LCP PageSpeed Checker
Estimate Largest Contentful Paint from server response, hero resource size, render delay, requests, and connection speed. Use the breakdown to decide whether to fix TTFB, compress the LCP asset, or reduce render-blocking work first.
Inputs
Cached hero asset
Repeat visit advantage
Estimated LCP
Planning modelRating
Target 2.5s or less for a Good score.
Top fixes
- • Reduce above-the-fold requests and combine assets.
LCP breakdown
| LCP factor | Estimate | Optimization signal |
|---|---|---|
| Server response | 0.25s | Improve cache hit rate, backend latency, CDN routing, and origin response |
| Hero download | 0.29s | Compress the LCP image, resize it, preload it, and serve a modern format |
| Render delay | 0.70s | Remove render-blocking CSS and reduce main-thread JavaScript before the hero paints |
| Total LCP estimate | 2.04s | Likely acceptable for a planning estimate |
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About LCP and PageSpeed
Largest Contentful Paint measures when the main content becomes visible. It is a Core Web Vital and a key signal of perceived load speed. Use this estimator to identify which part of the load is slowest.
Calculation method: estimated LCP equals server response time plus render delay plus hero resource download time plus request overhead. Hero download time is resource size in megabytes multiplied by 8 and divided by connection speed in Mbps, with a cache adjustment when the hero asset is already cached.
Worked example: if TTFB is 250 ms, render delay is 700 ms, the hero resource is 0.9 MB, connection speed is 15 Mbps, and there are 20 above-the-fold requests, the estimate combines server, rendering, download, and request costs. If the request overhead is largest, reduce above-the-fold scripts and tags before compressing smaller assets.
Quick guidance
- Keep hero images compressed and sized for the viewport.
- Preload critical assets and minimize render-blocking scripts.
- Use a CDN and caching to reduce server response time.
What is a good LCP score?
Good is 2.5 seconds or less. Between 2.5 and 4 seconds needs improvement, and above 4 seconds is poor. Treat the estimate as a planning signal, then confirm with field data or a lab tool.
Is this a real performance test?
No. This is a planning estimator based on inputs. Use Lighthouse, PageSpeed Insights, WebPageTest, Chrome DevTools, or real-user monitoring for lab and field validation.
What impacts LCP the most?
Slow server response, large hero images, render-blocking CSS, heavy JavaScript, font loading, and too many above-the-fold requests are common causes. The best first fix is usually the largest part of the breakdown.
About This Calculator
Free LCP PageSpeed checker. Estimate Largest Contentful Paint from TTFB, resource size, render delay, and network speed. Get instant performance rating and optimization tips.
Frequently Asked Questions
Is this LCP PageSpeed Checker a real Lighthouse test?
No. It is a planning estimator for Largest Contentful Paint. It helps you model likely pressure from TTFB, hero resource size, render delay, request count, and connection speed. Confirm final performance with Lighthouse, PageSpeed Insights, WebPageTest, Chrome DevTools, or real-user monitoring data.
What should I fix first when LCP is slow?
Fix the largest contributor in the breakdown first. If server response is high, improve caching, CDN routing, and backend latency. If hero download is high, compress and resize the LCP image. If render delay is high, remove render-blocking CSS and defer non-critical JavaScript.
What is a good LCP target?
A common target is 2.5 seconds or less for a good LCP experience. Between 2.5 and 4 seconds usually needs improvement, and above 4 seconds is poor. Field data matters most because user devices and networks vary.
What formula does the LCP PageSpeed Checker use?
The estimate is LCP = TTFB + render delay + hero resource download time + request overhead. Hero download time is approximated from resource size and connection speed, with a cache adjustment when the hero asset is cached.
Do my inputs leave the browser?
No. The estimate is calculated in your browser from the values you enter. The tool does not need to upload TTFB, resource size, render delay, connection speed, request count, or cache assumptions to SuperCalc.
The SuperCalc Editorial Team maintains calculator interfaces, formula notes, examples, and supporting explanations. Methods, assumptions, source links, and review depth vary by calculator and are documented on the relevant page where available.