Percentage Change Calculator

What this calculator does

This calculator computes the signed percent change between a starting value and an ending value. Positive when the value has grown, negative when it has shrunk, zero when it’s unchanged. The output is direction-aware: the sign of the number, the badge that says “Increase” or “Decrease”, and the plain-English description all agree, so there’s no room for a sign error to slip into a copy-paste.

How to calculate percent change

percent change = ((ending value − starting value) ÷ starting value) × 100

Three arithmetic steps and one interpretation step:

1. Subtract the starting value from the ending value to get the absolute change. The sign of this number carries the direction.
2. Divide by the starting value. The denominator is always the starting value — the same denominator the percent-increase and percent-decrease formulas use.
3. Multiply by 100 to convert the decimal into a percentage.
4. Read the sign: positive means the value went up, negative means it went down, zero means no change.

Worked example

A stock opens the week at $120 and closes at $132:

The same stock the following week opens at $132 and closes at $99:

Notice how the same stock now has a +10 % week and a −25 % week, not a net −15 % movement — sequential percent changes do not add. Compounding them gives 1.10 × 0.75 = 0.825, a 17.5 % net loss.

Real-world use cases

• Stock and crypto price tracking. Daily, weekly, year-to-date change. Direction varies, sign matters, percent change is the convention every market data API returns.
• Weekly metric reports. Sessions, conversion rate, churn, net-new signups. Anything that fluctuates wants a signed number with a comparable denominator from one week to the next.
• A/B test reporting. Variant lift over control. The convention is (variant − control) / control × 100. A negative result is a variant that lost.
• Lab measurements and quality control. Today’s reading vs yesterday’s; this sample vs the reference. Direction tells you whether a process is drifting up or down.
• Personal finance. Net worth change month over month, salary growth year over year.

Percent change vs increase vs decrease vs difference

Four formulas, four questions, often muddled.

• Percent change is signed, denominator = starting value. Use when direction isn’t known in advance, or when you want a single number that captures both magnitude and direction.
• Percent increase is the positive magnitude when the value has gone up. Denominator = starting value. Mathematically identical to a positive percent change.
• Percent decrease is the positive magnitude when the value has gone down. Denominator = starting value. Mathematically identical to the absolute value of a negative percent change.
• Percent difference is symmetric, denominator = average of the two values. Use when neither value is “first”. Never matches percent change for the same two numbers.

Common mistakes to avoid

Confusing percent change with percentage points. If a conversion rate moves from 5 % to 6 %, the percentage point change is +1, but the percent change is +20 %. Both are correct; they answer different questions. The news routinely mixes them up.

Adding percent changes across periods. A +20 % week followed by a −20 % week is not flat — it’s a 4 % net loss (1.20 × 0.80 = 0.96). Percent changes compound multiplicatively.

Comparing percent changes across different baselines. A 10 % increase on $100 is $10. A 10 % increase on $1 000 000 is $100 000. The percentage doesn’t tell you the dollars; report both in high-stakes contexts.

Reporting a percent change with a zero denominator. If the starting value is zero, growth is mathematically undefined (∞). Don’t paper over it with “100 %” or “N/A” — say “increased from zero” and report the absolute number.

Privacy

This calculator runs as a few arithmetic operations in JavaScript on your device. There are no fetch calls, no analytics on the values you enter, no server-side logging.