The law needs meters internally; other units are converted exactly (1 ft = 0.3048 m).
Illuminance
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Given a lamp's candela rating and a target lux level, find how far away that lux level occurs. Formula: d = √(I / E).
Distance
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Full beam angle, 0–180°. Narrow spot ≈ 10–40°, flood ≈ 60–120°.
Luminous flux
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Use the average lux over the whole area — not a single hot-spot reading.
Luminous flux
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01 · Candela vs lux vs lumens
Three SI photometric quantities people constantly mix up. Each answers a different question:
cd · luminous intensity
Candela
How concentrated light is in one direction. The "punch" of a beam.
Example: a thrower flashlight rated 50,000 cd reaches far; a 50,000-cd laser pointer and a 50,000-cd stadium light differ enormously in lumens.
lm · luminous flux
Lumens
The total light a source emits in all directions. The "amount" of light.
Example: an 800-lm bulb lights a room; whether it is a spot or a flood, the lumen total is the same.
lx · illuminance
Lux
How much light lands on a surface. The "brightness you feel" at a spot.
Example: 500 lux on an office desk; the same lamp gives 500 lux at one distance and 125 lux at double the distance.
| Quantity | Unit | Answers | Relation |
|---|---|---|---|
| Candela | cd | How intense is the beam? | I = Φ / Ω |
| Lumens | lm | How much light total? | Φ = I × Ω |
| Lux | lx | How bright is this surface? | E = I / d² = Φ / A |
02 · Worked examples
Example 1 · cd → lux
A spotlight is rated 1,000 cd. How bright is a wall 5 m away?
E = I / d² = 1000 / 5² = 1000 / 25 = 40 lux
Move the wall to 10 m and you get 1000 / 100 = 10 lux — quarter the distance change, quarter the light. That is the inverse-square law in action.
Example 2 · lux → cd (reverse)
You measure 40 lux at 5 m from a lamp. What is its candela rating?
I = E × d² = 40 × 5² = 40 × 25 = 1,000 cd
This is how reviewers estimate flashlight throw from a lux meter reading — distance must be recorded with the reading.
Example 3 · cd → lm via beam angle
An LED downlight is rated 950.36 cd with a 60° beam. What is its lumen output?
Ω = 2π(1 − cos(60°/2)) = 2π(1 − cos 30°) = 0.8418 sr
Φ = I × Ω = 950.36 × 0.8418 = 800 lumens
Φ = I × Ω = 950.36 × 0.8418 = 800 lumens
Same 950 cd in a 120° flood would be ~2,986 lumens — beam angle, not candela alone, sets the lumen total.
03 · Inverse-square quick chart
Lux at various distances from a 1,000 cd source (exact values):
| Distance | 1 m | 2 m | 3 m | 4 m | 5 m | 10 m |
|---|---|---|---|---|---|---|
| Lux | 1,000 | 250 | 111.11 | 62.5 | 40 | 10 |
Notice 2 m gives exactly ¼ of 1 m, and 10 m gives ¹⁄₁₀₀ — the square law, not a linear falloff.
04 · Methodology & honest limitations
Formulas used
- Inverse-square law: E(lx) = I(cd) / d², d in meters. Non-meter units convert exactly (1 ft = 0.3048 m, 1 in = 0.0254 m).
- Beam solid angle: Ω(sr) = 2π × (1 − cos(θ/2)), θ = full beam angle in degrees.
- Flux from intensity: Φ(lm) = I(cd) × Ω.
- Flux from illuminance: Φ(lm) = E(lx) × A(m²); 1 ft² = 0.09290304 m² exactly.
Limitations (read before trusting a number)
- Point-source assumption. The law is exact for a true point source. Real lamps have size — stay at least 5× the source's largest dimension away for the formula to hold well.
- No cosine law. Surfaces tilted away from the beam get less light (× cos of the incidence angle). This tool assumes the surface faces the source head-on.
- No reflections or losses. Room walls bounce light back (raising real lux); air, lenses, and diffusers absorb some (lowering it).
- Uniform-beam fiction. Real beams have hot spots and spill. Candela ratings usually quote the peak center-beam value, so edge lux will read lower.
- Datasheet candela is often peak. Comparing two products by candela alone is fair for throw, but not for total light — that is what lumens are for.
Scope boundary — what this tool is not
This tool is candela-centered and physics-based. Every conversion here needs a distance, a beam angle, or an area, because that is what the physics demands: candela ↔ lux (inverse-square law), candela ↔ lumens (beam solid angle), lux ↔ lumens (area).
For direct unit conversions that need no extra inputs — lux ↔ foot-candles, candela ↔ candlepower, lumens ↔ watts by bulb type, nits, phots — use our Lighting Unit Converter, a 19-unit hub built for that job.
A dedicated lumens ↔ lux area converter is also on our build queue for users who only need that pair.
05 · Frequently asked questions
How do you convert candela to lux?
