Lighting Calculator: Number of Luminaires for a Room (Lumen Method)

How many luminaires a room needs for a target light level, and what you get with a whole number of them.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

The lumen method is a quick way to estimate lighting. The light needed on the work surface is the target illuminance, in lux, times the floor area. The light you get is the output of each luminaire times the fraction that reaches the work surface, called the utilisation factor, times a maintenance factor that allows for dirt and ageing.

Dividing the light needed by the light delivered by one luminaire gives the number needed. The calculator rounds that up to a whole number, then works back to the lux you really get, the even spacing and the installed power for each square metre of floor.

The target level depends on the task and comes from the lighting standard or the client brief. The result is an average for an empty room, so uniformity, glare and emergency lighting need to be checked separately.

Formulas used

  • N = \frac{E\,A}{\Phi\,UF\,MF}
  • E = \frac{N\,\Phi\,UF\,MF}{A}
  • s = \sqrt{A/N}

Worked example

With the default numbers in the calculator above:

  1. Room area: 96.00 m2
  2. Luminaires needed (not rounded): 16.00
  3. Luminaires, rounded up: 16
  4. Illuminance with that many: 320.00 lx
  5. Spacing if laid out evenly: 2.45 m
  6. Installed power per floor area: 6.00 W/m2

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Target maintained illuminance (from the standard or the brief) 500 lx; Room length 10 m; Room width 6 m.

  1. Room area: 60.00 m2
  2. Luminaires needed (not rounded): 15.63
  3. Luminaires, rounded up: 16
  4. Illuminance with that many: 512.00 lx
  5. Spacing if laid out evenly: 1.94 m
  6. Installed power per floor area: 9.60 W/m2

Common questions

What lux do I need?

It depends on the task. Offices, workshops and warehouses need different levels. Take it from the lighting standard (AS/NZS 1680) or the client brief.

Where do I find the utilisation factor?

On the luminaire maker data. It depends on the room shape and the colours of the ceiling, walls and floor. About 0.5 to 0.7 is common for a typical room.

Is this enough for a real design?

It is an estimate for a plain rectangular room. A lighting design program checks uniformity, glare and layout, and emergency lighting is a separate matter.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: AS/NZS 1680 (interior and workplace lighting, licensed); the lumen method as given in lighting design handbooks; luminaire maker photometric data.

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