Percent difference between two numbers

About the percent difference calculator

Enter two numbers and this calculator reports how far apart they are, three ways at once:

• Absolute difference — |a − b|, plus the signed difference b − a and the direction of the move (increase, decrease, or no change).
• Percent difference (symmetric) — |a − b| ÷ |(a + b) / 2| × 100. The gap is measured against the midpoint of the two values, so neither value is privileged as "the" baseline and swapping a and b gives the same answer. Use this when the two numbers are just two measurements, with no before/after relationship — two lab readings, two quotes, two sensors.
• Percent change (directional) — (b − a) ÷ |a| × 100, the change from a to b measured against the starting value, reported in both directions along with the ratio b ÷ a. Use this when a really is the baseline: last month's revenue, the old price, the control group.

Set Measures to report to difference or change to show only one block, and Decimal places (0–10) to control display precision. The arithmetic itself is always done at full double precision; the setting only rounds what is printed. Everything runs locally in your browser — nothing is uploaded, and it keeps working offline once the page has loaded.

Worked example: 70 and 85

The two percentages differ because they divide by different things: 77.5 (the midpoint) versus 70 (the starting value). Both are correct answers to different questions, which is why they are shown side by side.

More examples

• 5 and 7 → percent difference 33.33% (mean 6, difference 2). Swap them and it is still 33.33%.
• 25 and 75 → percent difference exactly 100% — the symmetric measure hits 100% precisely when one value is three times the other, not when it doubles.
• 120 → 100 → percent change −16.67%, ratio 0.83. Reverse the direction and the same pair is a +20% change, because the baseline changed.
• −70 and −85 → percent difference 19.3548%, the same as the positive pair: magnitudes are compared, so a sign flip on both values changes nothing.
• 10 and 6 at 0 decimal places → percent difference 50%.

Limits and edge cases

• Percent difference is undefined when a + b = 0 (for example 5 and −5). The mean is zero, so there is nothing to measure the gap against. In all mode the measure is omitted with a note; in difference mode it is an error, because that is the measure you asked for.
• Percent change is undefined when a = 0. There is no baseline to divide by — any move away from zero is an infinite percent change. Same handling: a note in all mode, an error in change mode. When b = 0, only the reverse direction b → a and the ratio drop out.
• Values more than 10× apart get an advisory note. The symmetric percent difference saturates towards 200% for very unequal values, so it stops discriminating; percent change is the more informative measure there.
• 200% is the ceiling of the symmetric measure for two same-signed values, reached in the limit as one of them approaches zero.
• Negative values are accepted everywhere. Percent change divides by |a| so that the sign of the result always matches the sign of b − a, rather than flipping on a negative baseline.
• NaN and infinity are rejected with a message naming the field.
• Decimal places accept 0 to 10. Anything higher is an error rather than a silently clamped value.
• The measure is unit-free: both values must be in the same unit, and the result is a percentage either way. Percentages themselves are numbers, so comparing 20 and 30 percent gives a percent difference of 40% — a difference of 10 percentage points.

FAQ