What the calculator does
The lumen method is the standard way to get from a target light level to a number of luminaires. It asks how many lumens must land on the working plane to give the required lux over the room's area, then divides by the lumens each luminaire actually delivers there after the room has absorbed some (the utilisation factor) and after the luminaires have aged and gathered dust (the maintenance factor). The calculator does that, rounds up, and then proposes a rectangular grid, because 25 luminaires cannot be spread evenly over a 12 × 8 m room and a 4 × 7 grid can. It reports the illuminance the grid achieves, the spacing between luminaires against the mounting height, the lighting power density in W/m² and the luminaire efficacy in lm/W, so that the electrical load and the running cost are on the same page as the light level.
Formula
Room index k = L × W ÷ (Hm × (L + W))
N = E × A ÷ (Φ × UF × MF) → rounded up
Achieved E = Ngrid × Φ × UF × MF ÷ A
Rows = round(√(N × W ÷ L)), columns = N ÷ rows rounded up
Spacing-to-height ratio SHR = spacing ÷ Hm (typically ≤ 1.5)
Lighting power density = Ngrid × P ÷ A Efficacy = Φ ÷ P
where E is the target maintained illuminance in lux, A the floor area in m², Φ the luminous flux of one luminaire in lumens, UF the utilisation factor read from the luminaire's photometric table for the room index and the surface reflectances, MF the maintenance factor, and P the input power of one luminaire. The room index describes the room's shape as seen by the light: a large, low room has a high index and most of the light reaches the plane directly; a small, tall room has a low index and the walls take a bigger share. The spacing-to-height ratio compares the distance between luminaires with their height above the plane; beyond the luminaire's maximum SHR, usually about 1.5 for a diffuse fitting, the light between them falls off noticeably.
Worked example
An open-plan office in Lahore, 12 × 8 m with a 3.0 m ceiling and desks at 0.8 m, to be lit to 500 lx, the EN 12464-1 value for writing, typing and reading, with recessed 600 × 600 LED panels of 4 000 lm and 36 W. The panel's UF table gives 0.60 for the room index and light surfaces; the maintenance factor for a clean, air-conditioned office with LED is 0.8.
- Area 96 m². Mounting height above the plane Hm = 3.0 − 0 − 0.8 = 2.2 m. Room index k = 96 ÷ (2.2 × 20) = 2.18.
- Lumens needed on the plane: 500 × 96 = 48 000 lm. Each panel delivers 4 000 × 0.6 × 0.8 = 1 920 lm to the plane, maintained. N = 48 000 ÷ 1 920 = 25 luminaires.
- Grid: rows across the 8 m width = round(√(25 × 8 ÷ 12)) = round(4.08) = 4; columns along the 12 m length = 25 ÷ 4 rounded up = 7; 28 panels. Achieved illuminance 28 × 1 920 ÷ 96 = 560 lx, 12 % over the target.
- Spacing 12 ÷ 7 = 1.71 m along the length and 8 ÷ 4 = 2.00 m across; SHR = 2.00 ÷ 2.2 = 0.91, well inside 1.5.
- Power: 28 × 36 W = 1 008 W, a lighting power density of 10.5 W/m²; efficacy 4 000 ÷ 36 = 111 lm/W.
Twenty-five panels would fit a 5 × 5 grid, but in a 12 × 8 m room that puts them 2.4 m apart along the length and 1.6 m across, which shows as bright and dark bands; 4 rows of 7 keeps the two spacings close. The 12 % surplus is what rounding to a grid costs; a 3 600 lm panel in the same grid would give 504 lx, and a dimmable driver set to about 90 % would do the same.
Maintained illuminance for common spaces, EN 12464-1
| Space or task | Maintained illuminance Ēm |
|---|---|
| Offices: writing, typing, reading, data processing | 500 lx |
| Technical drawing | 750 lx |
| Conference and meeting rooms | 500 lx |
| Reception desk | 300 lx |
| Corridors and circulation areas | 100 lx |
| Stairs | 100 lx |
| Archives | 200 lx |
| Classrooms | 300 lx |
| Laboratories | 500 lx |
| Retail sales area | 300 lx |
| Kitchens | 500 lx |
| Industrial assembly: rough | 200 lx |
| Industrial assembly: medium | 300 lx |
| Industrial assembly: fine | 500 lx |
| Industrial assembly: precision | 750 lx |
| Warehouse storage, unmanned | 100 lx |
| Warehouse storage, continuously manned | 200 lx |
| Indoor car park, parking areas | 75 lx |
Assumptions and limitations
- The lumen method gives the average. It says nothing about uniformity, glare or the light on a vertical surface. EN 12464-1 requires a minimum uniformity (the ratio of minimum to average illuminance, 0.60 for most office tasks) and a UGR limit; both need a point-by-point calculation with the luminaire's photometric file in software such as DIALux or Relux.
