Room Lighting Calculator
Light fittings for a target lux by the lumen method, with watts and a grid.
Try before you buy.
- Free preview: the rules, dates and checks that apply to your figures, with every computed amount hidden.
- Locked until you unlock it: download and copy.
- Unlock: Pro pass, ₹179 for 30 days or ₹79 for 1 week, a one-time payment that never renews. Unlock: Pro pass, $5.99 for 30 days or $2.99 for 1 week, a one-time payment that never renews.
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Room Lighting Calculator
Use the lumen method to find how many light fittings a room needs. Enter the room’s length and width, the average illuminance you want in lux, each fitting’s light output in lumens, the utilisation factor (how much of that light reaches the working surface) and the maintenance factor (how much remains as fittings age and gather dirt). The result gives the number of fittings, rounded up, the lux they give, the total lumens and watts at your fittings’ efficacy, and a simple grid to place them evenly.
Choose the target lux for your room’s use; the calculator does not assume a standard level. The free preview shows the result rows with the values hidden; a Pro pass shows every value, with copy and text or JSON downloads.
How to use it
- Enter the room’s length and width and choose the unit.
- Enter the target average illuminance in lux and the lumens of one fitting.
- Enter the utilisation and maintenance factors (each above 0 and at most 1) and the fittings’ efficacy in lumens per watt.
- Select Calculate; with a Pro pass, copy the working or download it as text or JSON.
Formula
The number of fittings is N = E × A ÷ (Φ × UF × MF), where E is the target illuminance in lux, A the floor area in m², Φ the lumens of one fitting, UF the utilisation factor and MF the maintenance factor. N is rounded up, and the average achieved is N × Φ × UF × MF ÷ A. Watts = total lumens ÷ efficacy. The grid spreads the fittings over rows and columns in proportion to the room’s length and width. Units: NIST SP 811, appendix B.9.
Limitations
- The lumen method gives an average illuminance: it does not check uniformity, glare, emergency lighting or how each fitting spreads its light.
- The utilisation and maintenance factors must suit your room and your fittings; they come from the fitting maker’s data. An estimate, not a professional lighting design.
Privacy
On this device: nothing is sent
Everything is calculated on your device; nothing is uploaded or stored.
Frequently asked questions
What do I get without a pass?
Without a pass, Room Lighting Calculator shows the rules, dates and checks that apply to your figures, with every computed amount hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
What lux level should I use?
It depends on what the room is for, and lighting codes and guides publish recommended levels. Enter the level you need; the calculator does not assume one.
Which utilisation and maintenance factors should I enter?
The utilisation factor comes from the fitting maker’s tables for your room’s size and surface colours, the maintenance factor from how clean the room is and how often fittings are cleaned. The page starts with 0.5 and 0.8 as examples: replace them with your fittings’ figures.