Calculate total blocking time from long tasks on the main thread.
Only the part of each task beyond 50 ms counts as blocking, because shorter tasks do not perceptibly delay input. Splitting one 300 ms task into six 50 ms tasks removes the blocking time entirely. Total blocking time is the lab proxy for INP, which is why improving it in development usually improves real-user responsiveness.
Total Blocking Time
TBT = Σ(task duration − 50 ms) for every task longer than 50 ms
TBT = Σ(task duration − 50 ms) for every task longer than 50 ms Only the part of each task beyond 50 ms counts as blocking, because shorter tasks do not perceptibly delay input. Splitting one 300 ms task into six 50 ms tasks removes the blocking time entirely.
Total blocking time is the lab proxy for INP, which is why improving it in development usually improves real-user responsiveness.
This calculator takes 3 inputs: Number of long tasks, Average task duration, Longest task duration. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.