Period comparison calculator: percent change and percentage points

Published 11 October 2026 · Updated 16 October 2026 · KeySend

Enter last period's value and this period's value. The calculator returns the absolute change and the percent change. Tick the percentage box for rates such as CTR or conversion rate and it also returns the change in percentage points, so a move from 4% to 5% reads as +1 point and +25%.

How to fill in the form

The form accepts zero and positive numbers. Use the same metric, the same account or campaign, and the same currency for both values.

Put the earlier figure in Comparison Period value. This is the period you compare against, for example the previous 30 days.
Put the current figure in Reporting Period value.
If both values are percentages, tick the box and type the percentage as a whole number: 4 for 4%, not 0.04.
Select Calculate.

The formulas

Result	Formula	Example: 4% to 5%
Absolute change	Current − comparison	+1
Relative change (%)	(Current − comparison) ÷ comparison × 100	+25%
Percentage-point change	Current percentage − comparison percentage	+1 percentage point

Worked examples

Hypothetical conversion rate, 4% to 5%: the rate rose by 1 percentage point, which is a 25% relative increase. Writing "conversion rate rose 1%" leaves the reader to guess which one you mean.
Hypothetical CTR, 5% to 4%: a fall of 1 percentage point and a 20% relative decrease. The relative size differs from the 4% to 5% case because the starting value differs.
Hypothetical cost per conversion, 80 to 60: an absolute change of −20 and a relative change of −25%. Percentage points do not apply to money.
Hypothetical conversions, 0 to 5: the absolute change is +5. The relative change is undefined, because you cannot divide by zero.

When the comparison value is zero

The calculator shows the relative change as undefined. In a report, write the two values: "conversions went from 0 to 5." Do not call it a 100% increase. A 100% increase means the value doubled from a positive start.

A very small starting value has a similar problem. A CTR that moves from 0.1% to 0.2% is a 100% increase but only 0.1 percentage points. Show the clicks and impressions behind both rates so the reader can judge the size of the change.

Up is not always good

The calculator gives the sign of the change. You decide what it means. A −25% change in cost per conversion is usually welcome. A −25% change in conversion value is not. Say which direction is better when you write the result.

Check the comparison in Google Ads

In Google Ads, open the date range menu at the top right, turn on Compare, and choose Previous period, Previous year or Custom. Expand a column with the double arrows in its header to see the current value, the previous value, the numerical change and the percentage change. To calculate the rates themselves from totals, use the Google Ads metric calculator.

Calculate with your figures

The calculator code runs in your browser and does not send its inputs to a server.