Web Performance Budget Calculator
Define resource size budgets (JS, CSS, images, fonts) and Core Web Vitals targets. Enter your actual sizes to instantly see pass/fail status, estimated load times, and what's over budget.
Frequently Asked Questions
What is a web performance budget?▼
A performance budget is a set of limits on metrics that affect web performance — most commonly resource sizes (JavaScript, CSS, images) and Core Web Vitals. Teams use budgets to enforce performance standards in CI/CD pipelines: a PR that exceeds the budget fails the build. This prevents gradual performance degradation over time.
What are the recommended JavaScript bundle size budgets?▼
Google recommends keeping total JavaScript under 300KB (compressed). For a fast Time to Interactive on 3G, aim for under 170KB. In practice, a reasonable budget for a modern SPA is 200KB compressed JS, 50KB CSS, and 400KB images per page. Adjust based on your actual traffic — a developer tools site has a different audience than an e-commerce store.
What are Core Web Vitals and why do they matter for SEO?▼
Core Web Vitals (LCP, FID/INP, CLS) are Google's user experience signals used directly in search ranking. LCP (Largest Contentful Paint) measures perceived load speed, FID/INP measures interactivity, and CLS measures visual stability. Pages scoring 'Good' on all three have a ranking advantage. As of 2024, INP (Interaction to Next Paint) has replaced FID as an official CWV.
How do I estimate page load time from file sizes?▼
Rough formula: load time (ms) = (total size in KB × 8) / connection speed in Kbps × 1000. A 500KB page on 4G (25 Mbps) takes about 160ms for raw transfer — but actual Time to Interactive is 3-10× longer due to DNS, TCP, TLS handshake, parsing, and rendering. Use this calculator for budgeting guidance, not precise predictions.
What tools can measure my actual Core Web Vitals?▼
Use Google PageSpeed Insights for field and lab data, Chrome DevTools (Lighthouse panel) for local testing, and Web Vitals Chrome extension for real-time measurement. For CI/CD integration, use Lighthouse CI. For production monitoring, Google Search Console's Core Web Vitals report shows real-user data.
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