Watts to Lux Calculator

About the Watts to Lux Calculator

A watts to lux calculator predicts the illuminance level in lux that a specified wattage of lighting will produce over a given area, using the luminous efficacy of the chosen bulb type. This forward-direction lighting design calculation is used to estimate whether a proposed lighting installation will meet illuminance targets before purchase and installation — avoiding costly re-specification. A 100W LED panel in a 10 m² room produces very different lux than the same wattage in a 100 m² warehouse, and the calculator makes both cases transparent. By showing lux output for different bulb types at the same wattage, it also quantifies the practical benefit of switching from halogen or CFL to LED: upgrading from a 100W halogen array (producing 200 lux in 10 m²) to 100W of LED (producing 1,000 lux in 10 m²) gives 5× the light for identical electrical consumption. The result is also shown in footcandles for US-based lighting design. Relevant for architects, electricians, interior designers, horticulturalists, photographers, and anyone planning or evaluating room lighting across the USA, UK, Canada, and Australia.

How It Works

Step 1: lm = W × Efficacy (lm/W). Step 2: Lux = lm / Area (m²). Combined: Lux = (W × Efficacy) / Area. Example: 60W of LED lighting (100 lm/W) in a 15 m² bedroom. Step 1: lm = 60 × 100 = 6,000 lumens. Step 2: Lux = 6,000 / 15 = 400 lux. This meets the CIBSE recommendation of 100-300 lux for bedrooms and approaches office levels — a reasonably bright bedroom. The same 60W in halogen (20 lm/W): lm = 60 × 20 = 1,200 lm; Lux = 1,200/15 = 80 lux — dim for any task. Footcandles: 400 lux / 10.764 = 37.2 fc (matches IESNA residential recommendation of 20-50 fc for living spaces). In practice, lux levels vary significantly across a room due to fixture height, beam distribution, and surface reflectance — the calculation gives average maintained lux for design purposes.

Tips & Best Practices

• ✓Dimmer controls: LED dimmers reduce wattage but may not reduce lumens proportionally. A poorly-designed LED dimmer at 50% power may produce 60-70% of full lumens. Quality leading-edge or trailing-edge dimmers with compatible LED bulbs typically achieve near-linear dimming: 50% power ≈ 50-55% lumens.
• ✓Fixture efficiency: the luminaire itself has a light output ratio (LOR) — the fraction of lamp lumens that actually exits the fixture. Open troffer fluorescents: LOR ≈ 0.85. Recessed downlights: LOR ≈ 0.65-0.75. Indirect pendants: LOR ≈ 0.50-0.60. Multiply the watts-to-lux result by LOR for a more realistic illuminance prediction.
• ✓Sports facility lighting: outdoor courts and fields typically require 100-200 lux for recreational use, 300-500 lux for competition, 750-1,500 lux for TV broadcasting. Calculating watts to lux helps specify floodlight quantities for any sports lighting project.

Who Uses This Calculator

Homeowners predicting whether a new ceiling fixture will provide adequate lighting before installation. Lighting designers estimating average lux levels for preliminary project planning. Photographers and videographers calculating whether portable LED panels will achieve target exposure at a given scene area. Horticulturalists verifying grow light intensity before scaling a cultivation system.