Web Font Performance Calculator

Assess the load cost of web fonts and the saving from subsetting.

About Web Font Performance Calculator

Fonts block text rendering unless font-display is set to swap, so their load time directly delays contentful paint. Subsetting to the required character set is the single largest saving available. Each additional weight or style is a separate file and a separate request, which is why limiting to two or three is standard practice.

Formula

Web Font Performance

Font load time = subset size ÷ bandwidth + one round trip

subsetting: Removing unused glyphs, typically saving 60 to 80 per cent
font-display: Controls whether text is visible while fonts load

Worked example

With the default values below, Web Font Performance Calculator returns Font Load Time of 161. Change any input above and every figure here is recalculated in your browser.

Inputs used

Number of font filesminimum 0: 4
Average font file sizeminimum 0 KB: 90 KB
Saving from subsettingaccepts 0–100 %: 65 %
Connection speedminimum 0.01 Mbps: 10 Mbps
Round-trip latencyminimum 0 ms: 60 ms

Results

Font Load Time: 161 ms
Total Font Weight: 360.0 KB
Weight After Subsetting: 126.0 KB
Time Saved by Subsetting: 187 ms
Recommendation: Font payload is still large — subset more aggressively

Step by step

1Values used
2Web Font Performance
3Font Load Time= 161 ms
4Total Font Weight= 360.0 KB
5Weight After Subsetting= 126.0 KB
6Time Saved by Subsetting= 187 ms
7Recommendation= Font payload is still large — subset more aggressively

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 Web Font Performance calculated?

Font load time = subset size ÷ bandwidth + one round trip Fonts block text rendering unless font-display is set to swap, so their load time directly delays contentful paint. Subsetting to the required character set is the single largest saving available.

Why does Web Font Performance matter?

Each additional weight or style is a separate file and a separate request, which is why limiting to two or three is standard practice.

What values do I need to enter?

This calculator takes 5 inputs: Number of font files, Average font file size, Saving from subsetting, Connection speed, Round-trip latency. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.