Bulb Count Calculator

Example data table

Formula used

The calculator estimates the required bulb count using the lumen method: N = (E × A) ÷ (F × UF × MF).

• N = number of bulbs (rounded to a whole number)
• E = target illuminance (lux). If you enter foot-candles, it is converted to lux.
• A = floor area (m²). If you use ft², it is converted to m².
• F = luminous flux per bulb (lumens)
• UF = utilization factor (captures fixture and room efficiency)
• MF = maintenance factor (captures dirt and lamp aging)

How to use this calculator

1. Select Design to estimate bulbs needed for a target level.
2. Choose your unit system, then enter room dimensions or an area override.
3. Enter your target brightness and bulb lumen rating from the packaging.
4. Set UF and MF based on room reflectance, fixture type, and maintenance.
5. Optionally enter bulbs per fixture to estimate fixture count.
6. Press Calculate to see results above the form.
7. Use the CSV and PDF buttons to export the latest report.

Lighting planning guide for construction spaces

1) Why bulb counts matter on site

Temporary and permanent lighting both influence productivity and safety. Underlit corridors increase trip risk, while overlit areas waste energy and create glare. A bulb count estimate gives a fast starting point for procurement, fixture layout, and power planning when drawings are still evolving.

2) Understand illuminance targets

Illuminance is measured in lux (or foot-candles). Typical indoor guidance ranges from 100–200 lux for circulation zones, 300–500 lux for offices and general work, and 500–1,000 lux for detailed tasks like finishing, inspection, or bench work. Outdoor task lighting often needs higher local levels due to contrast.

3) Area and geometry inputs

The calculator uses floor area as the baseline because it correlates with average light on the working plane. For rectangular rooms, enter length and width; for irregular footprints, use the area override from a takeoff. When you enter ft², it is converted to m² for consistent results.

4) Lumen rating and lamp selection

Lumens describe total light output from each bulb. Modern LED lamps commonly range from 450 to 1,600 lumens, while higher-output lamps and integrated fixtures can exceed that. Choose a lumen value from the manufacturer label, then keep the same bulb type when comparing options.

5) Utilization factor (UF) as efficiency

UF captures how much of the lamp output actually reaches the work plane. It varies with fixture optics, mounting height, room reflectance, and spacing. Light-colored walls and efficient luminaires raise UF; dark finishes, high ceilings, and wide spacing lower it. Typical practical inputs fall between 0.4 and 0.8.

6) Maintenance factor (MF) for real life

MF accounts for depreciation from dust, temperature, aging, and lens yellowing. In clean indoor areas, MF may approach 0.9–1.0. On construction sites with dust or outdoor exposure, 0.7–0.85 is common. Using MF prevents the design from meeting targets only on day one.

7) Fixtures, circuits, and energy checks

After bulbs are estimated, fixtures are derived using bulbs per fixture so you can size quantities quickly. Add optional wattage and hours to approximate daily and monthly energy. These estimates help coordinate temporary power, breaker loading, and generator capacity, especially when schedules shift.

8) Interpreting results and next steps

Use the “Bulbs needed” value as a planning baseline, then refine with layout, mounting height, and glare control. If the achieved lux looks marginal, raise lumen output, improve UF with better fixtures, or increase quantity. Export CSV or PDF for submittals, site notes, and cost tracking.

FAQs

1) What is the difference between lux and foot-candles?

Lux and foot-candles both measure illuminance. One foot-candle equals about 10.764 lux. This calculator converts automatically so you can work in either unit without manual conversions.

2) Which UF and MF values should I start with?

For typical indoor spaces, start with UF 0.6 and MF 0.8. Use lower values for dusty areas, dark finishes, high ceilings, or wide spacing, and higher values for efficient fixtures and clean environments.

3) Does this replace a full lighting design?

No. It is a fast planning tool using the lumen method. Final designs should consider fixture photometrics, mounting height, uniformity, glare, and task locations, especially for compliance-critical projects.

4) Why does rounding up matter?

Rounding up provides a buffer against real-world losses and layout constraints. Rounding down can drop illuminance below the target once dirt, aging, and imperfect spacing are considered.

5) Can I use integrated LED fixtures instead of bulbs?

Yes. Enter the fixture’s lumen output as “lumens per bulb,” then set bulbs per fixture to 1. The math works the same as long as the lumen value represents one installed light source.

6) How do I handle irregular rooms or open plans?

Use the area override from your takeoff or drawing. If zones have different tasks, run separate calculations for each zone and combine counts, then adjust layout to balance uniformity.

7) Why is achieved lux shown even in Design mode?

Achieved lux reflects the rounded bulb count and your chosen factors. It helps you see the safety margin above the target and compare how changes to UF, MF, or lumens affect performance.