Room Lighting Calculator

Plan room lighting by estimating total lumens, LED watts, and a simple fixture count. Enter room size and your lighting goal, then use advanced options for CU, LLF, ceiling height, daylight, and a basic layout visualisation.

Target illuminance: –
This changes the assumed work surface height and the target brightness.
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How to use the Room Lighting Planner

This calculator estimates the total light output (lumens) your room may need. It also suggests fixture counts and shows a simple layout diagram.

  1. Select the room type:
    Choose the closest match. This sets a typical illuminance target (lux) for that space.
  2. Choose a measurement unit:
    Select Feet, Inches, or Metres. Use the same unit for width, length, and ceiling height.
  3. Enter room width and length:
    Enter the inside dimensions (wall to wall). Both must be positive numbers.
  4. Set your lighting goal:
    Choose Low, Standard, or Bright. The "Target illuminance" line updates to show the lux target being used.
  5. Pick the main activity area:
    Choose General room, Desk or table, Kitchen bench, or Vanity mirror. This adjusts the assumed work surface height and usually increases the target brightness for task areas.
  6. Choose room finishes and fixture type:
    Finishes affect how much light is effectively used in the room. Fixture type affects the utilisation estimate (CU) and the example fixture plan.
  7. Optional: open Advanced options:
    Use this if you want a more technical estimate.
    • Ceiling height: Leave blank to use the default (8 ft / 2.4 m).
    • Override target illuminance (lux): If entered, it replaces the preset target.
    • Daylight in this room: Adjusts the recommendation up or down based on natural light.
    • Light loss factor (LLF): Accounts for dust and ageing. Lower LLF increases the recommended lumens.
    • Coefficient of utilisation (CU): Auto estimate uses room geometry and fixture type. Manual lets you enter your own CU (0.30 to 0.85).
    • LED efficacy (lumens per watt): Used to estimate total LED watts.
    • Fixture planning method: Keep the default to estimate fixture count from a lumen rating, or choose the known-count option to divide the recommendation across your fixtures and bulbs.
    • Existing fixtures and lumens: If entered, you will see an "Existing vs recommended" bar.
  8. Click "Calculate":
    You will see:
    • Recommended total light output: total lumens for the whole room.
    • Target illuminance: the lux (and foot-candles if using feet or inches).
    • Estimated LED power: total watts based on the lm/W assumption.
    • Model factors used: CU, LLF, and room index K.
    • Fixture plan: an estimated fixture count, example counts, or per-bulb shopping guidance when you enter a known fixture and bulb count.
    • Room layout visualisation: a simple grid showing fixture positions. If the count is very large, it caps the drawing and notes it.
    • Existing vs recommended: shown only if you entered existing fixture count and lumens per fixture.

Tip: If you are planning downlights, the calculator also shows a rough spacing range based on mounting height. Use it as a starting point, then adjust for beam angle and glare.

Example Room Layout Visualisation

Methodology and sources

What this calculator is doing

This calculator estimates the total light output (lumens) needed to reach a target illuminance (lux) on a chosen work plane. It uses a simplified lumen method with coefficient of utilisation (CU) and light loss factor (LLF).

Step 1: Room area and unit conversions

You enter room width and length in feet, inches, or metres. The calculator converts both to metres and computes area:

Area (m2) = width_m * length_m

If you are using feet or inches, the calculator also shows area in square feet using:

Area (ft2) = Area (m2) * 10.7639

When foot-candles are shown, they are computed from lux using:

foot-candles = lux / 10.7639

Step 2: Target illuminance (lux)

The base target illuminance comes from the built-in room table for the selected room type and lighting goal (low, standard, bright). The "Main activity area" then applies a multiplier:

  • General room (floor level): 1.00
  • Desk or table: 1.15
  • Kitchen bench: 1.30
  • Vanity mirror: 1.40

If you enter "Override target illuminance (lux)", that value replaces the calculated target (if it is a positive number). The built-in room targets are planning presets rather than quoted IES recommendations; use an applicable project standard or lighting professional's target when compliance or task-critical lighting matters.

