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JavaScript Payload Calculator

Assess JavaScript payload size and its parse and execution cost.

About JavaScript Payload Calculator

Compression reduces transfer time but not parse time, because the engine works on the expanded code. That is why bundle size matters twice, and why the parse cost dominates on mobile. JavaScript is byte-for-byte the most expensive resource type, costing far more per kilobyte than images because it must be parsed and executed as well as downloaded.

Formula

JavaScript Payload

Cost = compressed size ÷ bandwidth + uncompressed size ÷ parse rate × CPU throttle

parse cost
Depends on uncompressed size, since the engine parses the expanded code
mobile budget
About 170 KB compressed is the commonly cited target

Worked example

With the default values below, JavaScript Payload Calculator returns Total JavaScript Cost of 3,216. Change any input above and every figure here is recalculated in your browser.

Inputs used

Uncompressed bundle sizeminimum 0 KB
900 KB
Compression savingaccepts 0–100 %
70 %
Parse rateminimum 0.01 KB/ms
1.2 KB/ms
Mobile CPU slowdownaccepts 1–10 x
4 x
Connection speedminimum 0.01 Mbps
10 Mbps

Results

Total JavaScript Cost
3,216 ms
Transfer Time
216 ms
Parse and Execute Time on Mobile
3,000 ms
Compressed Size
270.0 KB
Verdict
Above budget — consider code splitting

Step by step

  1. 1Values used
  2. 2JavaScript Payload
  3. 3Total JavaScript Cost= 3,216 ms
  4. 4Transfer Time= 216 ms
  5. 5Parse and Execute Time on Mobile= 3,000 ms
  6. 6Compressed Size= 270.0 KB
  7. 7Verdict= Above budget — consider code splitting

Assumptions and limits

  • The result is derived only from the 5 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 1 decimal place, 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 JavaScript Payload calculated?

Cost = compressed size ÷ bandwidth + uncompressed size ÷ parse rate × CPU throttle Compression reduces transfer time but not parse time, because the engine works on the expanded code. That is why bundle size matters twice, and why the parse cost dominates on mobile.

Why does JavaScript Payload matter?

JavaScript is byte-for-byte the most expensive resource type, costing far more per kilobyte than images because it must be parsed and executed as well as downloaded.

What values do I need to enter?

This calculator takes 5 inputs: Uncompressed bundle size, Compression saving, Parse rate, Mobile CPU slowdown, Connection speed. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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