Shower sizes and clearances

The code minimum, read carefully#

IRC P2708.1 asks for three things at once, and people usually remember only the first. The compartment must have at least 900 sq in (0.58 m2) of interior floor area, it must be able to contain a 30 in (76 cm) circle, and it must keep both of those from the top of the threshold up to a point 70 in (178 cm) above the drain.

The circle is the clause that catches people. A 20 x 45 in (51 x 114 cm) tray has 900 sq in of area and fails, because no 30 in circle fits inside it. The 70 in height is the clause that catches renovations: a sloping ceiling, a boxed soil stack or a bench that starts below 70 in can shrink the compliant volume even when the tray is legal.

Then two more numbers. The finished door opening must give at least 22 in (56 cm) clear for egress, and there must be 24 in (61 cm) of clear floor in front of the opening. NKBA's planning figure for that outside space is 30 in (76 cm) the full width of the shower, and the reasoning is the same as everywhere else in bathroom fixture clearances: code stops you being trapped, the guideline lets you dry yourself.

Sizes that actually work#

Footprint	Metric	Verdict
30 x 30 in	76 x 76 cm	Code minimum. Elbows touch the walls washing hair.
32 x 32 in	81 x 81 cm	Common tray size. Still tight for an adult.
36 x 36 in	91 x 91 cm	The real minimum. One adult, arms up, no contact.
36 x 48 in	91 x 122 cm	Comfortable. Room to bend and to keep a dry corner.
36 x 60 in	91 x 152 cm	The tub replacement footprint. Walk-in with a screen.
42 x 60 in	107 x 152 cm	Comfortable walk-in, takes a 15 in bench.
60 x 36 in roll-in	152 x 91 cm	Accessible roll-in, ADA 608.2.2 minimum.
60 x 60 in wet room	152 x 152 cm	Two people or a seated user with a helper.

The step from 30 in to 36 in is worth more than the extra 6 in suggests. At 30 in your shoulders (18 to 20 in / 46 to 51 cm) leave 5 in each side, which your elbows use up the moment you raise your arms. At 36 in the elbows clear. Everything above 36 in buys bending room and dry storage rather than standing room.

Walk-in and wet room geometry#

A doorless entry needs 32 to 36 in (81 to 91 cm) of opening. The 22 in code figure describes an egress door, not a gap you carry a towel through. It also needs length, and this is the part that gets designed by hope.

Two related rules for wet rooms. Keep the whole floor falling to the drain, not just the shower zone, and tank the walls to at least 78 in (198 cm) plus a hand's width beyond the wet area. And put the towel rail and the door outside the 30 in splash radius, or accept a wet towel every morning.

Kerbs, curbless floors and drains#

A tiled kerb is normally 4 to 6 in (10 to 15 cm) above the shower floor. The finished threshold has to sit below the sides and back of the receptor so water runs to the drain rather than over the edge, and a proprietary tray does that for you.

Curbless is a floor build problem, not a tiling problem:

• Fall. 1/4 in per foot (2%) toward the drain. Over a 36 in (91 cm) run that is 0.75 in (19 mm); over 60 in (152 cm) it is 1.25 in (32 mm).
• Linear drain. Set against the back wall or the entry, it gives a single-plane fall, which is why large-format tile works with a linear drain and rarely works with a centre drain (a centre drain needs four planes and a diagonal cut at every corner).
• Recess. The fall plus the tray or mortar bed has to come out of the floor structure. You cannot notch joists for it: IRC R502.8 limits notches to one sixth of the joist depth and bans them from the middle third of the span. The usual answers are a sloped former on top of the existing deck with the whole bathroom floor raised, or engineered joists dropped at the design stage.
• Threshold. Keep any level change to 1/2 in (13 mm) maximum, bevelled above 1/4 in (6 mm), which is the accessible standard and is worth meeting whether or not you need it. More numbers of that kind are in accessible clearances at home.

Inside the enclosure#

Bench. 17 to 19 in (43 to 48 cm) high, 15 in (38 cm) deep. That range is the ADA 610.3 seat height and also just normal seat height, and 15 in is the depth at which a thigh is supported without the front edge cutting in. A bench 15 in deep across the end of a 36 x 60 in shower leaves 36 x 45 in (91 x 114 cm) to stand in, which is still comfortable. In a 36 x 36 in shower, a folding seat is the only honest option.

Niche. The free size is the stud bay: 14.5 in (37 cm) clear at 16 in (41 cm) centres, 22.5 in (57 cm) at 24 in (61 cm) centres. Depth is 3 to 3.5 in (76 to 89 mm) in a 2x4 wall after waterproofing, which takes a shampoo bottle but not a large one lying down. Set the sill at 42 to 48 in (107 to 122 cm) so you reach it without bending, slope the sill about 1/4 in (6 mm) over its depth, and never cut a niche into an exterior wall or the wet wall carrying the supply pipes.

Heights. Showerhead at 78 to 84 in (198 to 213 cm), with 80 in (203 cm) a good default and 84 in for a household with someone over 6 ft (183 cm). Mixer valve at 38 to 48 in (97 to 122 cm) above the floor, which is the ADA 608.5 reach range and happens to be where a standing adult's hand goes; for a tub and shower combination the valve drops to 28 to 32 in (71 to 81 cm) so it can be reached from the tub. A slide rail runs 24 to 36 in (61 to 91 cm) long with the lower bracket at 40 to 48 in (102 to 122 cm); ADA 608.6 requires a hose of at least 59 in (150 cm) where a hand shower serves as the accessible outlet.

Tubs and alcove tolerances#

Tub	Metric	Notes
60 x 30 in	152 x 76 cm	The default alcove tub.
60 x 32 in	152 x 81 cm	Same length, noticeably wider inside.
66 x 32 in	168 x 81 cm	Comfortable for a 6 ft user.
72 x 36 in	183 x 91 cm	Two people, or a genuine soak. Check floor loading.
67 x 31 in freestanding	170 x 79 cm	Allow 4 to 6 in behind and 6 in each side to clean.

Alcove tolerance is where tub installations fail. Frame the opening 0.25 to 0.5 in (6 to 13 mm) longer than the nominal tub length and shim, because a 60 in tub is usually manufactured at 59.75 in (1518 mm) and a wall built dead on 60 in with 0.5 in board on both ends leaves you 1 in (25 mm) short. Check the alcove for square as well as length: a corner out by 1/4 in (6 mm) shows as a tapering tile joint along the whole rim, and nothing about the tub will absorb it.

A filled 72 x 36 in tub with an occupant is roughly 90 to 110 US gallons (340 to 420 litres) of water, about 800 to 950 lb (360 to 430 kg) spread over 18 sq ft. That is within normal residential floor design but worth confirming on an upper floor with long joist spans, particularly for a cast iron or stone resin tub.

The order to size a shower in#

1. Start from the outside. Mark the 24 in (61 cm) code clear space, or 30 in (76 cm) if you have it, in front of the opening. If it collides with the toilet or the vanity, the shower is in the wrong place and no tray size will fix it.
2. Set the entry width next, 22 in minimum with a door, 32 to 36 in with no door.
3. Then the footprint. 36 x 36 in before you consider anything smaller. Drop to a 30 in dimension only after you have tried moving a wall.
4. Check the 70 in rule against ceiling slopes, boxings and any bench or ledge.
5. Fix the head, the valve and the panel last, and place the valve so it can be reached from outside the spray. If you are also planning the ceiling fittings above it, the wet zone rules are covered in bathroom mirror and lighting heights.