About the Bulb Quantity Calculator
How many bulbs or fittings a room needs, and a sensible spacing grid for them.
Also known as: how many light fittings, downlight spacing calculator, number of bulbs needed, spotlight quantity.
How to use this calculator
- Enter room length, room width, ceiling height, target illuminance and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
fittings = target lux × area ÷ utilisation ÷ lumens per fitting
Worked example
Using the values pre-filled in the calculator above:
- Room length = 5 m
- Room width = 4 m
- Ceiling height = 2.4 m
- Target illuminance = 300 lux
- Lumens per fitting = 500
- Utilisation factor = 0.6
the calculator returns:
- Fittings required: 20
- Suggested grid: 4 rows × 5 columns
- Spacing along the length: 1 m
- Spacing across the width: 1 m