Watt or lumen, which should I look at?

Always lumen! Watt only tells you how much power the bulb draws, not how much light. Old incandescent: ~15 lm/W. Halogen: ~20 lm/W. LED: 80–120 lm/W (typically 100). A 60W incandescent (~800 lm) matches an 8–10W LED. Rule of thumb: desired lumen ÷ 100 ≈ LED wattage. Watt used to be a proxy for brightness because incandescents were standardised, but that no longer holds.

What are typical lux levels?

Direct sunlight: 30,000–100,000 lux. Overcast day: 1000–10,000. Good office: 300–500. Living room: 100–300. Restaurant: 50–100. Moonlight: 0.1–1. Starlight: 0.001. The human eye adapts over an enormous range (12 orders of magnitude), so "bright enough" depends strongly on the task. Reading and fine work need at least 500 lux; a bedroom is comfortable at 50–100 lux for a soft feel.

What does beam angle do?

The same lumen output in a narrow beam gives higher candela and lux at the centre, but hits a smaller area. A wider beam gives even lighting over a larger area with lower centre lux. Typical angles: 15° = spot (retail display, accent), 40° = flood (general ceiling), 120° = wide flood (work light). A 1000-lm bulb in a 10° beam gives ~30,000 lux at 1 m; the same bulb in 120° gives ~250 lux.

What’s the theoretical max lumen per watt?

683 lm/W at 555 nm (green) - the peak of human eye sensitivity and how candela itself is defined. For white light covering the whole visible range, the upper limit is ~250 lm/W. Best commercial LEDs reach ~200 lm/W in labs and 100–160 in typical bulbs. For comparison: sunlight is ~93 lm/W (a lot of energy is UV/IR, "wasted" for the eye); incandescents ~15 lm/W.

Luminosity Converter - Lumen, Lux, Candela

How lumen, lux, and candela relate

Lumen measures total luminous flux, candela measures intensity in one direction, and lux measures how much light hits a surface. The relationship depends on the beam angle (solid angle) and the distance to the surface.

Solid angle: Ω = 2π · (1 − cos(θ/2)), where θ is the beam angle
cd = lm / Ω
Lux: lx = cd / r² (distance r in meters)

A beam angle of 360° corresponds to a source emitting in all directions (Ω = 4π steradians), like a typical light bulb.

About this tool

The luminance calculator converts between lumen (total light output), lux (illuminance on a surface = lumen/m²) and candela (intensity in one direction = lumen/steradian). Add distance and beam angle and you get practical answers like "how many lux does my desk get from a 1000-lumen LED with a 40° beam at 2 m height?". Useful for choosing LED bulbs, planning office and studio lighting, comparing torches, and picking grow lights for plants. All computed locally.

How to use it

Pick which values you know (e.g. lumen and distance).
Enter the numeric values.
Add beam angle if relevant (spot vs flood).
See lux, candela and total lumen update in real time.

Examples

LED spot over desk

Input1000 lm, 40° beam, 2 m distance

Output~830 lux at the centre, 2100 cd

Office general lighting

InputTarget: 500 lux over a 10 m² work surface

OutputNeeds ~5000 lm effective (typically 6000–8000 lm installed)

EN 12464-1 recommends 500 lux for office work, 750 lux for detailed technical drawing, 300 lux for corridors. Allow ~30% loss from reflectance and fixture efficiency.

Common use cases

Choosing LED bulbs by lumen instead of watts.
Planning lighting for office, workshop or studio.
Comparing torches (lumen, candela and "throw" distance).
Setting up photo and video lighting for correct exposure.
Picking grow lights for plants (PAR/PPFD, related to lumen).

Frequently asked questions

Watt or lumen, which should I look at?: Always lumen! Watt only tells you how much power the bulb draws, not how much light. Old incandescent: ~15 lm/W. Halogen: ~20 lm/W. LED: 80–120 lm/W (typically 100). A 60W incandescent (~800 lm) matches an 8–10W LED. Rule of thumb: desired lumen ÷ 100 ≈ LED wattage. Watt used to be a proxy for brightness because incandescents were standardised, but that no longer holds.
What are typical lux levels?: Direct sunlight: 30,000–100,000 lux. Overcast day: 1000–10,000. Good office: 300–500. Living room: 100–300. Restaurant: 50–100. Moonlight: 0.1–1. Starlight: 0.001. The human eye adapts over an enormous range (12 orders of magnitude), so "bright enough" depends strongly on the task. Reading and fine work need at least 500 lux; a bedroom is comfortable at 50–100 lux for a soft feel.
What does beam angle do?: The same lumen output in a narrow beam gives higher candela and lux at the centre, but hits a smaller area. A wider beam gives even lighting over a larger area with lower centre lux. Typical angles: 15° = spot (retail display, accent), 40° = flood (general ceiling), 120° = wide flood (work light). A 1000-lm bulb in a 10° beam gives ~30,000 lux at 1 m; the same bulb in 120° gives ~250 lux.
What’s the theoretical max lumen per watt?: 683 lm/W at 555 nm (green) - the peak of human eye sensitivity and how candela itself is defined. For white light covering the whole visible range, the upper limit is ~250 lm/W. Best commercial LEDs reach ~200 lm/W in labs and 100–160 in typical bulbs. For comparison: sunlight is ~93 lm/W (a lot of energy is UV/IR, "wasted" for the eye); incandescents ~15 lm/W.

Technical background

Candela (cd) is the SI base unit for luminous intensity, defined via a monochromatic 540 THz source (green, 555 nm) with luminous efficacy 683 lm/W. Lumen (lm) is luminous flux: 1 lm = 1 cd · 1 sr (steradian). Lux (lx) is illuminance: 1 lx = 1 lm/m². Relations: for a point source E [lx] = I [cd] / r² [m²] (inverse-square law). Total flux from a source with intensity I over solid angle Ω [sr] is Φ [lm] = I · Ω. A full sphere = 4π sr ≈ 12.57 sr, so an isotropic 100-cd source emits 4π · 100 ≈ 1257 lm. For a cone with half-angle θ, solid angle Ω = 2π(1 − cos θ) sr. Photometric quantities are weighted by eye sensitivity V(λ) peaking at 555 nm; radiometric quantities (W) are the raw physical energy flow. All in IEEE 754 double, no server calls. Real beams are rarely perfect cones. Manufacturers give the "full-width half-maximum" angle where intensity drops to 50%, not where the light "ends".