What this site is for
KelvinPlan answers two questions that almost every lighting decision comes down to: how much light does this room need, and what colour should it be.
Everything here is domestic. We do not cover commercial, industrial, emergency or stage lighting, all of which are governed by codes and photometric requirements this site does not attempt to represent.
Where the numbers come from
Every figure on this site traces to a published source, and the page says which one where it matters.
| Figure | Source |
|---|---|
| Footcandle targets by room | IES recommended illuminance for residential spaces |
| Downlight spacing = ceiling height ÷ 2 | Installation guides published by Halo, Lithonia and Juno |
| Bathroom extraction airflow | HVI guidance and ASHRAE 62.2 |
| Colour rendering (Ra, R9, R1–R15) | CIE 13.3 method; TM-30 where quoted |
| Lumen maintenance (L70, LM-80, TM-21) | IES LM-80 and TM-21 |
| Flicker limits | IEEE 1789 recommended practice |
| Reflectance values | Standard published surface reflectance ranges |
Where a number is a convention or a rule of thumb rather than a measurement — and several of the most useful ones are — the page says so in those words rather than dressing it up as a standard.
Manufacturer figures vary, sometimes widely. Where they do, we give the range rather than picking a point inside it and presenting it as the answer.
How the calculators work
The five calculators on this site compute in code rather than reproducing a lookup table, and the working is shown on the page so you can check it against your own situation rather than taking the output on trust.
Each one is tested across its full input range before it goes live: for impossible values, for missing results, and for answers that contradict the explanation printed beside them.
If a calculator cannot give an honest answer for the numbers you entered — a room shape it does not model, an input outside the range the underlying source covers — it says so instead of producing a figure.
Who writes this, and how
Articles are researched and written for this site, then edited before publication. Explanatory prose is drafted with AI assistance; the numbers are not. Every figure comes from the sources in the table above or from the calculators' own code, and no value on this site is produced by a language model.
We state that distinction plainly rather than making a blanket claim in either direction, because the distinction is the part that affects whether you can rely on what you read here.
What we do not do is publish a number we cannot source. Where the honest answer is that it depends on something we cannot see from here, the page says that.
What we are not
Not electricians. Anything involving new circuits, work near water, or building regulations is a job for a qualified person in your jurisdiction, and the requirements differ between them.
Not a shop. There are no affiliate links on this site, no sponsored placements and no paid product recommendations. Where a product category is named, it is because the specification matters, not because anyone paid for it.
Advertising
The site carries display advertising. Advertisers have no involvement in what is published here, no advance sight of it, and no ability to influence it. Advertising is separated from editorial in the layout so it is clear which is which.
Corrections
If something here is wrong, tell us: [email protected]. We would rather hear it than not.
A wrong figure in a calculator is fixed in the code, which corrects it everywhere that calculator is used. A source that has changed — a revised datasheet, an updated standard — is corrected with a note recording what changed and when. Substantive corrections are dated on the page.