A mezuzah goes on the right-hand doorpost as you walk into the room, set in the upper third of the doorway. Ashkenazim affix it on a slant with the top tilted inward; Sefardim place it vertically. You say the bracha "...likboa mezuzah" right before fixing it. Most living-space doorways with two doorposts and a lintel are obligated, while rooms like a bathroom are exempt. For any edge case, ask your rav.
Which doorways need a mezuzah, and which are exempt?
Doorways of dwellings, courtyards, barns, and storehouses are obligated in a mezuzah (Yoreh De'ah 286:1). A doorway only carries the obligation if it has two doorposts and a lintel across the top (YD 287:1). So a normal room entrance in a home generally needs one.
Some doorways are exempt: a bathroom, bathhouse, tannery, or mikveh, because they are not made for dignified dwelling (YD 286:4), and a room smaller than four amos by four amos (very roughly six to eight feet square) is exempt as well (YD 286:13). Whether a particular small room or borderline case is fully exempt or obligated without a bracha is a genuine machlokes, so don't pasken it yourself, ask your rav.
Which side of the doorway does a mezuzah go on?
The mezuzah goes on the right-hand side as you enter the room, not as you exit it. This is a clear din, not a custom: placing it on the left is pasul (YD 289:2). The way to figure out "right" for any given doorway is to stand where you'd normally walk in and look to your right.
For an interior doorway between two rooms where it isn't obvious which is the "entrance," determining the direction of entry can get subtle. When it isn't clear-cut, that's a question for your rav rather than a guess.
How high on the doorpost does the mezuzah go?
The mezuzah is placed at the start of the upper third of the doorway, measuring from the bottom up. Placing it higher than that is still valid, as long as it stays at least a tefach (a handbreadth) below the lintel at the top of the doorway (YD 289:2).
Notice that the source defines the spot in proportions and a tefach, not inches or centimeters. Anyone handing you an exact "X inches off the floor" number is converting, not quoting the halacha. Measure the upper third of your specific doorway and place it there; if the height is awkward on a particular frame, ask your rav.
Does a mezuzah go on a slant or straight up and down?
This depends on minhag. The Sefardi custom is to affix it vertically, straight up and down. The Ashkenazi custom is on a slant, with the top tilted inward toward the room (Rema, YD 289:6). The meticulous approach behind the slant is that it fulfills both opinions at once, the view that it should stand vertical and the view that it should lie horizontal.
Because this is minhag and not a fixed din, follow your family or community practice. If you're unsure which custom is yours, ask your rav.
What bracha do you say when putting up a mezuzah?
Before you fix the mezuzah to the doorpost, you say the bracha ending "...likboa mezuzah" (YD 289:1). The bracha comes right before the act of affixing it.
If you're putting up several mezuzos at once, you don't repeat the bracha for each doorway. One who affixes two or three mezuzos in one go says a single bracha covering all of them (YD 289:1). Moving into a new home and doing the whole place in one session is exactly this situation. The exact mechanics of intent and interruption between doorways are worth confirming with your rav.
Can you take the mezuzos with you when you move out?
If a renter is moving out and another Jew is moving in, he may not remove the mezuzos when he leaves (Bava Metzia 102a; YD 291:2). They stay on the doorposts for the next Jewish resident.
This is a frequent surprise for people leaving a rental, so plan for it: budget for a fresh set in your new place rather than counting on bringing the old ones. A mezuzah is also a meaningful housewarming or new-home gift for that reason, and a kosher handwritten mezuzah from kmezuzah.com is a fitting way to outfit a new home. The details of which situations require leaving them behind, and what happens with a non-Jewish landlord or buyer, vary, so ask your rav for your specific move.
Common questions
- Does every door in the house need a mezuzah?
- Most living-space doorways that have two doorposts and a lintel are obligated, such as the doorways of a dwelling and its rooms. Bathrooms, bathhouses, and similar non-dignified spaces are exempt, and very small rooms can be exempt as well. Because borderline rooms are disputed, confirm any uncertain doorway with your rav.
- How do I know which side is the right side?
- Stand where you would normally walk into the room and look to your right; the mezuzah goes on that doorpost. The right-as-you-enter side is a din, and the left side is pasul. For an interior doorway where the direction of entry isn't obvious, ask your rav.
- Do I say a separate bracha for each mezuzah?
- No. When you affix two or three mezuzos at once, you say a single bracha ("...likboa mezuzah") that covers all of them, rather than repeating it at each doorway. This is the typical case when setting up a new home in one session.
- Can I take my mezuzos when I move out of a rental?
- If another Jew is moving in after you, you may not remove the mezuzos when you leave; they stay for the next resident. Plan to buy a fresh set for your new home. Situations involving a non-Jewish landlord or buyer differ, so ask your rav.
- Should my mezuzah be slanted or vertical?
- It depends on your custom. The Ashkenazi minhag is to slant it with the top tilted inward; the Sefardi minhag is to place it vertically. Both are valid customs, so follow your family or community practice, and ask your rav if you're unsure which is yours.