Downlight Calculator

How to use the Downlight Calculator

1. Enter the room length and width.
2. Select the room type to apply recommended lux levels automatically.
3. Choose the downlight wattage and beam angle.
4. View the optimal grid layout with spacing distances from walls and between fittings.

Why use this tool

• Eliminates guesswork by applying lighting design principles to produce an even wash.
• Prevents dark spots near walls by calculating the correct offset from the perimeter.
• Works for kitchens, bathrooms, living rooms, and commercial spaces with different lux targets.

Good downlight spacing is about more than aesthetics. Too few fittings and you get dark patches; too many and you waste energy and create an over-lit, flat-looking room. The rule of thumb is to place downlights at half the spacing distance from the walls, with even gaps between fittings across the room. This calculator applies that principle using the beam angle and mounting height to determine the effective spread of each fitting, then generates a grid layout you can mark out on the ceiling.

Part of Builder Tools Calculators — free construction tools for UK tradespeople.