Core Web Vitals Estimate Calculator

Work out core web vitals estimate instantly with clear inputs, formula shown and shareable results.

About Core Web Vitals Estimate Calculator

Largest contentful paint is modelled as time to first byte plus the transfer time for above-the-fold assets plus client render time, with Google's thresholds at 2.5 and 4.0 seconds. Transfer time dominates on slow connections, which is why cutting above-the-fold weight moves LCP more than any amount of server tuning.

Formulas

Transfer time

ms = above-the-fold KB x 8 / Mbps

LCP estimate

LCP = TTFB + transfer time + render time

Worked example

With the default values below, Core Web Vitals Estimate Calculator returns Estimated largest contentful paint of 1,720. Change any input above and every figure here is recalculated in your browser.

Inputs used

Page weight above the foldminimum 1 KB: 700 KB
Connection speedminimum 0.1 Mbps: 5 Mbps
Time to first byteminimum 1 ms: 200 ms
Client render timeminimum 0 ms: 400 ms

Results

Estimated largest contentful paint: 1,720 ms
LCP in seconds: 1.720 s
Time spent transferring assets: 1,120 ms
Core Web Vitals rating: Good

Step by step

1Values used
2Transfer time
3LCP estimate
4Estimated largest contentful paint= 1,720 ms
5LCP in seconds= 1.720 s
6Time spent transferring assets= 1,120 ms
7Core Web Vitals rating= Good

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Core Web Vitals Estimate figure worked out?

ms = above-the-fold KB x 8 / Mbps. Largest contentful paint is modelled as time to first byte plus the transfer time for above-the-fold assets plus client render time, with Google's thresholds at 2.5 and 4.0 seconds.

Why does this matter?

Transfer time dominates on slow connections, which is why cutting above-the-fold weight moves LCP more than any amount of server tuning.

What do I need to enter?

Enter page weight above the fold, connection speed, time to first byte, client render time. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.