- The utilisation factor must come from the luminaire's own table, read against the room index and the ceiling, wall and floor reflectances. The default of 0.60 is a placeholder for a recessed panel in a light office; a dark warehouse with a high-bay fitting may be 0.4 or less.
- Luminaire lumens, not lamp lumens. Modern UF tables and datasheets quote the flux leaving the luminaire; older tables quoted bare-lamp lumens and folded the light output ratio into the UF. Do not mix the two.
- No daylight, no controls, no emergency lighting. Daylight and occupancy sensing reduce the hours, not the installed number; emergency lighting is designed separately to EN 1838.
- An empty rectangular room. Shelving, partitions, machines and tall furniture shadow the plane; split L-shaped rooms into rectangles and treat racking aisles as their own rooms.
- The spacing check is a rule of thumb. The 1.5 × Hm limit suits a general diffuse luminaire; the datasheet's maximum SHR governs, and the first row should sit about half a spacing from the wall.
Frequently asked questions
What is the difference between lux and lumens?
Lumens measure the light a source emits; lux measures the light that lands on a surface. One lux is one lumen spread over one square metre, so the same 4 000 lm panel gives more lux over a desk directly below it than over a corridor floor further away, and a room twice the size needs twice the lumens for the same lux. Standards specify lux because that is what the task sees; datasheets specify lumens because that is what the luminaire makes.
Why include a maintenance factor?
Because the standard's values are maintained illuminance: the level that must still be there at the end of the maintenance cycle, after the LEDs have lost some output, the diffusers have collected dust and the walls have dulled. Designing for day one means falling below the requirement by year three. The factor is a product of lumen depreciation, luminaire dirt and room surface dirt over the cleaning interval; CIE 97 sets out how to build it.
Why not just count the bulbs I have now and add a few?
Because the room, not the count, decides. Two luminaires of the same wattage can differ by half in lumens; the same luminaire delivers 70 % of its light to the plane in a low, white room and 40 % in a tall, dark one; and a fitting that was adequate when new may be at two-thirds of its output today. The lumen method takes ten minutes and replaces all of that guessing with two datasheet numbers and the room dimensions.
Why are LED "watt equivalents" meaningless?
A watt is power in, not light out. "100 W equivalent" was a way of telling shoppers an LED bulb was about as bright as the incandescent bulb it replaced, roughly 1 500 lm, but luminaire efficacies now range from under 80 to over 180 lm/W, so two "equivalent" products can differ by more than double in light. Specify lumens for the light and lm/W for the efficiency, and check both against the datasheet, not the box.
What is UGR?
The Unified Glare Rating is the CIE measure of discomfort glare from the luminaires in a room, on a scale that runs roughly from 10 (imperceptible) to 30 (intolerable). EN 12464-1 sets a limit for each activity: 19 for offices, 16 for technical drawing, higher for corridors and industry. It depends on luminaire luminance, its size and position relative to the observer, and the room's brightness, so it comes from a software calculation with the luminaire's photometric file, not from this calculator. A low-glare panel with a microprismatic diffuser is the usual answer for offices with screens.
References
- EN 12464-1:2021, Light and lighting — Lighting of work places — Part 1: Indoor work places — maintained illuminance, UGR, uniformity and colour rendering by task
- SLL Code for Lighting, Society of Light and Lighting (CIBSE), 2022 — lumen method, room index, utilisation factor tables and spacing-to-height ratio
- CIE 97:2005, Guide on the maintenance of indoor electric lighting systems — maintenance factor and its components
- IES Lighting Handbook, 10th edition, Illuminating Engineering Society, 2011 — the North American reference for the same method
- IEC 60598-1, Luminaires — Part 1: General requirements and tests — luminaire safety, not photometry