A/B Testing Core Web Vital Calculator
Compare control and variant Core Web Vitals to estimate whether an A/B test improves loading, responsiveness, and visual stability.
Overview
Use this A/B Testing Core Web Vital Calculator to compare a control page with a test variant using Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). It shows the percentage change in each metric and an equal-weight combined index to help identify the likely performance direction of the test.
How it works
The calculator measures the percentage reduction from the control to the variant for each Core Web Vital. It also creates a combined index by comparing LCP with 2.5 seconds, INP with 200 milliseconds, and CLS with 0.1, then averaging those three ratios. A lower index is better: an index of 1.00 is equivalent to being at the selected good thresholds on average. The overall improvement is the reduction in this index from control to variant.
How to use this calculator
- 1Enter the control LCP value in seconds.
- 2Enter the variant LCP value from the same measurement source.
- 3Add the control and variant INP values in milliseconds.
- 4Add the control and variant CLS scores.
- 5Review the individual changes and overall Core Web Vitals improvement.
Example Calculation
Control LCP
2.8
Variant LCP
2.4
Control INP
240
Variant INP
180
Control CLS
0.12
Variant CLS
0.08
Overall CWV improvement
+24.4%
The variant improves LCP by about 14.3%, INP by 25.0%, and CLS by 33.3%. Its combined Core Web Vitals index is about 25.5% lower than the control.
Frequently asked questions
What Core Web Vitals does this calculator compare?
It compares Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS), the three current Core Web Vitals.
Is a positive improvement percentage good?
Yes. A positive percentage means the variant's metric is lower than the control's. Lower values are generally better for LCP, INP, and CLS.
What is a good Core Web Vitals score?
Common good thresholds are LCP of 2.5 seconds or less, INP of 200 milliseconds or less, and CLS of 0.1 or less. Performance should also be assessed for relevant user segments.
What does the combined CWV index mean?
It is an equal-weight comparison of each metric against its good threshold. Lower is better, and 1.00 indicates that the average normalized result is at the thresholds used by this calculator.
Does a better CWV result prove that my A/B test won?
No. Better Core Web Vitals do not by themselves prove a conversion or revenue effect. Review statistical significance, guardrail metrics, and the test's primary business outcome separately.
Should I use lab data or field data?
Use the same data type for both versions. Field data reflects real-user conditions, while lab data is useful for controlled debugging and early checks.
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Assumptions and warnings
Assumptions
- The control and variant measurements are collected over comparable dates, devices, locations, and traffic sources.
- Lower LCP, INP, and CLS values are treated as better user-experience outcomes.
- The combined index gives LCP, INP, and CLS equal weight relative to the commonly used good thresholds.
- Results compare reported metric values and do not test statistical significance or business conversion impact.
Warnings
- Core Web Vitals can vary with traffic mix, device type, network conditions, and measurement method.
- Use a sufficiently large and representative sample before making rollout decisions.
- This calculator is an estimate and does not replace a statistical A/B test analysis.