Convert between lumens, candela, lux, and foot-candles. Calculate luminous efficacy, illuminance at distance, and compare light sources.
The lumen is the SI unit of luminous flux — the total amount of visible light emitted by a source in all directions, weighted by the human eye's sensitivity. It is the most common metric for comparing light bulbs, flashlights, projectors, and any light-emitting device. Understanding lumens and their relationship to candela, lux, and foot-candles is essential for lighting design, photography, and energy-efficient building planning.
Candela measures luminous intensity in a specific direction (lumens per steradian), while lux and foot-candles measure illuminance — the light falling on a surface (lumens per square meter or per square foot). The beam angle determines how the total lumens are distributed: a narrow-beam spotlight concentrates lumens into a small solid angle for high candela, while an omnidirectional bulb spreads lumens evenly.
This calculator converts between all four photometric units, computes luminous efficacy (lumens per watt) to evaluate energy efficiency, and provides illuminance at any distance. Reference tables for common light sources and an illuminance-vs-distance table make it a complete tool for lighting designers, photographers, and anyone comparing light output specifications.
Use this calculator when you need to move between lumens, candela, lux, and foot-candles without losing track of beam angle or distance.
It is useful for room-lighting estimates, flashlight comparisons, fixture selection, and checking whether a spec sheet is describing total output or on-axis intensity. It also helps separate total flux from directional intensity when two products list similar lumens but very different beam patterns.
Lumens from candela: Φ = I × Ω, where Ω = 2π(1 − cos(θ/2)). Illuminance: E = I/d² (lux). Foot-candles: fc = lux/10.764. Efficacy: η = lumens/watts.
Result: 255 cd, 63.7 lux at 2 m, efficacy 80 lm/W
An 800 lm LED bulb at 10W has efficacy = 80 lm/W. With a 120° beam, the solid angle is about 2pi(1-cos60°) = pi sr, so I = 800/pi ≈ 255 cd. At 2 m, illuminance is 255/4 ≈ 63.7 lux.
Photometric units answer different questions. Lumens describe total visible output, candela describes how concentrated that output is in a direction, and lux describes how much of that light reaches a surface.
The most common mistake is comparing products by lumens alone when the beam angles are very different. Another is ignoring distance in illuminance estimates: doubling the distance cuts lux to one quarter, so throw and mounting height matter in practical lighting design. For room lighting, surface reflectance and fixture distribution also affect whether the delivered light feels even in practice. That is why a raw lumen number rarely tells the whole lighting story by itself.
Lumens measure light output; watts measure energy consumption. Efficacy (lm/W) connects them — modern LEDs produce 80-120 lm/W versus ~15 lm/W for incandescent bulbs.
General lighting: 300-500 lux (28-46 fc). Task lighting: 500-1000 lux. Multiply target lux by room area (m²) to get total lumens needed.
One foot-candle = one lumen per square foot ≈ 10.764 lux. Still common in US lighting standards, building codes, and photography.
Candela measures peak beam intensity, which better indicates throw distance. A narrow-beam flashlight can have high candela with modest lumens.
The theoretical maximum is 683 lm/W (monochromatic green, 555 nm). For white light, the practical limit is about 250-350 lm/W depending on color rendering.
Narrower beam = less solid angle = higher candela for the same lumens = higher lux along the beam axis. A 30° spotlight is much brighter on-axis than a 120° flood.