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How to calculate needed lumens in a room?

Posted by kate1234 (My Page) on
Sun, Mar 4, 07 at 20:25

What would the formula be to figure out how many total lumens you might want in a room? I realize there won't be one answer to a given sized room - that it will depend on

how bright one likes a room to be (bright but on a dimmer),

how good your eyesight (we're getting older),

how light the surfaces are (light oak floor, white painted cabinets, white tile backsplash, stainless appliances, white ceiling, light tan paint in just a few places, and dark shiny granite).

The kitchen is 10' x 12.5' with an 8' ceiling. I will have undercabinet lighting plus cans and I am just trying to figure out how many lumens those cans should have all together.

Follow-Up Postings:

RE: How to calculate needed lumens in a room?

Hi Kate,

I saw your question addressed to me on Dinger's thread, too, but the weekend's been busy. The method I've used is something I came up with on my own, and I'm just an enthusiastic amateur; I'm sure a degreed lighting engineer would have a much more sophisticated approach. I think lumens are primarily useful for gauging ambient light, and that task lighting on countertops is better addressed with footcandle calculations. Anyhow, I think of ambient light as the light bouncing randomly around the room from one surface to another, illuminating the whole, general space rather than a particular surface. I estimated the surface area of the walls, floor and ceiling of my own little galley kitchen (450 square feet) and the nominal lumens generated by the half-dozen 50PAR20 bulbs that I've got in the ceiling (3300 total) and came up with a figure of 7.3 lumens per square foot (walls and ceiling included, remember; this is not just floor area) as a data point to describe my own kithen. Since I sometimes wish I had a bit more light in my kitchen, I figured it might be good to shoot for a bit more than 7.3 lm/sf, so I started using (in a purely intuitive, guesstimate sort of way) a figure of 8.5 lm/sf as a target when suggesting lighting arrangements for other people who asked.

This is a blunt and highly subjective instrument, meant only to avoid ending up with a room that resemebles a cave or an operating theater. As you said, surface reflectances, personal preference and the age of the viewer (I'm 36), not to mention other issues like fixture efficacy and available natural light, can play significant parts, and none of these are mathematically accounted for in this approach. But, then, to account for them mathematically would require a lot of data that isn't typically available in the context of these online chats about particular rooms in distant parts of the country or world.

So, your 10' by 12.5' by 8' kitchen has a walls/floor/ceiling surface area of around 610 square feet. My little 8.5 lm/sf target suggests you might want to build in the capacity to generate at least 5185 total lumens. A basic 50 watt PAR 30 lamp produces about 660 lumens, so I'd think in terms of using about 8 of them to light up that room. This would you give you more general light than I've got, and most of your surfaces are lighter than mine (I've got a lot of earth tones), so that's probably still a decent number despite the fact that you're "getting older."

RE: How to calculate needed lumens in a room?

I been also looking for this question for my ceiling light and got some answers from one forum. Got a little bit encourage and they send me some hints on how.

This link might be useful:

RE: How to calculate needed lumens in a room?

Lighting requirements depend on the type of work being performed (how finely one has to see and what the object's contrast is against its background) and the worker's age. I have seen tables for these factors on the www but cannot provide a reference. A search may prove illuminating.


RE: How to calculate needed lumens in a room?

A quick web search puts ies minimum recommendations at 30FC for residental kitchens yet some califonia city that quoted that ies figure requires a minimum of 40 fc design. Which is smart because todays fluorescent lights, their advertised initial lumen output degrades over time, fairly quick on many of them.

another fwiw;
In general a quick way to roughly estimate lumen requirements, which amazingly works pretty well with normal light color ceil and walls, is foot candles will after losses be 50% of the total lamp rated lumens. If that makes any sense to ya.

Another words for general lighting at 40 fc;
10 x 12.5 room = 125 sq ft x 40 fc = 5000 total fc x 2 because lumens are only 50% efficient coming out of a fixture and bouncing around = 10,000 total lamp lumens required to get that 40 foot candles & should be pretty decent lighting.

However - I seriously doubt you actually need that many lumens because there really is no reason to light up every square foot of the kitchen area with light levels suitable for food preporation or to wash dishes. You're are going to want those levels concentrated on your counter top with task lighting. The work area's is what really counts, not the whole of the room.

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