It's not as simple as you might think...
You may think that the answer to this question is simple...It goes on the right side. Indeed it is often that simple. For example the Mezuzah on the front door of your house will always be on the right side as you enter.
Where it can get more confusing is with the Mezuzahs on the internal doors of your home. For example the door between the kitchen and the dining room or between the living room and the dining room. In a situation where you can move through your house in many different ways, it is not always clear which side of a door is the right side.
In this article I will give a very technical guide to how to determine the right side of any door in your home according to Halacha (Jewish Law). If you find it to be too confusing, don't panic, just call your local Rabbi and I am sure he will be happy to help you work it out.
Evaluate according to a list of criteria:
There is a list of factors that you must evaluate to determine which side of the door is the right side. You should evaluate the situation according to each one of these criteria in order. As soon as you have clearly met one of them, you can use that to determine which side is the right side and ignore the rest of the list. If the first one is not clear then move on to the second one. If it is not clear then move on to the third one etc. until you have a determination.
Here's the list:
Internal vs External - Look at the floor plan of your house. Consider how far-removed each room is from the outside of the house. Number each room according to how many stages removed it is from the outside. When it comes to the Mezuzah, we always go from an external room to an internal room.
So if the door you are considering goes from a room which is 2 rooms removed from the outside to one which is 3 rooms removed, the Mezuzah will go on the right side entering the room which is 3 rooms removed. That is to say from the external room to the internal room. If both rooms are equal, move on to the next criteria...
Flow of Traffic - If it is clear to you that there is a certain common flow of traffic in between these rooms, then you would put the Mezuzah on the right side as you enter the way that is most common to enter. Only use this criteria if there is a clear flow of traffic. If it is not clear then move on to the next criteria...
Room Usage - Mezuzahs are meant to go on rooms that you "live in". So if one room is used more for "living" than that would be considered the room you are entering. For example, if there was a door between an office and a dining room, and all of the previous criteria were equal, you would place the Mezuzah on the right side as you enter the dining room. This is because eating is more of a basic function of a living space than working. If this is not clear, then move on to the next criteria...
Which way the door swings - If all of the above are equal then look at which way the door swings. You would place the Mezuzah on the right side of the room into which the door swings. Interestingly, according to the Chabad custom, you look at this last criteria first and only afterwards at the previous three.
What if it is still not clear?
If you have carefully considered all of these things and both rooms are still 100% equal then you have a problem. I have actually never seen a situation like this in the real world, but in theory there is an argument in Halacha regarding what to do in such a situation.
One opinion says you would put the Mezuzah on whichever side you want. They say that the Torah isn't really concerned with the Mezuzah being on the right side. It is mainly concerned that it not be on the left side. In a case where there is no left side, you can put it on either side.
The second opinion is that you would put a Mezuzah on each side.
We're here to help!
In any case, if you have such a situation, its probably time to call in the experts. Give a call to your local Rabbi or Torah Scribe. You can also always feel free to write me with any Mezuzah related questions and I will do my best to try to help you work them out. Aaron@MezuzahStore.com