Lighting design
Lighting calculator (lux and number of fittings)
Estimate how many light fittings a room needs to reach a target lux level, using the lumen method. Handy for quoting a job.
Lumen method: N = (lux x area) / (lumens x UF x MF), with a utilisation factor of 0.6 and maintenance 0.8 assumed. A design guide; confirm with the fitting's photometric data.
The lumen method in one screen
The number of fittings a room needs is the total light it must receive divided by the usable light each fitting delivers. That is the lumen method, and it gets you a sensible fitting count for a quote in seconds.
What the factors mean
- Target lux: the light level for the task. Around 100 to 150 for living spaces, 300 for kitchens and bathrooms, 500 for an office or workshop.
- Utilisation factor (UF): how much of a fitting's output actually reaches the working plane after the room eats some. 0.6 is a reasonable default for a normal room.
- Maintenance factor (MF): lamps dim and dust builds up, so design for the depreciated output. 0.8 is typical.
From count to layout
Once you have the number, spread the fittings evenly so the light is uniform, not bright in the middle and dim in the corners. For an exact design use the fitting's photometric file, but for pricing and a first layout this is enough. It pairs with the cost guide when you put the quote together.
FAQs
How many downlights do I need in a room?
Work out the room area, pick a target lux (about 150 for living spaces, 300 for a kitchen), then divide the total lumens needed by the output of one fitting after utilisation and maintenance factors. This calculator does it for you.
What is the lumen method?
Number of fittings = (target lux x floor area) / (lumens per fitting x utilisation factor x maintenance factor). It estimates how many luminaires a space needs to reach a light level.
What lux level should a room be?
Roughly 100 to 150 lux for lounges and bedrooms, 300 for kitchens and bathrooms, and 500 for offices, workshops and detailed tasks.