Step 3: Mounting height and room index K

Ceiling height is taken from "Ceiling height" if provided. If blank, it defaults to 2.4 m. An entered ceiling height must be a positive number. The calculator assumes a work plane height (in metres):

  • Desk or table: 0.76 m
  • Kitchen bench: 0.90 m
  • Vanity mirror: 1.00 m
  • General room (floor level): 0.00 m

Mounting height is then:

Hm = ceiling_m - workplane_height_m

If Hm is too small, the calculator clamps it to a minimum of 0.5 m to avoid unrealistic values.

The room index K is calculated as:

K = (L * W) / (Hm * (L + W))

where L and W are the room dimensions in metres.

Step 4: Coefficient of utilisation (CU)

CU represents how much of the emitted lumens end up as useful light on the work plane. In "Auto estimate" mode, CU is estimated from fixture type and room index K using a simple stepped model, then adjusted for room finishes:

  • Mostly light finishes: +0.05
  • Mixed finishes: +0.00
  • Mostly dark finishes: -0.07

CU is clamped to stay between 0.30 and 0.85. In "Enter my own CU" mode, enter a value from 0.30 to 0.85, or leave it blank to keep the automatic estimate. Values outside this range show validation.

Step 5: Light loss factor (LLF)

LLF accounts for lamp ageing, dirt, and real-world losses. The dropdown provides fixed options (0.90, 0.80, 0.70). The default is 0.80.

Step 6: Daylight adjustment

"Daylight in this room" applies a multiplier to the required lumens:

  • Average daylight: 1.00
  • Little or no daylight: 1.10
  • Good natural daylight: 0.85

Step 7: Total required lumens

The required total lamp lumens are computed as:

Required lumens = (target_lux * area_m2) / (CU * LLF) * daylight_factor

The calculator rounds up to the next whole lumen.

Step 8: Estimated LED watts

If you do not enter LED efficacy, it defaults to 90 lumens per watt. Watts are estimated as:

Watts = ceil(required_lumens / lumens_per_watt)

Step 9: Fixture count and spacing guidance

If you enter "Lumens per bulb or fixture", the calculator estimates the fixture count as:

Fixture count = ceil(required_lumens / lumens_per_fixture)

If you instead enter a known fixture and bulb count, the calculator keeps the same recommended total and divides it evenly:

Total bulbs = fixtures * bulbs_per_fixture

Lumens per bulb = required_lumens / total_bulbs

The per-bulb watt estimate uses the selected lm/W value. Treat lumens as the main package-shopping value because real LED wattage varies.

If you leave that blank, it shows example counts using typical per-fixture lumens by fixture type:

  • Recessed downlights: 600, 800, 1100 lm
  • Ceiling fixture (direct): 1200, 2000, 3000 lm
  • Pendant (direct): 800, 1100, 1600 lm
  • LED panel: 3000, 4000, 5000 lm
  • Indirect (bounce light): 1500, 2500, 3500 lm

For recessed downlights, it also suggests a spacing range based on mounting height:

Spacing range (m) = 0.8 * Hm to 1.2 * Hm

That range is converted back into your selected length unit for display.

Step 10: Existing vs recommended (optional)

If you enter "Existing fixtures" and "Existing lumens per fixture", the calculator estimates current total lumens and compares them to the recommendation:

Existing total = existing_count * existing_lumens

Percent of recommended = (existing_total / required_lumens) * 100

Visualisation logic

The diagram places fixture dots on a simple grid inside a scaled room rectangle. It uses the estimated fixture count (or the example mid-count) and caps the drawing at 36 fixtures to keep the diagram readable. The grid dimensions are chosen from the square root of the fixture count to keep the layout roughly square.

Limits of this estimate

This is a planning estimate. Real results depend on fixture photometrics, beam angle, glare control, ceiling reflectance, surface reflectances, obstructions, and how the fixtures are actually spaced. If you need an accurate design, use manufacturer IES files with lighting software or get a lighting designer to model the room.

Sources