Use the inverse-square law: lux = candela / distance^2, with distance in meters. For example, a 1,000-candela spotlight at 5 meters gives 1,000 / 25 = 40 lux on the target. Distance is required — candela and lux measure different things, so there is no fixed conversion factor between them.
What is the difference between candela and lux?
Candela (cd) measures luminous intensity: how concentrated light is in one direction, like the brightness of a flashlight beam. Lux (lx) measures illuminance: how much light actually lands on a surface, like the brightness of your desk. Same lamp, different concepts: candela describes the source, lux describes the receiving surface.
How do you convert lux back to candela?
Reverse the formula: candela = lux x distance^2 (distance in meters). For example, if you measure 40 lux at 5 meters from a lamp, its intensity is 40 x 25 = 1,000 candela. You must know the measurement distance — without it, lux cannot be converted to candela.
Why does the distance matter so much in candela to lux conversion?
Because of the inverse-square law: light spreads over an area that grows with the square of the distance. Double the distance and you get one-quarter of the lux, not half. A 1,000-cd lamp gives 1,000 lux at 1 m, 250 lux at 2 m, 111 lux at 3 m, and only 40 lux at 5 m. This calculator applies that law exactly.
Can you convert candela to lux without knowing the distance?
No — and any calculator that claims to is hiding an assumption. Candela is direction-dependent (per unit solid angle); lux is per unit area at a specific place. Without a distance there is nothing to divide by. If a datasheet gives candela, the lux always depends on how far away you measure.
What is the difference between candela, lux, and lumens?
Three different quantities: candela (cd) = luminous intensity, the concentration of light in a direction (the flashlight beam). Lumens (lm) = luminous flux, the total light output in all directions (the whole bulb). Lux (lx) = illuminance, the light landing on a surface (the desk brightness). Relations: lumens = candela x beam solid angle; lux = lumens / area; lux = candela / distance^2.
How do you convert candela to lumens?
Multiply by the beam's solid angle: lumens = candela x Omega, where Omega = 2 x pi x (1 - cos(beam_angle/2)). A 950-cd lamp with a 60-degree beam gives 950 x 0.8418 = 800 lumens. The beam angle is required — the same candela rating produces very different lumen totals for a narrow spotlight versus a wide floodlight.
How do you convert lux to lumens?
Multiply by the illuminated area: lumens = lux x area in square meters. If a room floor averages 300 lux over 12 m^2, the total flux reaching it is 3,600 lumens. This assumes the lux value is the average over that whole area, not a single hot spot.
How many candela is 1 lux?
It depends on distance: candela = lux x distance^2. At 1 meter, 1 lux = 1 candela exactly. At 2 meters, 1 lux = 4 candela. At 10 meters, 1 lux = 100 candela. There is no single answer without a distance, which is why this calculator asks for one.
Is candela per square meter (cd/m2) the same as lux?
No. Candela per square meter is luminance — how bright a surface looks (also called nits; your phone screen brightness is rated in nits). Lux is illuminance — how much light falls onto a surface. A white wall and a black wall under the same lux have very different cd/m2 because they reflect differently.
Should I use candela or lumens when comparing flashlights or LEDs?
Use both, for different questions. Lumens tell you total output (how much light the device makes). Candela tells you throw (how far the beam reaches — candela determines lux at distance). A 1,000-lumen floody headlamp and a 1,000-lumen thrower flashlight have wildly different candela, so only candela predicts brightness at 50 meters.
Why do different candela-to-lux calculators give different answers?
Usually one of three reasons: (1) a hidden assumed distance or beam angle, (2) mixing units — feet versus meters in the distance changes the answer by ~10.76x if handled wrongly, or (3) one calculator uses the candela rating while another uses lumens with an area assumption. This tool shows every input and the exact formula used, so nothing is hidden.
How accurate is the inverse-square law?
Very accurate for point-like sources measured at distances much larger than the source itself (at least 5x the source's largest dimension). It breaks down close to large sources — e.g., right under a 2-meter LED panel — and it ignores reflections, atmospheric loss, and the cosine law for angled surfaces. Treat results as precise for spotlights and distant lamps, approximate for room interiors.
What beam angle should I assume if the datasheet does not give one?
Do not assume — ask or measure. Typical full-width-half-maximum values are 10-40 degrees for spotlights, 60-120 degrees for downlights and bulbs, but the real emission pattern matters more than the marketing number. If you must estimate, this calculator lets you solve backwards: enter lumens and candela to derive the effective beam angle.
Does this calculator send my data anywhere?
No. This tool is 100% client-side: every calculation runs in your browser's JavaScript engine, nothing is uploaded, and there are no analytics beacons, tracking pixels, or external requests. You can disconnect from the internet and it keeps working.
How is this different from the Lighting Unit Converter on this site?
This tool is candela-centered and physics-based: it converts between candela, lux, and lumens using the inverse-square law and beam solid angle, which require distance, angle, or area inputs. The Lighting Unit Converter (#286) is a general unit hub: lux to foot-candles, candela to candlepower, lumens to watts by bulb type, and similar direct unit conversions that need no extra inputs.