Lighting — Lux & Lumens
The lumen method in three parts: how many lumens and luminaires a target illuminance demands, what an installation that already exists actually delivers maintained and on day one, and where the luminaires go — room index, maximum spacing from the spacing-to-height ratio, a suggested grid with wall offsets, and the installed power density per 100 lux. Metric and imperial, fully offline.
Open Lighting — Lux & Lumens →What is the lighting calculator?
A free lumen-method lighting calculator that runs entirely in your browser, in metric or imperial. The first tab answers the design question: for a room of a given size and a target illuminance, how many lumens are needed and how many luminaires that comes to, using Φ = E × A / (UF × MF). It then shows what the rounded-up whole number of luminaires actually delivers, which is rarely the target exactly. The second tab works the other way, for an installation that already exists: enter the room, the luminaire count and their output and it gives the maintained illuminance, the initial illuminance on day one before any depreciation, and how far short of a target you are in lumens. The third tab handles the layout — the height above the working plane, the room index, the widest spacing the spacing-to-height ratio allows, a suggested grid with its wall offsets, and the installed power density per 100 lux. Nothing is uploaded.
How to use Lighting — Lux & Lumens
- Describe the room — Length and width, or the floor area directly if the room is not rectangular. Pick a space type to fill in a typical maintained illuminance, or type your own target — each entry carries how well corroborated it is.
- Enter the two factors honestly — The utilisation factor comes from your luminaire's photometric data at your room index and surface reflectances. The maintenance factor is a loss factor; open the four-factor panel to build it from lamp lumen maintenance, survival, luminaire maintenance and room surface maintenance rather than picking a round number.
- Check which lumens your UF belongs to — A classic CIBSE-style table is referenced to lamp lumens; a modern LED photometric file is referenced to luminaire lumens. The selector relabels the lumens field to match. Getting this pairing wrong is worth twenty to forty per cent.
- Lay them out — On the Layout tab, enter the ceiling height, any suspension and the working-plane height. The tool works out the height above the working plane, the room index, the maximum spacing your spacing-to-height ratio allows, and a grid — then tells you whether that grid's actual spacing passes in both directions.
Frequently asked questions
What is the difference between maintained and initial illuminance?
Maintained illuminance is the design figure: what the room still gives at the end of the maintenance cycle, after lamps have depreciated and surfaces have dirtied. Initial illuminance is what it gives on day one, before any of that. The maintenance factor is the ratio between them, so a room designed to 500 lx maintained with an MF of 0.8 starts life at 625 lx. Standards quote maintained values, which is why this tool does too.
Where does the room index come from?
It is computed, not looked up: RI = L × W / ((L + W) × Hm), where Hm is the height of the luminaires above the working plane. It is twice the floor area divided by the wall area between those two planes, which is why a long thin room and a square room of the same floor area have very different indices — and very different utilisation factors. Computing it means it is right at any shape, including well outside the 0.75 to 5.00 span a table tabulates.
Why does the tool not fill in the utilisation and maintenance factors for me?
Because between them they swing the answer by a factor of two, and neither is a property of the room alone. The utilisation factor depends on your luminaire's light distribution, the room index and the ceiling, wall and floor reflectances. The maintenance factor depends on the lamp, the cleaning interval and the environment. Indicative tables are offered, clearly labelled and editable, but nothing is applied on your behalf.
What is installed power density per 100 lux?
The watts per square metre needed to deliver 100 lux. It is the figure that lets you compare two schemes fairly, because raw watts per square metre rewards a scheme that simply under-lights the room. It is metric by definition, so it is reported in W/m² even when you are working in feet and footcandles. The tool also shows the achieved luminaire efficacy against the 95 lm per circuit watt general-lighting figure in Approved Document L — which is a UK figure, and guidance rather than a verdict.
How firm are the tables in this tool?
Less firm than in its sibling electrical calculators, and the tool says so on screen. The illuminance figures follow EN 12464-1 but several rows are a mapping onto a table row that does not carry that name; the maintenance-factor and spacing-to-height rows are engineering rules of thumb rather than published table entries; and the utilisation-factor sample is one published table for one fluorescent luminaire. Every one of them fills an editable field and none is applied automatically. Check them against the standard and against your own photometric data.
Tips
- Round the luminaire count up, then check what that whole number actually gives — the answer is almost never the target exactly, and a 5% over-provision is usually cheaper than a clever fraction.
- If the spacing fails only in one direction, changing the grid shape often fixes it without adding a single luminaire.
- An average illuminance that meets the target can still be an unusable room: this method says nothing about glare, uniformity or how the light is modelled.