Free calculator · Engineering
Rate your Core Web Vitals and estimate payback
Enter your field Core Web Vitals and your site's traffic, conversion rate, order value and margin. The calculator rates each metric against Google's thresholds, then shows what a conversion lift you choose would add in revenue and profit, how long the project takes to pay back, and the lift it needs to pay back within a year.
How this is calculated
The calculator does two separate things. It rates your field 75th-percentile LCP, INP and CLS against Google's published thresholds, and it estimates the value of a relative conversion lift that you choose. The ratings are not used in the money: no conversion-per-millisecond formula is applied.
Step by step
- Rate each metric: good at or below the good threshold, poor above the poor threshold, needs improvement in between.
- Revenue today per month = sessions × conversion rate × average order value.
- Extra revenue per month = revenue today × the relative lift you choose. Extra gross profit = extra revenue × gross margin.
- Payback in months = project cost ÷ extra gross profit per month.
- Lift needed to pay back within 12 months = project cost ÷ (12 × revenue today × gross margin).
- First-year profit after the project = 12 × extra gross profit − project cost.
Default assumptions
Assumptions marked adjustable can be changed in the calculator; the others are fixed parts of the model.
| Assumption | Default | Sources |
|---|---|---|
| Sessions per month (example)adjustable | 150,000 a month | None |
| Conversion rate (example)adjustable | 2% | None |
| Average order value (example)adjustable | $120 | None |
| Gross margin (example)adjustable | 40% | None |
| Field p75 LCP (example)adjustable | 3.20 s | |
| Field p75 INP (example)adjustable | 250 ms | |
| Field p75 CLS (example)adjustable | 0.12 | |
| Expected relative conversion liftadjustable | 2% | |
| Project cost (example)adjustable | $30,000 one-off | None |
| LCP good threshold | 2.50 s | |
| LCP poor threshold | 4 s | |
| INP good threshold | 200 ms | |
| INP poor threshold | 500 ms | |
| CLS good threshold | 0.10 | |
| CLS poor threshold | 0.25 |
What this doesn’t model
- The conversion lift is your assumption; the calculator doesn't predict it from your metrics.
- Case studies are cited as examples from other sites and are not used in the maths.
- The lift is applied to every session entered and starts immediately; real changes take time to roll out and measure.
- Search ranking, bounce rate, repeat visits and lifetime value aren't modelled.
- Traffic, prices and margin are held flat; ongoing maintenance costs aren't included.
Sources
- Google (web.dev), How the Core Web Vitals metrics thresholds were defined (21 May 2020). Accessed . At the 75th percentile: LCP good ≤ 2,500 ms, poor > 4,000 ms; INP good ≤ 200 ms, poor > 500 ms; CLS good ≤ 0.1, poor > 0.25. Last updated 7 May 2025.
- Google (web.dev), Web Vitals (4 May 2020). Accessed . Recommends measuring the 75th percentile of page loads, segmented across mobile and desktop; a page passes when all three metrics meet the good threshold.
- Google (web.dev), Interaction to Next Paint (INP). Accessed . INP at or below 200 ms is good, above 200 ms and at or below 500 ms needs improvement, above 500 ms is poor.
- Google (web.dev), Cumulative Layout Shift (CLS). Accessed . Good CLS values are 0.1 or less; poor values are greater than 0.25.
- Google (Chrome for Developers), Overview of CrUX (Chrome UX Report). Accessed . Field data collected from real Chrome users. PageSpeed Insights and Search Console report a site's p75 values from it.
- Google (web.dev), Vodafone: A 31% improvement in LCP increased sales by 8% (17 Mar 2021). Accessed . An A/B test on one landing page with paid traffic split 50/50. One site's result, cited as an example, not used in the maths.
- Deloitte Ireland (commissioned by Google), Milliseconds make millions (24 Mar 2020). Accessed . Across retail, travel, luxury and lead-generation brands over four weeks, a 0.1 s mobile speed improvement went with 8.4% more retail conversions. Observational; cited as an example, not used in the maths.
Last reviewed by the QuantmHill engineering team. Found an error?
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Frequently asked questions
Google's web.dev guidance rates the 75th percentile of field page loads: LCP is good at or below 2.5 seconds and poor above 4 seconds; INP is good at or below 200 milliseconds and poor above 500; CLS is good at or below 0.1 and poor above 0.25. Values in between need improvement. A page passes when all three are good.
Because no published formula turns a faster LCP into a reliable conversion lift for every site. Results depend on the site, its audience and how the test was run. The calculator keeps the lift as your assumption and shows the break-even lift, so you can judge whether the project is worth it even at a modest result.
Vodafone reported on web.dev that a 31% LCP improvement led to 8% more sales in an A/B test on one landing page. Deloitte's Milliseconds Make Millions study, commissioned by Google, linked a 0.1 second mobile speed improvement with 8.4% more retail conversions. These are examples from other sites, not inputs to the maths.
PageSpeed Insights and the Core Web Vitals report in Search Console show your 75th-percentile LCP, INP and CLS from the Chrome UX Report, which is collected from real Chrome users. Your own real-user monitoring works too. Lab scores from a single test run are not the same thing.
Payback is the project cost divided by the extra gross profit per month (extra revenue × gross margin). The break-even lift is the relative lift at which 12 months of extra gross profit equals the project cost.
Effects beyond conversions, such as search visibility, bounce rate or repeat visits; the time a lift takes to appear; ongoing maintenance; and changes in traffic or prices. It also treats the lift as applying to every session you enter.
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