Online Tool Store
· 5 min read
How to Convert Between Nits and Luminance Units
Manesh Jayawardhana
CIO & Co-founder
A projector spec sheet quotes foot-lamberts. The monitor next to it is rated in nits. A cinema standard document uses candelas per square metre. You need to know whether these three numbers describe comparable brightness, and the units are all different for historical rather than technical reasons.
Luminance conversion is straightforward arithmetic once you know which quantity you’re actually dealing with — and the most common mistake isn’t the conversion, it’s converting the wrong quantity entirely.
What luminance actually measures
Luminance is how bright a surface appears — light emitted or reflected from a surface, per unit area, in a given direction. It’s what you measure when you point a meter at a screen.
The SI unit is the candela per square metre (cd/m²). A nit is the same thing under a different name: 1 nit = 1 cd/m² exactly. That’s not an approximation or a convention, they’re identical, which is why display specs use the two interchangeably.
The others are older units. A foot-lambert (fL) is used in projection and cinema work, and equals roughly 3.426 cd/m². A stilb is one candela per square centimetre, so 1 stilb = 10,000 cd/m² — an enormous unit rarely seen outside older literature.
The critical distinction is that luminance is not illuminance. Illuminance measures light falling onto a surface, in lux or foot-candles, and it’s a different physical quantity. You cannot convert between them without knowing the reflectance of the surface involved.
Why people get stuck here
- Confusing luminance with illuminance. Lux and nits get treated as convertible constantly, and they aren’t without additional information.
- Lumens in the mix. Projector specs quote lumens, which is total light output — converting that to screen luminance requires the screen size and gain.
- Peak versus sustained. A display rated at 1,000 nits peak may sustain far less across a full white field. The two figures answer different questions.
- Foot-lamberts in cinema standards. Projection standards are written in fL while equipment is often specified in cd/m².
- Assuming a spec number is measured. Manufacturer figures are best case; a meter tells you what the device actually does in your room.
| Unit | Equals in cd/m² | Where You’ll See It |
|---|---|---|
| Nit | 1 | Display and phone specifications |
| cd/m² | 1 | SI standard, technical documents |
| Foot-lambert | ≈ 3.426 | Projection and cinema standards |
| Stilb | 10,000 | Older optics literature |
What a good conversion workflow looks like
Identify the quantity first
Before converting anything, confirm you’re dealing with luminance and not illuminance or luminous flux. If the unit is lux or lumens, you have a different quantity and conversion needs extra information.
Keep the precision you need
The foot-lambert relationship isn’t a round number. Rounding 3.426 to 3.4 introduces about 0.8% error, which is fine for a rough comparison and not for calibration work.
Note the measurement conditions
A luminance figure means little without knowing what was measured — full white field, a specific test pattern, peak or sustained, and at what point on the screen.
Common mistakes to avoid
- Converting lux to nits directly. It needs the surface’s reflectance, and without that the conversion is meaningless.
- Comparing peak specs to sustained measurements. They’re different numbers describing different conditions.
- Ignoring screen gain in projection. A gain-1.3 screen produces meaningfully different luminance from a gain-1.0 screen with the same projector.
- Using a phone light sensor as a luminance meter. Phone sensors measure illuminance at the sensor, which is the other quantity.
- Rounding early in a multi-step calculation. Keep full precision until the final figure.
How to do it with Luminance Converter
Online Tool Store’s Luminance Converter converts between luminance units in your browser, with no account and nothing uploaded.
- Confirm your source figure is luminance — cd/m², nits, foot-lamberts, or stilb — and not lux or lumens.
- Enter the value and select the source unit.
- Read the converted figure, keeping more decimal places than you think you need if it feeds into another calculation.
- Note the measurement conditions alongside the number, since the figure alone doesn’t carry them.
- Round only at the end, once.
The Illuminance Converter handles the other quantity, and the Luminosity Converter covers astronomical use. Everything else is in the tools directory.
Frequently asked questions
Are nits and cd/m² the same thing?
Yes, exactly. “Nit” is an informal name for the candela per square metre, and 1 nit equals 1 cd/m² precisely. Display marketing tends to prefer nits because it’s shorter; technical documents prefer cd/m².
Can I convert lux to nits?
Not directly. Lux measures light arriving at a surface and nits measure light leaving one, so the conversion depends on how much of the arriving light that surface reflects. With a known reflectance you can relate them; without it, you can’t.
Why do projector specs use foot-lamberts?
Historical convention in the cinema and projection industry, where the standards were written in imperial units and have stayed that way. The underlying physics is identical — a foot-lambert is about 3.426 cd/m².
Final thought
Check which quantity you have before you convert anything. Nits, cd/m² and foot-lamberts convert cleanly between each other; lux and lumens are different measurements wearing similar-sounding names.