How many lumens per square foot do I need?
In the first step of your calculations, you need to choose the type of area and activity that you want to illuminate. Simply select one of the options from the list, and our lighting calculator will automatically determine the optimal level of illumination in lux or foot candles (that is, how much light should be incident on the surface). Intuitively, ambient light in the bedroom shouldn’t affect our preparations to sleep time and won’t be as intensive as the light required for sewing.
The next thing you need to determine is the illuminated area. In the case of a bedroom or a bathroom, it will simply be the total area of the room. If you’re trying to figure out LED lighting for your kitchen counter, the illuminated area will be calculated as the length of the counter multiplied by its width.
Once you know all these values, the foot candle calculator will determine how many lumens you need in total. We use the following equation:
lumens = lux * area
You can use illumination units of lux or foot candles. If you want to recalculate between these units, remember that one foot-candle equals 10.764 lux.
Lighting calculation formula
Once you know how many lumens you need, you can start figuring out how many light bulbs will suffice to illuminate your surface. To do it, use the formula below:
bulbs = lumens / BL
BL stands for the number of lumens that a light bulb emits. You can usually find this number on the bulb packaging. It’s a much better indicator of the bulb’s luminosity than the wattage, as LED lights often need less power than regular light bulbs.
Using the foot candle calculator: an example
- Choose the area of the house that you want to illuminate. Let’s say you’re planning the lighting for your kitchen, including the kitchen counter.
- Check the optimal lighting level. For the whole kitchen, it is 108 lux, and for the counter (detailed tasks) – 538 lux.
- Determine the dimensions of your illuminated space. The whole kitchen is a rectangle of length 4 m and width 2.5 m, so you can calculate the area as
A₁ = 4 * 2.5 = 10 m²
The counter is 4 meters long and 60 cm wide:
A₂ = 4 * 0.6 = 2.4 m²
- Multiply the required illumination level by the area to figure out how many lumens you need:
L₁ = 108 * 10 = 1080 lumens
L₂ = 538 * 2.4 = 1291 lumens
- Then, choose the type of a light bulb you want to use. Let’s assume you’re using a standard bulb that emits 800 lumens for the kitchen and small LED lights that emit 200 lumens each above the counter.
- Divide the total number of lumens by the efficiency of the bulb and round up to figure out the number of bulbs you need:
n₁ = 1080 / 800 => 2 bulbs
n₂ = 1291 / 200 => 7 bulbs
You will need two light bulbs (800 lumens each) to illuminate the whole kitchen and additional 7 LED lights (200 lumens each) above the counter.
If you’re wondering how much you’ll pay for electricity when using all of this lighting, take a look at the electricity cost calculator!