Luminance Converter (nits, cd/m², foot-lamberts)

Understanding the luminance converter — concept and principle

Luminance tells you how bright a surface looks: a phone or TV screen, a cinema screen, a lit sign or a sheet of paper under a lamp. Display makers quote it in nits, which is simply candela per square metre (cd/m²), while cinema projection traditionally uses foot-lamberts (fL). Older optics texts also use stilbs, lamberts and candela per square inch. This converter moves a value between all of them.

Nits are candela per square metre: 14 fL × 3.4263 = 47.97 nits.

How it is calculated

Every unit is converted into cd/m² (nits) first, then into the target unit:

1 fL = (1/π) cd/ft² ≈ 3.4262591 cd/m²
1 stilb (sb) = 1 cd/cm² = 10,000 cd/m²
1 lambert (L) = (1/π) cd/cm² ≈ 3,183.0989 cd/m²
1 cd/in² = 1 ÷ 0.0254² ≈ 1,550.0031 cd/m²

Result = value × (cd/m² in source unit) ÷ (cd/m² in target unit).

The principle

Luminance is luminous intensity per unit of projected area in a given direction, so its SI unit is cd/m². The stilb and cd/in² are the same quantity per square centimetre or per square inch. The lambert and foot-lambert come from a different idea: they describe a perfectly diffusing (Lambertian) surface. Such a surface lit with 1 lumen per square foot and reflecting all of it has a luminance of 1 fL, which equals 1/π cd/ft². The π appears because a diffuse surface spreads its light over a hemisphere and its total output is π times its luminance. In SI terms, a matte surface of reflectance ρ under illuminance E lux has luminance L ≈ ρ × E ÷ π cd/m².

Worked example

Default inputs: value 14, from foot-lamberts, to nits.
1 fL = 1 ÷ (π × 0.3048²) = 3.4262591 cd/m².
14 × 3.4262591 = 47.96762739 nits (cd/m²).
Other units: 0.0047967627 sb, 0.0150694746 L and 0.0309467945 cd/in².

How to read the result

14 fL, about 48 cd/m², is the reference white level for digital cinema projection, which is why cinema screens look dim beside a TV. Standard-dynamic-range video is commonly mastered for about 100 cd/m², office monitors often run around 200 to 350 nits, and HDR TVs and recent phones can exceed 1,000 nits in small highlights. A white sheet of paper with 80% reflectance under 500 lux has a luminance of roughly 0.8 × 500 ÷ π ≈ 127 cd/m².

Common mistakes

Confusing luminance (cd/m², how bright a surface looks) with illuminance (lux, how much light lands on it).
Comparing a display's peak HDR brightness with another's full-screen brightness.
Forgetting the 1/π in foot-lamberts and lamberts.

Keep in mind: Measured luminance depends on viewing angle, the measured area and the content shown. Manufacturer brightness figures are often peak values under specific test conditions and may not match what you see.

Frequently asked questions

How many nits is 1 foot-lambert?

About 3.426 cd/m². To go from nits to fL, divide by 3.426.

Is a nit the same as cd/m²?

Yes. "Nit" is an informal name for 1 cd/m².

Why are cinema screens only about 48 nits?

Cinema is watched in a dark room, where a lower white level looks bright and comfortable and matches the mastering reference.

How do I convert lux to nits?

Only for a known surface: for a matte surface, nits ≈ reflectance × lux ÷ π.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · NIST Special Publication 811 (2008), Appendix B · CIE S 017, ILV: International Lighting Vocabulary

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.