Web Core Vitals Score
Assess LCP, FID, CLS and get performance recommendations. Enter values for instant results with step-by-step formulas.
Worked Examples
Example 1: E-commerce Product Page Optimization
Problem: An online store has: LCP 3.8s, FID 150ms, CLS 0.18, TTFB 1200ms. Identify priorities and expected improvement.
Solution: Current Assessment:\nLCP: 3.8s → Needs Improvement (threshold: 2.5s)\nFID: 150ms → Needs Improvement (threshold: 100ms)\nCLS: 0.18 → Needs Improvement (threshold: 0.1)\nTTFB: 1200ms → Needs Improvement (threshold: 800ms)\n\nCore Web Vitals: FAILING ❌\n\nPriority Order:\n1. LCP (3.8s) - Highest impact on both UX and SEO\n Actions: Optimize hero image, implement CDN, preload LCP element\n Target: 2.0s\n\n2. CLS (0.18) - Layout shifts hurt conversions\n Actions: Add image dimensions, reserve ad space, font-display swap\n Target: 0.05\n\n3. TTFB (1200ms) - Affects all other metrics\n Actions: Server caching, database optimization, CDN\n Target: 600ms\n\n4. FID (150ms) - Interactivity\n Actions: Code splitting, defer non-critical JS\n Target: 80ms\n\nExpected improvement timelin
Result: All 4 metrics need improvement | LCP and CLS priority | TTFB affects all metrics
Example 2: News Publisher Mobile Performance
Problem: A news site scores well on desktop but mobile shows: LCP 4.5s, FID 280ms, CLS 0.3. Heavy on ads and dynamic content.
Solution: Mobile-Specific Issues:\nLCP: 4.5s → POOR (mobile devices slower)\nFID: 280ms → POOR (heavy JS execution)\nCLS: 0.3 → POOR (ads and lazy content)\n\nRoot Cause Analysis:\n1. LCP issue: Large hero images not optimized for mobile\n2. FID issue: Third-party ad scripts blocking main thread\n3. CLS issue: Ads loading without reserved space\n\nMobile-Specific Fixes:\n\n1. LCP Optimization\n- Serve mobile-sized images (srcset)\n- Preload critical assets\n- Reduce hero image quality for mobile\n- Lazy load below-fold images\nTarget: Under 2.5s\n\n2. FID Optimization\n- Defer ad loading until after LCP\n- Use async/defer for all scripts\n- Implement lazy ad loading (intersectionObserver)\n- Consider header bidding timeout reduction\nTarget: Under 100ms\n\n3. CLS Optimization\n- Reserve ad slot dime
Result: Mobile failing all CWV | Ad optimization critical | Balance UX vs ad revenue
Example 3: SaaS Application Dashboard
Problem: A SaaS dashboard has: LCP 2.2s (good), FID 50ms (good), CLS 0.05 (good), but INP is 450ms. Users report slowness.
Solution: Current Status:\nLCP: 2.2s → GOOD ✓\nFID: 50ms → GOOD ✓\nCLS: 0.05 → GOOD ✓\nINP: 450ms → POOR ❌\n\nCore Web Vitals: PASSING ✓\nBut INP becoming Core Web Vital in March 2024!\n\nINP Analysis:\nFID measures FIRST interaction\nINP measures ALL interactions\n\nUser reports 'slowness' = ongoing interactivity issues\n\nCommon INP problems in dashboards:\n1. Heavy data tables with slow rendering\n2. Chart libraries blocking main thread\n3. Filter/sort operations not debounced\n4. Real-time updates causing jank\n\nINP Optimization Strategy:\n\n1. Event Handler Optimization\n- Debounce rapid interactions (typing, scrolling)\n- Use requestAnimationFrame for visual updates\n- Break long tasks with setTimeout(0)\n\n2. Rendering Optimization\n- Virtual scrolling for large lists\n- Pagination instead o
Result: Passing CWV now but INP failing | Fix before March 2024 | Dashboard interactions need optimization
Frequently Asked Questions
What are Core Web Vitals?
Core Web Vitals are Google's metrics for measuring user experience: LCP (loading performance), INP (interactivity — replaced FID in March 2024), and CLS (visual stability). They became a ranking factor in 2021 and are measured on real users via Chrome User Experience Report (CrUX).
What's a good LCP score?
LCP (Largest Contentful Paint) should be under 2.5 seconds for 'good', 2.5-4.0s is 'needs improvement', and over 4.0s is 'poor'. LCP measures when the largest content element becomes visible—typically a hero image or heading.
Why is my CLS score bad?
High CLS (Cumulative Layout Shift) usually comes from: images without dimensions, ads or embeds that load late, dynamically injected content, web fonts causing text reflow, or animations that trigger layout shifts. Always reserve space for dynamic content.
How do Core Web Vitals affect SEO?
Core Web Vitals are a confirmed Google ranking factor. However, they're one of many factors—content relevance still dominates. Passing CWV provides a tiebreaker advantage when competing pages have similar content quality. Poor CWV can hurt rankings, especially on mobile.
What tools measure Core Web Vitals?
Lab tools: Lighthouse, PageSpeed Insights, WebPageTest. Field data: Chrome User Experience Report (CrUX), Search Console Core Web Vitals report. Real User Monitoring: Google Analytics, various RUM providers. Field data matters most for rankings.
Do Core Web Vitals matter for all pages?
Yes, CWV are evaluated per-page, not site-wide. However, if CrUX doesn't have enough data for a specific page, Google may use page-type or site-level data. Focus optimization on high-traffic, high-value pages first.