Lighting Lux Calculator — Fixture Count and Lighting Load
Work out how many fittings an area needs to reach the average lux your design basis sets, from the fitting's lumens, the utilisation factor and the maintenance factor — and the lighting load that follows, for circuits, panels and the energy budget.
What this tool estimates
How many fittings an area needs to reach the average maintained illuminance you target, by the lumen method and rounded up to whole fittings — and the installed load in kW and W/m² that follows.
What it cannot decide
- What lux the space needs — take it from your design basis, such as the recommended values in IS 3646 (Part 1) or NBC 2016 Part 8 for the task.
- Uniformity, glare, mounting height, spacing or aiming. Only a photometric layout settles those.
- Emergency and escape lighting, which is a separate system with its own requirements.
- Whether the installed W/m² meets an energy code that applies to your building.
Result
An average-illuminance estimate only. Uniformity, glare, mounting height, spacing and emergency lighting need a photometric layout and their own design.
| Step | Value |
|---|
| UF | MF | Fittings | kW |
|---|
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
How this calculator works — and where it stops
What it calculates
How many fittings an area needs to reach the average maintained illuminance you target, by the lumen method — rounded up to whole fittings — and the installed lighting load in kW and W/m², plus the average illuminance the rounded-up count gives.
Inputs
- Area in m² or ft².
- Target average maintained illuminance in lux, from your design basis — the tool has none built in. IS 3646 (Part 1) and NBC 2016 Part 8, Section 1 give recommended values by space and activity.
- Luminous flux per fitting in lumens, from the datasheet, on the same basis as the utilisation factor.
- Utilisation factor (UF), from the fitting's photometric data for the room's proportions and surface reflectances — more than 0, at most 1.
- Maintenance factor (MF) for lumen depreciation and dirt over the cleaning cycle — IS 3646 (Part 1) calls it the light loss factor; more than 0, at most 1.
- Input power per fitting in watts, including the driver.
Method
- The area is converted to m² (1 ft² = 0.09290304 m²).
- Lumens needed = target lux × area ÷ (UF × MF).
- Fittings = lumens needed ÷ lumens per fitting, rounded up to a whole number.
- Installed load (kW) = fittings × watts ÷ 1,000; power density (W/m²) = fittings × watts ÷ area.
- Average maintained illuminance with the rounded-up count = fittings × lumens × UF × MF ÷ area — the lumen-method relation in IS 3646 (Part 1):1992, clause 6.2.2.
- Sensitivity: the count and the load are recomputed with UF and MF each 0.1 either side, kept within 0 to 1.
Assumptions
- The fittings are spread so that an average means something: the lumen method gives the average over the working plane, not the minimum at any point.
- The UF belongs to this fitting, its lumens and this room. A UF from another luminaire or another room shape does not transfer.
- All fittings are identical and all count towards the general lighting.
Limitations
- Average illuminance only: uniformity, glare, mounting height, spacing and aiming need a photometric layout.
- Emergency and escape lighting is a separate system with its own requirements and is not sized here.
- The count is not a layout — rows, spacing and the room's shape can need more fittings than the minimum.
- It does not test the W/m² against an energy code. ECSBC 2024 (BEE) covers commercial and office buildings with a connected load of 100 kW or more or a contract demand of 120 kVA or more (§2), and §7.3.1 caps their installed interior lighting power; whether it applies to your building depends on the state's notification and the project, so check it per project.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Area 100 m²
- Target 300 lux average maintained
- 10,000 lm and 100 W per fitting
- UF 0.6, MF 0.8 — test inputs, not recommendations
Working
- Lumens needed = 300 × 100 ÷ (0.6 × 0.8) = 62,500 lm.
- Fittings = 62,500 ÷ 10,000 = 6.25, rounded up to 7.
- Installed load = 7 × 100 W = 700 W = 0.7 kW, or 7 W/m² over 100 m².
- Average maintained illuminance with 7 fittings = 7 × 10,000 × 0.6 × 0.8 ÷ 100 = 336 lux.
Result. 7 fittings and 0.7 kW installed, giving an average of 336 lux maintained against the 300 lux target.
Sensitivity — what moves the answer
- UF and MF divide the count directly: with MF 0.7 the same room needs 8 fittings (0.8 kW); with MF 0.9, 6 (0.6 kW).
- Across UF 0.5–0.7 and MF 0.7–0.9 the load runs from 0.5 kW (5 fittings) to 0.9 kW (9 fittings) — the photometric data and the cleaning regime matter as much as the fitting.
- Rounding up adds light: 7 fittings give 336 lux, 12% above the 300 lux target. At small counts a fraction of a fitting is a large share of the result.
How engineers use the result
To put a first number on fitting count and lighting load for the budget, circuit and DB planning and the transformer and energy estimates — and to test a supplier's proposal by recomputing its count from the same lumens, UF and MF.
When a professional design must replace it
Before fittings are ordered: a lighting designer should produce a photometric layout with the actual luminaire files, room geometry, reflectances and mounting height, check uniformity and glare against the design basis, design the emergency lighting separately, and confirm any energy-code limit that applies.
Sources
- IS 3646 (Part 1):1992 (BIS), Code of practice for interior illumination — Part 1: General requirements and recommendations for working interiors — clause 6.2.2 gives the lumen-method formula with utilisation factor and light loss factor; Table 1 lists recommended service illuminance by interior and activity. A 2025 revision of Part 1 is listed by standards distributors — use the edition your design basis names
- National Building Code of India 2016 (BIS), Part 8 Building Services, Section 1 — Lighting and Natural Ventilation — includes recommended levels of illuminance for different spaces and activities (BIS, Guide for Using NBC 2016); no value from it is built in
- Energy Conservation and Sustainable Building Code 2024 (ECSBC 2024), Bureau of Energy Efficiency — §2 scope, §7.3.1 interior lighting power — applicability depends on state notification and the project; checked 6 Oct 2026
Questions people ask before using it
What do I need before using the lighting lux calculator?
The area to be lit; the average maintained illuminance your design basis sets for the task (IS 3646 (Part 1) and NBC 2016 Part 8 tabulate recommended values by space and activity); the chosen fitting's lumen output and input watts from its datasheet; its utilisation factor for your room's proportions and surface reflectances, from the manufacturer's photometric data; and a maintenance factor for lumen depreciation and dirt over your cleaning cycle.
Which site conditions change the number of fittings?
Mounting height and the room's proportions, which change the utilisation factor; dark or dirty walls, ceilings and floors; dust, smoke or oil mist that dirties fittings between cleanings; obstructions such as racking, cranes and ducts that block light; daylight you plan to rely on; and the task itself — fine inspection work needs far more light than a store aisle.
What should I send an electrical engineer for a lighting design review?
The layout with room uses, mounting heights and obstructions, the lux targets and their source, the proposed fittings' datasheets and photometric files, surface finishes, the cleaning and maintenance regime you can actually run, the existing lighting schedule if this is a retrofit, and the panel or DB schedule the new load will connect to.
Optional: ask an engineer to look at it
The result above is yours with no form. If you want an engineer to check it against your drawings, schedules or bills, send a request. Nothing is sent until you press the button.
Request saved
It reached our team with exactly what the preview showed. An engineer will use the number you gave if they need your drawings or data.
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Got your number — what next?
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