Bathtub vs Shower Footprint Calculator

What a bathtub and a shower take up on the floor

Floor space is the first thing that goes in a small bathroom, and the tub is usually the largest single object in the room. The math is simple once you stop measuring the room and start measuring the fixture: length in inches times width in inches, divided by 144, gives square feet. A standard alcove tub at 60 by 30 inches works out to 1,800 square inches, which is 12.5 square feet. That is a quarter of the floor in a 5 by 8 foot bathroom.

A shower is not automatically smaller. The footprint depends on the base, not the fixture type. A 36 by 36 inch shower base is 9 square feet, so it saves 3.5 square feet against a standard tub. A 30 by 60 inch shower base is 12.5 square feet, exactly the same as the tub it replaces, and a 42 by 42 inch corner shower at 12.25 square feet saves almost nothing. The shape changes how the room feels, but the number on the floor plan barely moves.

This calculator takes both footprints and puts them side by side, then shows the difference as square feet and as a share of your bathroom floor. It also checks the shower against the 900 square inch minimum that most US plumbing codes apply to a shower compartment.

Standard tub sizes and the floor they need

Almost every tub sold in the US falls into a handful of sizes. The 60 by 30 inch alcove tub is the default in homes built after 1960, because it fits a 5 foot rough opening between three walls. Longer tubs exist for primary bathrooms, and wider tubs exist for soaking, but each step up costs floor area fast.

Those numbers are the tub shell. The rough opening is bigger. A 60 inch tub needs a 60 inch finished opening, and the framed cavity is usually 60.5 to 61 inches to allow for the apron and the tile flange. Add a tile surround and the finished face can push 1 to 2 inches into the room on each side. When you plan a swap, measure the finished opening, not the tub you saw in the showroom.

A drop-in tub is the expensive case. It needs a deck around it, and a 72 by 36 inch tub with a 6 inch deck on the exposed sides occupies closer to 7 by 3.5 feet, or 24.5 square feet. That is more than half of a small bathroom floor, which is why drop-ins usually show up in rooms of 50 square feet or more.

How small a shower can be and still work

US plumbing codes set a floor for shower size. Wisconsin's rule SPS 384.20(5)(L)4, which follows the shower section of the International Residential Code, requires 900 square inches of finished interior area and the ability to fit a 30 inch diameter circle inside the compartment. The measurement is taken 24 inches above the top of the threshold and has to hold up to 70 inches above the drain.

That 900 square inch floor is smaller than most people expect. A 30 by 30 inch stall is exactly 900 square inches and passes. So does a 32 by 29 inch corner unit. The code is a safety minimum, not a comfort standard, and a shower that only meets the minimum feels cramped to most adults.

The federal accessibility rules sit in a different place. The 2010 ADA Standards for Accessible Design require a 36 by 36 inch transfer shower or a 30 by 60 inch roll-in shower in covered buildings, and those dimensions come with clear floor space in front. A home is not bound by ADA, but the numbers are a useful reference for what an adult body needs.

Elbow room matters more than area alone. A 30 by 60 inch shower has 12.5 square feet but only 30 inches of width, so you cannot raise both elbows at once without touching a wall. A 36 by 36 inch shower has less area at 9 square feet but feels roomier because the short side is 36 inches. If comfort is the goal, look at the smallest dimension first and the total area second.

Worked example: a 5 by 8 foot bathroom

Take the most common small bathroom in the US: 5 feet by 8 feet, or 40 square feet of floor. It holds a 60 by 30 inch tub, a toilet and a 24 inch vanity, and the tub sits in a three wall alcove at the far end.

• Tub footprint: 60 x 30 = 1,800 square inches = 12.5 square feet, which is 31.3% of the floor.
• Shower swap, 36 x 36: 1,296 square inches = 9 square feet, or 22.5% of the floor. The room gains 3.5 square feet, which is 8.8% of the floor and roughly a 22 by 23 inch square.
• Shower swap, 30 x 60: 1,800 square inches = 12.5 square feet. The room gains nothing on paper, but the 24 inches of freed wall can hold a cabinet.
• Shower swap, 42 x 42 corner: 1,764 square inches = 12.25 square feet. The gain is 0.25 square feet, or 36 square inches.

The 3.5 square feet from the 36 by 36 swap is the number that changes a layout. It is enough to turn a 24 inch vanity into a 30 inch vanity, or to swing the door inward without hitting the shower. It is not enough to add a second sink, which needs about 6 feet of counter and 30 inches of standing room.

