A/B Testing Core Web Vital (Monthly) Calculator
Estimate how an A/B test may change the number of monthly visits with a good Core Web Vitals experience.
Overview
Use this A/B Testing Core Web Vital (Monthly) Calculator to translate control and variation Core Web Vitals pass rates into an estimated monthly experience impact. Enter eligible traffic, the test split, and the percentage of sessions rated good for each experience.
How it works
The calculator splits eligible monthly sessions between the control and variation using the selected allocation. It multiplies each group’s session count by its good Core Web Vitals rate to estimate good visits during the test. For the rollout estimate, it applies each rate to all eligible monthly sessions and subtracts the control result from the variation result. The rate change is shown in percentage points, not as a relative percentage increase.
How to use this calculator
- 1Enter the monthly number of sessions eligible for the experiment.
- 2Set the percentage of traffic assigned to the variation.
- 3Add the observed good Core Web Vitals rate for the control.
- 4Add the observed good Core Web Vitals rate for the variation.
- 5Review the estimated good-visit change at full rollout and the blended test rate.
Example Calculation
Monthly eligible sessions
100000
Variation traffic allocation
50%
Control good CWV rate
65%
Variation good CWV rate
72%
Estimated monthly additional good CWV visits
+7,000 visits
With a 65% control rate and a 72% variation rate, a full variation rollout is estimated to create 7,000 additional monthly visits with good Core Web Vitals. The blended rate in a 50/50 test is 68.5%.
Frequently asked questions
What is a good Core Web Vitals rate?
It is the share of measured sessions or page views that meet the good threshold for the Core Web Vitals criteria used in your reporting. Use the same definition for both test groups.
Why does the calculator use percentage points?
Percentage points show the direct difference between rates. For example, moving from 65% to 72% is a 7 percentage-point increase.
Does this calculator prove that the variation caused the change?
No. It converts the rates you enter into an estimated traffic impact. You should assess sample size, test duration, confidence, and possible confounding factors separately.
What traffic should I enter?
Enter monthly sessions that are genuinely eligible to see either test experience. Excluding ineligible traffic makes the estimate more representative of the rollout audience.
Can I use field data from different devices or countries?
Yes, but use comparable segments for the control and variation. If performance differs substantially by device, geography, or page type, calculate each segment separately for a clearer estimate.
What does a negative additional good-visits result mean?
It means the variation's entered good CWV rate is lower than the control rate, so a full rollout would be expected to reduce good experiences at the stated traffic level.
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Assumptions and warnings
Assumptions
- The control and variation good CWV rates are based on comparable, representative measurement periods.
- Eligible monthly session volume remains broadly similar after a full rollout.
- A good CWV rate is treated as the percentage of sessions meeting your chosen Core Web Vitals assessment criteria.
- The calculation estimates observed experience volume and does not establish whether a test result is statistically significant.