To select a light bulb very first step is defining the need for lighting. The lighting is categorised into two sections which are ambient lighting and task lighting.

Ambient lighting is general for the room lighting where you may mount a bulb at the ceiling of the room. On the other hand task lighting is different. It is for the task specific and you can define that lighting amount yourself according to the work you will be doing.

For most of the cases the following assumptions will be working and we will check also how to calculate. Please be aware that these are the rough estimations and it is always better to use the national regulations if there is any in your area. (Be aware for SI using countries 1 fc is about 10,76 lx and 1 foot = 0,3048 meters)

PLACE	FOOT CANDLES
Kitchen ambient	40 – 50
Kitchen task	70 – 80
Bedroom	10 – 20
Bathroom	70 – 80
Living room	10 – 20
Dining room	30 – 40
Hallway	5 – 10

So how to calculate?

You have found the foot candle for your room. So what is next? The next is lumen. Lumen is used to measure light. To find lumen you need to multiply your foot candle requirmemt with your room area which is in foot. So in this case if your bathroom is 100 square foot, then you need 100×80 = 8000 lumens. (Here for my metric friends 1 foot = 0.3048 ?)

Now actually lighting is a personal issue sometimes and you may find these values very birght or dark. In this cases it is ok to fine adjust it with %10-20 so that it suits your comfort. On the other hand it is the best if you use dimmer light switches where you can dim the light according to your needs.

How do you like your light? Soft or cool? ♨️?

Another important thing is the light appearance in other words how do you want your bulb? ?

You may love it softer(warmer) or bright (cooler )or none of them which is daylight. This is indicated by Kelvin of it but no need to define it here in this text’s scope since this is an introduction. The only thing you need to know at this point for kelvin is that you need to select softer for areas where you have a rest and more bright for the areas where you are mostly active. For example for kitchen bright may be ok but for living room or bedroom you may like soft which makes you feel more comfortable and ready to have a rest. ?

But how much watt is it? ?

You cannot directly say this lumen is this watt. ?

But you can convert it if you know the luminous efficacy of the bulb (lumens/ watt) Here at this point there are defult values which may benefit you to calculate .

Bulb type	Average Lumens / watt
Halogen lamp	20
Flourescent	60
LED	90
Tungsten incandescent	15
Mercury vapor	50
High pressure sodium	115
Low pressure sodium	150

As you can see these are the average values for your rough estimations in your lighting design. You will get more detailed information and specific values from your bulb provider’s catalog.

From here you can also get hint about which bulb type is better. Which one gives more light per watt. This is the point where you will get. If efficiency is your point then search for luminoius efficacy in your catalog for bulbs.

If we come to our previous example, we found out that our bathroom need 8000 lumen. So if we use LED lighting we will need 8000/90 = 88.88 watt. So you may go for 89 watt or 8000 lumen. And from market you can buy at least this amount of wattage or lumens bulbs. ?