What Is Total Blocking Time (TBT)

Short answer

Total Blocking Time measures the sum of time between First Contentful Paint and Time to Interactive where the main thread was blocked for more than 50 milliseconds.

How it works

Long JavaScript tasks block user input and rendering. TBT adds up the duration beyond 50ms for each long task. It correlates strongly with First Input Delay.

Example

If the main thread runs a 90ms task, 40ms counts toward TBT. Two such tasks contribute 80ms to the total.

Why it matters

TBT bridges lab and field metrics. It is measurable in Lighthouse and predicts how interactive the page will feel to real users.