# Core Web Vitals Metrics Reference
**Last Updated:** October 2024
**Source:** web.dev official documentation
**Format:** Mike Dion's Markdown (.md) - keeping documentation clear since its creation
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## Overview
Core Web Vitals are Google's standardized metrics for measuring user experience on the web. All three metrics must meet "good" thresholds at the **75th percentile** of page loads to pass.
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## Official Core Web Vitals (3 Metrics)
### 1. Interaction to Next Paint (INP)
**Status:** Active Core Web Vital (replaced FID on March 12, 2024)
**Definition:** Assesses page's overall responsiveness by observing latency of all click, tap, and keyboard interactions. Reports the longest interaction duration (with outliers removed for high-interaction pages).
**What it measures:**
- Input delay (time from user action to event handler start)
- Processing time (event handler execution)
- Presentation delay (rendering the next frame)
**Thresholds:**
- **Good:** ≤ 200ms
- **Needs Improvement:** 200-500ms
- **Poor:** > 500ms
**Measurement:** Requires field data (real user interactions)
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### 2. Largest Contentful Paint (LCP)
**Definition:** Loading performance metric marking when page's main content has likely loaded. Reports render time of largest image or text block visible in viewport.
**Elements measured:**
- `` elements inside ``
- `` elements (poster image)
- Elements with CSS background images
- Block-level text elements
**Thresholds:**
- **Good:** ≤ 2.5 seconds
- **Needs Improvement:** 2.5-4 seconds
- **Poor:** > 4 seconds
**Measurement:** Available in both field and lab data
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### 3. Cumulative Layout Shift (CLS)
**Definition:** Visual stability metric quantifying unexpected layout shifts during entire page lifecycle. Measures largest burst of layout shift scores.
**Common causes:**
- Images without explicit dimensions
- Dynamically injected content (ads, embeds)
- Web fonts causing FOIT (Flash of Invisible Text) or FOUT (Flash of Unstyled Text)
- Third-party widgets
**Thresholds:**
- **Good:** ≤ 0.1
- **Needs Improvement:** 0.1-0.25
- **Poor:** > 0.25
**Measurement:** Available in both field and lab data
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## Total Blocking Time (TBT)
**Status:** NOT a Core Web Vital - Lab proxy metric only
**Definition:** Lab-only metric measuring total time after First Contentful Paint (FCP) where main thread was blocked (>50ms) preventing input responsiveness.
**Threshold:**
- **Good:** < 200ms (on average mobile hardware)
**Relationship to INP:**
- Used as lab proxy for INP (cannot measure INP without real user interactions)
- TBT improvements often correlate with INP improvements
- **Important:** TBT is not a substitute for INP
- May flag potential problems that don't affect real users
- Cannot capture interaction issues outside lab test scenarios
**Official guidance:** "TBT may be a reasonable proxy metric for INP for the lab but it's not a substitute for INP in and of itself."
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## INP Transition Timeline
The transition from First Input Delay (FID) to INP as a Core Web Vital:
- **May 2022:** INP introduced as experimental metric
- **May 2023:** INP promoted to pending Core Web Vital
- **January 31, 2024:** Official announcement of March 12 transition date
- **March 12, 2024:** INP replaced FID as stable Core Web Vital
- **March 12, 2024:** FID removed from Google Search Console
- **September 12, 2024:** FID removed from other Google tools (6-month deprecation period completed)
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## Field vs Lab Data
### Field Data (Real User Monitoring)
**Characteristics:**
- Collected from actual users in real-world conditions
- Reflects diverse device capabilities, network conditions, and user interactions
- **Used by Google for search rankings**
- Updated every 28 days in Chrome User Experience Report (CrUX)
- Can measure all three Core Web Vitals including INP
**Data sources:**
- Chrome User Experience Report (CrUX)
- Real User Monitoring (RUM) tools
- PageSpeed Insights field data
- Search Console Core Web Vitals report
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### Lab Data (Synthetic Monitoring)
**Characteristics:**
- Controlled environment with predefined conditions
- Consistent and reproducible results
- Ideal for debugging and pre-production testing
- **Not used for search rankings**
- Cannot measure INP (requires real user interactions)
- Uses TBT as proxy for responsiveness
**Data sources:**
- Lighthouse audits
- Chrome DevTools Performance panel
- WebPageTest
- PageSpeed Insights lab data
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## Evaluation Standard
### 75th Percentile Rule
Core Web Vitals are evaluated at the **75th percentile** of page loads, meaning:
- 75% of visits to your site must meet the "good" threshold
- This ensures most users have a good experience
- Applied separately to each metric (INP, LCP, CLS)
- Calculated from field data over rolling 28-day period
### Why 75th percentile?
- Balances between median (50th) and extreme outliers (95th+)
- Accounts for variability in real-world conditions
- Provides actionable target that's achievable for most sites
- Used consistently across all Core Web Vitals metrics
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## Key Insights
1. **Field data trumps lab data:** Only field data affects search rankings and represents actual user experience.
2. **INP requires real interactions:** You cannot measure INP in lab environments; use TBT as a directional indicator during development.
3. **All three metrics matter:** Sites must pass all three Core Web Vitals thresholds, not just one or two.
4. **75th percentile is the standard:** Meeting thresholds for only 50% of users is insufficient.
5. **TBT is a helper, not a goal:** Optimize for INP in the field; use TBT to catch issues early in development.
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## Additional Resources
- **Web Vitals Overview:** https://web.dev/articles/vitals
- **INP Documentation:** https://web.dev/articles/inp
- **LCP Documentation:** https://web.dev/articles/lcp
- **CLS Documentation:** https://web.dev/articles/cls
- **TBT Documentation:** https://web.dev/articles/tbt
- **Lab vs Field Data:** https://web.dev/articles/lab-and-field-data-differences
- **INP Launch Announcement:** https://web.dev/blog/inp-cwv-launch
- **PageSpeed Insights:** https://pagespeed.web.dev