Scale the same math to a larger room and the percentage drops. In a 60 square foot bathroom, the same 3.5 square feet is 5.8% of the floor, and the room already had space to spare. The swap pays off most in rooms under 45 square feet, which is where the door swing, the vanity and the towel bar all compete for the same 3 feet.

One more check before you buy: the drain. A tub drain sits at one end, usually 12 to 18 inches from the end wall. A 36 by 36 shower pan centers its drain, so the old drain lands near a corner of the new pan and the plumbing has to move. A 60 by 30 shower pan can reuse the tub drain in many cases, which is why the swap that frees no floor is also the cheapest one to build.

What the swap gives back, and what it takes

The floor area is only one line on the ledger. A tub to shower conversion changes storage, resale, safety and cost, and those often decide the question more than 3.5 square feet does.

Storage. A tub alcove has three walls and no built-in storage. A shower usually gets a niche, and the 24 inches of wall freed by a shorter base can take a linen cabinet 24 inches wide and 12 inches deep, which adds about 2 cubic feet of storage. That is often worth more to a household than the floor area.

Resale. Real estate agents in most US markets still expect at least one tub in a house with three or more bedrooms. If the hall bath has a tub, converting the primary bath is low risk. If this is the only tub, a buyer with small children may walk, and the discount can run past the cost of the conversion.

Safety. A curbless shower with a bench and a grab bar is the standard recommendation for aging in place. The National Association of Home Builders puts the cost of a basic aging-in-place remodel in the low thousands, against tens of thousands for a fall-related hospital stay. A tub with a 16 inch rim is the hardest fixture in the house to step over.

Cost. A prefab shower and surround runs about $1,500 to $6,000 installed in most markets. A tiled walk-in with a new pan, drain and waterproofing runs $6,000 to $15,000. Moving the drain adds several hundred dollars and, in a slab-on-grade house, a jackhammer.

Water. A full tub holds 40 to 60 gallons. A 2.5 gallon per minute shower head uses 20 gallons in 8 minutes. The swap usually cuts water use, but a rain head at 2.5 gpm for 20 minutes uses 50 gallons, which erases the gain.

The space a tape measure misses

Two bathrooms with the same fixture list can feel completely different, because the usable floor is not the same as the open floor. Four things eat the difference.

Wall build-up. Tile, backer board, mortar and waterproofing add 3/4 to 1-1/4 inches per wall. On a 60 by 30 inch alcove that can take 2 inches off the interior width, and the shower base has to be ordered to the finished opening, not the framed one.

Door swing. A 30 inch door swinging into a 40 square foot bathroom sweeps about 5 square feet, and code generally forbids a door from swinging into the clear floor space in front of a tub or shower. A pocket door costs $1,000 or more to add but returns the full swing area.

Clearance in front. Codes and good practice want 24 inches of clear floor in front of a shower door and 30 inches in front of a vanity. A shower that fits the alcove can still block the vanity if the room is only 5 feet wide.

The curb and the threshold. A 4 inch curb reduces the usable depth of a 36 inch shower to about 32 inches at the entry. A curbless design needs the floor dropped 2 to 3 inches, which is a framing change, not a finish change.

Add those four and the honest footprint of a 36 by 36 shower is closer to 40 by 40 inches of floor you cannot use for anything else. That still beats a 60 by 30 tub, which claims a 60 by 30 alcove plus the wall you cannot hang a cabinet on. The gap narrows, but it does not close.

Measuring and checking before you commit

Five measurements settle most of the question, and they take about ten minutes with a tape and a notepad.

• The finished opening. Measure wall to wall at the widest point and at the floor. Older rooms are rarely square, and a 1/2 inch difference decides whether a prefab pan fits.
• The drain center. Measure from each side wall to the drain. Compare it with the drain location on the pan you want.
• The door swing. Note which way the door opens and how far it travels.
• The clear floor. Mark 24 inches in front of the shower and 30 inches in front of the vanity. If the marks overlap, the layout needs work.
• The route. A 60 inch tub does not fit through a 30 inch door or around a tight stair. Measure the path before you order.

Then run the code check. The shower interior needs 900 square inches and a 30 inch minimum dimension, the drain needs a slope of 1/4 inch per foot to the curb, and the waterproofing needs to turn up the wall at least 2 inches above the curb. Local amendments vary, and a permit inspector has the final word, so confirm the numbers with your local building department before demolition day.

Figures here are planning estimates from nominal fixture sizes. Confirm the exact dimensions of the fixture you buy, and check local code before removing a tub.