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

> The lumen method works out how many light fittings a room needs to reach a target average illuminance: N = (E × A) / (Φ × UF × MF). It gives a fitting count, a connected load and a lighting power density for early planning, before a lighting designer runs a full point-by-point calculation.

## 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

| Assumption | Default | Notes |
| --- | --- | --- |
| Target illuminance | 500 lux | General office tasks in EN 12464-1 |
| Fitting output | 4,000 lm | A typical 2 ft × 2 ft LED panel class; use your datasheet |
| Fitting power | 36 W | Including driver |
| Utilisation factor | 0.65 | Read from the fitting's UF table |
| Maintenance factor | 0.8 | Clean office, LED fittings |
| Height above desk | 7 ft | Ceiling height minus desk height, about 0.75 m |
| Power density check | 9.5 W/m² | ECBC 2017 building-area figure for offices, base level |

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:

| Step | Calculation | Result |
| --- | --- | --- |
| Room in metres | 12.19 m × 9.14 m | 111.5 m² |
| Light delivered by one fitting | 4,000 × 0.65 × 0.8 | 2,080 lm |
| Fittings needed | 500 × 111.5 / 2,080 | 26.8, so 27 |
| Illuminance achieved | 27 × 2,080 / 111.5 | about 504 lux |
| Connected load | 27 × 36 W | 972 W |
| Lighting power density | 972 / 111.5 | 8.7 W/m² |
| Room index | 111.5 / (2.13 × 21.34) | 2.45 |

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.

## Frequently asked questions

### How many lux does an office need?

It depends on the task. Lighting standards such as EN 12464-1 and IS 3646 tabulate maintained illuminance by activity, and 500 lux is the figure EN 12464-1 gives for general office tasks such as writing, typing, reading and data processing. Circulation areas need less and detailed tasks more. Your lighting designer confirms the value for each space.

### What is the utilisation factor?

The utilisation factor is the share of a fitting's light that actually reaches the working plane. It depends on the fitting's light distribution, the room's proportions and the reflectance of the ceiling, walls and floor. Manufacturers publish a UF table for each fitting; read the value off it at your room's room index.

### What is the maintenance factor?

Light output falls over time as LEDs age and dust settles on fittings and room surfaces. The maintenance factor allows for that loss so the target is still met before the next clean or replacement. A value around 0.8 is a common starting point for clean offices with LED fittings; dirtier spaces or longer cleaning intervals need a lower one.

### Why is my lighting power density too high?

Either the fittings give too few lumens for each watt, or too much of their light is lost before it reaches the desk. A more efficient fitting, a fitting with a better utilisation factor for the room, or lighter room finishes all bring the power density down without reducing the light.

### Does hitting the lux target mean the lighting is good?

No. Average lux says nothing about glare, uniformity, colour rendering or controls. A room can meet its average and still be uncomfortable to work in. The calculator sizes the quantity of light; a lighting designer checks its quality.

## Sources

- [IS 3646 (Part 1)](https://standards.bis.gov.in/website/published-standards/department-wise) — Code of practice for interior illumination
- [EN 12464-1](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Light and lighting: lighting of indoor work places
- [ECBC 2017](https://beeindia.gov.in/) — Lighting: interior lighting power

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Source: https://hagerstone.com/calculators/lighting-lux-calculator