Watts to Lux Calculator

How to use the watts to lux calculator

What this calculator does

This calculator estimates illuminance from electrical power, a lamp's luminous efficacy, and the area receiving the light. It can also reverse the same relationship to estimate the power needed for a target illuminance. Lux is an SI unit of illuminance: the Illuminating Engineering Society defines one lux as one lumen per square meter. The result is an idealized conversion. It does not predict the exact reading of a light meter because real fixtures lose and redirect light through reflectors, lenses, beam angles, mounting geometry, room surfaces, dirt, aging, and obstructions.

When to use it

Use the calculator to compare lamp technologies at the same wattage, estimate the average lux available over a known floor or work area, approximate wattage for a target lux level, or translate a spherical coverage radius into an equivalent illuminated surface area. It is useful for early planning and sanity checks before reviewing manufacturer photometric files or performing a point-by-point lighting design.

How to calculate

1. The calculator opens with a complete demonstration: a 12 W LED lamp, 90 lm/W efficacy, and 10 m² area produce 108 lx. The matching Excel workbook is immediately available.
2. Choose Illuminance (lux) or Power (watts) under Solve for. The selected unknown becomes read-only while the quantity you must supply becomes editable.
3. Select a Light source. Presets fill Luminous efficacy; choose the custom option to type a manufacturer value.
4. Choose Surface area or Spherical radius under Geometry input, then enter the active geometry value. Unit selectors convert the current value rather than merely relabeling it.
5. Read the primary result, the summary cards, the formula line, and the calculation table. Select Download Excel to export the current validated model.
6. Reset clears the demonstration and all semantic data fields. The Excel button then remains unavailable until a complete valid input set is entered again.

Input guide

Solve for is required and accepts either illuminance or power. Switching it changes which field is calculated. Light source is required; each preset supplies a representative efficacy, while Custom luminous efficacy makes the efficacy field editable. Power accepts a positive decimal in W or kW, such as 12 W. Do not enter “12W” inside the field or use a decimal comma; use the adjacent unit selector and a decimal point. Higher power increases lumens and lux linearly when efficacy and area stay constant.

Luminous efficacy is measured in lumens per watt and must be positive. A realistic custom example is 110 lm/W. The U.S. Department of Energy explains why lumens, rather than watts alone, should be compared across lamp types. A higher efficacy means more estimated luminous flux from the same electrical power. Use a luminaire or lamp datasheet value when precision matters; a technology preset is only a representative planning assumption.

Geometry input determines whether Surface area or Spherical radius is editable. Surface area accepts a positive decimal in m² or ft², for example 10 m². Radius accepts a positive decimal in m or ft, for example 0.892 m; the calculator uses A = 4πr². Increasing area or radius lowers average lux for the same lamp output. Illuminance uses lx or fc. In power mode it is a required positive target, such as 300 lx; in illuminance mode it is calculated. One foot-candle equals 10.76391 lux.

Output guide

Estimated illuminance is the primary output in illuminance mode, expressed in lux or foot-candles. It is an estimate of average luminous flux per area, not a guaranteed minimum at every point. Required power is the primary output in power mode and is the ideal wattage implied by the target lux, efficacy, and area. Luminous flux is power multiplied by efficacy, shown in lumens. Surface area and Spherical radius show the canonical geometry in the selected units. Luminous efficacy repeats the active conversion factor. The four header pills summarize the active solve mode, selected source and efficacy, current geometry, and validation status. The Calculation map restates the exact current equation and its modeling limitation, while the concise live summary announces the primary result to assistive technology. The Calculation details table lists each quantity, its formula role, its current value, and whether it is an input or derived value.

Worked example

The startup example uses 12 W and 90 lm/W, so estimated luminous flux is 12 × 90 = 1,080 lm. Spread uniformly over 10 m², illuminance is 1,080 ÷ 10 = 108 lx. The equivalent spherical radius is √(10 ÷ 4π) = 0.892 m. These exact values appear in the first-open controls, result cards, table, and workbook checkpoints.