Increase / Decrease Calculator removes the guesswork by keeping four variants in separate modes, each labelled for what you actually have in hand.

Most use cases fall into one of two buckets: working forward (you have a before and an after, and want the percentage) or working backward (you have an endpoint and a rate, and need to recover the starting figure). Both directions are covered. Monthly revenue, exam scores, stock prices, unit costs — the arithmetic is identical regardless of domain.

Worked Example: A Quarterly Sales Figure

Load Sample in % Change (A → B) mode fills the fields with 240 and 312. The formula applied is ((312 − 240) / |240|) × 100, which gives 30% — a 30% increase. The breakdown line reads 30% increase (from 240 to 312) so the direction is always spelled out rather than left to the sign alone.

• Switch to New Value After Change and load its sample (starting value 85, change −15%) to see the reverse direction: 85 × (1 + (−15/100)) = 72.25.
• Reverse: Find Original solves for what value, after a 10.5% increase, produces 1326. The answer is 1326 / 1.105 = 1200. This is the mode price-tag shoppers need when a sticker says "sale price after 12% off" and they want to know the original.
• Difference as a % answers a subtly different question: what fraction of the reference value is the raw gap? With 4800 and 5040 the gap is 240, which is exactly 5% of 4800. The denominator is always the reference (A), not the comparison.

How Each Formula Differs

All four modes ultimately involve a ratio, but what sits in the denominator changes the meaning of the result.

• % Change divides by |A|, the absolute value of the original. This ensures that a move from −50 to −25 reports +50%(correctly, a halving of the negative magnitude) rather than −50%, which the raw-value denominator would give.
• Find Original inverts the multiplication: original = result / (1 + P/100). A −100% entry is blocked with the message A −100% change means the original became 0 — no single original.
• Results are trimmed to 4 decimal places, with trailing zeros removed, so 30.0000 prints as 30 and 72.2500 prints as 72.25.
• The expandable Show formulas panel at the bottom of the tool lists the algebraic expression for all four modes. Most first-time visitors scroll straight past it.

Edge Cases: Zero, Negatives and Very Large Numbers

• Any mode that divides by A raises Original value cannot be zero — division by zero. when A is 0. Percentage change from zero is undefined, not infinite.
• Negative starting values are fully supported. Going from −200 to −100 is a 50% increase, not a decrease, because the magnitude halved. The |A| denominator handles this without any sign-correction branch.
• Very large inputs (beyond JavaScript's Number.MAX_SAFE_INTEGER, approximately 9 quadrillion) will silently lose precision in the mantissa. For figures of that scale, exact integer arithmetic is the right tool and this one is not it.
• All computation runs locally. Neither input value nor any result is transmitted anywhere.