Free · Home economics calculators 🏡

Room Lumens & Bulb Count from Target Lux

Estimate lumens and whole bulbs from rectangular area and target illuminance, with retained room presets and a custom target.

  • Formula & worked example
  • Private in your browser
  • No signup
Go to tool

Calculator inputs

Loading calculator…

How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

A 4 m by 3 m area at a custom target of 200 lux needs 2,400 lumens at the surface in this ideal model. Dividing by 800 lumens per bulb gives three bulbs.

Lumens = target lux × area in m²; bulbs = ceiling(lumens / lumens per bulb)

Worked example

Enter these known values and leave the other values blank.

Lighting type
Custom lighting type
Target illuminance
200 lx
Length
4 m
Width
3 m
Lumens per bulb
800 lm / bulb
Area
12 m²
Required lumens
2400 lm
Ideal whole-bulb count
3 bulbs

Assumptions and limitations

  • One lux is one lumen per square meter. The calculation assumes uniform delivery of all entered bulb lumens to the modeled surface.
  • The retained room presets are illustrative targets, not a certification that lighting meets a local standard. Choose the custom option to supply a reviewed target.
  • Fixture losses, room reflectance, mounting height, beam shape, daylight and maintenance losses are not modeled. Actual installed light output can require a different fixture count.
  • Bulb count rounds upward to a whole number. Enter the total lumens from one bulb or fixture, not its electrical wattage.

Common questions

Why is the bulb count rounded up?

A fractional bulb cannot deliver the required modeled output. Rounding upward ensures that the entered per-bulb output times the count reaches the target total in this ideal calculation.

References

Foot-candles use the exact international square foot rather than the rounded captured illuminance factor. The international acre is exactly 43560 square international feet; Bookify replaces the rounded captured area factor.

Explore home economics calculators 🏡 or browse all everyday life calculators.