Lighting Calculator: Fittings, Lux and Lighting Load for a Room

What it calculates

Given a room, a target light level and the fitting you plan to use, the calculator gives:

• the number of fittings needed to reach the target average illuminance;
• the average illuminance actually achieved once that number is rounded up;
• the connected lighting load in watts and kilowatts;
• the lighting power density (W/m²), checked against a figure you choose;
• a starting grid of rows and columns, with approximate spacing;
• the room index, which you need to look up the utilisation factor.

Inputs

Room dimensions can be entered in feet or metres. The calculation runs in square metres, because lux is lumens on each square metre.

Target average illuminance is the maintained lux level the space needs. Take it from the task tables in IS 3646 or EN 12464-1, or from your lighting designer.

Light output per fitting is the luminaire's rated output in lumens from its datasheet. Use the luminaire figure, not the output of the bare LED chips, which is always higher.

Power per fitting is the total wattage including the driver.

The assumptions panel holds the utilisation factor, the maintenance factor, the height of the fittings above the desk and the power density you want to check against.

Method

The lumen method sets the light delivered to the working plane equal to the light the room needs:

N = (E × A) / (Φ × UF × MF)

• N is the number of fittings;
• E is the target average illuminance in lux;
• A is the area of the room in m²;
• Φ is the light output of one fitting in lumens;
• UF is the utilisation factor;
• MF is the maintenance factor.

N is rounded up, and the achieved illuminance is worked back from the rounded number. The connected load is N times the power per fitting, and the lighting power density is that load divided by the floor area.

The room index, K = (L × W) / (Hm × (L + W)), is calculated from the length, width and the height of the fittings above the working plane. It does not change the result directly; it tells you which row of the manufacturer's UF table to read.

Assumptions

The power density check uses the ECBC 2017 building-area method figure for office buildings at the base ECBC level. ECBC+ and SuperECBC buildings, and other building types, have lower or different figures. Check the current edition for the value that applies to your project.

Limitations

• It gives an average, not a distribution. The lumen method assumes light is spread evenly. It cannot show dark corners, bright spots under fittings or the uniformity across desks.
• It does not assess glare. Fitting choice, shielding and position control glare, and none of them appear in the formula.
• It treats the room as an empty rectangle. Tall furniture, partitions and deep beams block light and need a point-by-point calculation.
• The UF is only as good as the table you read it from. Using a generic value for a narrow-beam or highly directional fitting can be badly wrong.
• It ignores daylight. Daylight and its controls reduce energy use but are not part of the sizing here.

Worked example

An open office 40 ft × 30 ft, lit to 500 lux with 4,000 lm, 36 W panels, UF 0.65 and MF 0.8:

The calculator suggests a starting grid of 5 rows of 6, which is 30 positions. In practice the designer would either use all 30 positions, giving about 560 lux and more even light, or adjust the spacing to place 27. At 8.7 W/m² the layout sits below the 9.5 W/m² check value.

How engineers use it

A lighting designer uses the lumen method at concept stage to set a fitting count and a load for each space. The load goes to the electrical designer for circuit and distribution board sizing, and to the HVAC designer, because every watt of lighting becomes heat the air conditioning has to remove.

The count also feeds the reflected ceiling plan, where fittings are coordinated with sprinklers, smoke detectors, diffusers and access panels. That coordination often moves fittings, which is why the final layout is checked with lighting software rather than the formula.

When a professional design must replace it

Use the calculator to compare fittings, to budget a fitting count and to check that a proposed layout is in the right range. Replace it with a point-by-point calculation in lighting software before fittings are ordered, so that uniformity, glare and the effect of furniture and partitions are checked on the real layout. Where the project must show energy code compliance, the lighting power figures go into the compliance documents prepared by the project's consultants.