Glare Index Calculator

Work out glare index instantly with clear inputs, formula shown and shareable results.

About Glare Index Calculator

The unified glare rating formula, 8 log10(0.25/Lb x sum of L² omega / p²), weights each glare source by the square of its luminance and its solid angle, divided by the square of a position index that discounts sources away from the line of sight. Office standards typically require UGR of 19 or below.

Formula

Unified glare rating

UGR = 8 log10[(0.25 / Lb) x sum(L² omega / p²)]

Lb: Background luminance
L: Source luminance
omega: Solid angle
p: Guth position index

Worked example

With the default values below, Glare Index Calculator returns Unified glare rating of 12.32. Change any input above and every figure here is recalculated in your browser.

Inputs used

Luminance of the light sourceminimum 1 cd/m²: 5,000 cd/m²
Background luminanceminimum 0.1 cd/m²: 200 cd/m²
Solid angle of the source at the eyeminimum 0 sr: 0.01 sr
Guth position indexminimum 1: 3

Results

Unified glare rating: 12.32
Source to background luminance ratio: 25.0
Glare assessment: Very low glare — suitable for technical drawing and precision work
Glare summation term: 27,777.7778

Step by step

1Values used
2Unified glare rating
3Unified glare rating= 12.32
4Source to background luminance ratio= 25.0
5Glare assessment= Very low glare — suitable for technical drawing and precision work
6Glare summation term= 27,777.7778

Assumptions and limits

The result is derived only from the 4 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 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Glare assessment for a real installation requires photometric data for the luminaires and calculation at the actual observer positions.

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

Preliminary estimate only. Glare assessment for a real installation requires photometric data for the luminaires and calculation at the actual observer positions.

Frequently Asked Questions

How is the Glare Index figure worked out?

UGR = 8 log10[(0.25 / Lb) x sum(L² omega / p²)]. The unified glare rating formula, 8 log10(0.25/Lb x sum of L² omega / p²), weights each glare source by the square of its luminance and its solid angle, divided by the square of a position index that discounts sources away from the line of sight.

Why does background luminance appear in the denominator?

Glare is a contrast phenomenon. The same lamp is far more uncomfortable against a dark ceiling than a bright one, so raising background luminance genuinely reduces discomfort.

What values do I need to enter?

This calculator takes 4 inputs: Luminance of the light source, Background luminance, Solid angle of the source at the eye, Guth position index. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.