I’m interested in converting two small closets into an additional bath and would like to know the smallest square footage needed to accommodate a full bath.
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You could probably squeeze a bathroom in to 20 square foot area. The bath tub is 5 ft by 2ft so there is 10 square feet and you need another 2-3 square feet for the sink and another 2-3 for the toilet. You might want to leave a little walk room. If you are really tight on space you could skip the tub, tile the entire bathroom floor and walls and just do a shower. Design it so it will not have to be an enclosed shower and you could really scale back on space.
The most practical size is 5’ x 7’ or 35 sf.
I would agree that you really don’t want to go much smaller than about 30-35 sq. ft for it to be considered a full bath. For true functionality, that is about as small as you probably want to go. The biggest variable is whether you need a full bathtub or if you can use a corner shower unit. That knocks off about 7 sq. ft alone.
5’ x 7’ is the smallest practical size for a code-complying full bath with a tub. If you have a tub at one end, protruding 30" into the room, you can put a toilet and small vanity into the remaining space.
If you replace the tub with a shower, use a tiny sink, and install a wall-mount toilet, you might get as small as 3’6" x 6’6". The tiniest full baths usually don’t obey code requirements for space in showers, around the toilet, etc.
Check first with your local planning / building department because rules can vary. Also look for fixtures that don’t protrude into the room. ajmadison.com has a nifty tool for checking out plumbing fixtures. 
Rhoda
35 sq. ft ( At least 7 x 5)
There are few different requirements and spacing needed from the shower, toilet etc. that are criticall in building a new bathroom.
This article below gives a good descritpion of bathroom guidelines and restrictions.
I once did one under the basement stairs. 24 Sq ft
For a house, you should select the components you want to incorporate intot he bathroo, then lay it out on graph paper. A corner shower would take less space, as would a shower curtain vs a glass door. A wall hung sink will take less space, compared to a vanity sink.
Cheaper motorhomes and travel trailers cram a full bath into 12 square feet.The floor is the shower pan & the toilet sits on it. The drain in in the floor in front of the toilet. You have to dry off the sink, toilet and walls after using the shower - the entire bathroom it literally the shower.
5’ by 7’ is the smallest size for a functional bathroom. This would generally include a 21" vanity and a 30’’ toilet space, plus a 32’’ tub. This adds up to 83’’.
For a small single family home it could be 5’ x 7’
5’ X 7’ is pretty much the smallest I would go with in order to have a completely functional bathroom which includes a 5’ tub. I believe thats also code when having a full size tub. I have done smaller bathrooms 4’ 6 X 7 but that only had a a 4’ shower. tight but it worked.
This is a great question! We recently built a bathroom in a 5 x 7’ space which included a 32" x 32" corner shower, enclosed with frameless glass wall and door, medium size pedestal sink and a corner toilet. What was particularly challenging about this bathroom is that we had to incorporate 2 doors and one medium window in the design.
Another recent design build was a 5’ x 9’ bathroom space which included a 5’ tub with shower, pedestal sink, toliet and a wall mount IKEA storage cabinet. There are a couple of pictures of that space on our Buildzoom page if you are interested in checking it out.
Dickens Thomas
Dickens Thomas Construction