Total Blocking Time Calculator

Calculate total blocking time from long tasks on the main thread.

About Total Blocking Time Calculator

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.

Formula

Total Blocking Time

TBT = Σ(task duration − 50 ms) for every task longer than 50 ms

long task: Any main-thread task exceeding 50 ms
blocking time: Only the portion beyond 50 ms counts

Worked example

With the default values below, Total Blocking Time Calculator returns Total Blocking Time of 420. Change any input above and every figure here is recalculated in your browser.

Inputs used

Number of long tasksminimum 0: 6
Average task durationminimum 0 ms: 120 ms
Longest task durationminimum 0 ms: 380 ms

Results

Total Blocking Time: 420 ms
Rating: Needs improvement
Blocking From the Longest Task: 330 ms
Total Task Time: 720 ms
Milliseconds to Save to Reach Good: 220 ms

Step by step

1Values used
2Total Blocking Time
3Total Blocking Time= 420 ms
4Rating= Needs improvement
5Blocking From the Longest Task= 330 ms
6Total Task Time= 720 ms
7Milliseconds to Save to Reach Good= 220 ms

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Total Blocking Time calculated?

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.

Why does Total Blocking Time matter?

Total blocking time is the lab proxy for INP, which is why improving it in development usually improves real-user responsiveness.

What values do I need to enter?

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.