How to measure for a shower kit, including the numbers most people forget

What you need before you start

A 25-foot tape measure, a 4-foot level, a pencil, your phone camera and a sheet of paper. A framing square helps but is not essential. Measure in inches and write fractions to the nearest eighth. Rounding every number to the nearest inch is the most common reason a kit that “should fit” does not.

Measure twice, on two different days if you can, or have a second person read the tape. Write each number on a quick sketch of the room viewed from above, with the door, the window, the toilet and the plumbing wall marked. That sketch is worth more to a supplier than any single measurement.

Step 1: decide which opening you are measuring

There are two openings in every shower space and they differ by the thickness of the wall finish. The finished opening is wall surface to wall surface, the space you see today. The rough opening is stud to stud, what you will have once the old tile, backer board or surround is stripped out. Depending on the design, a kit is sized to one or the other. Flanged bases and panels that fasten to framing usually work from the rough opening; some remodel systems are made to sit inside the existing finished walls.

If the walls are still covered, measure the finished opening now and estimate the rough opening by checking the wall thickness at an outlet box, the edge of a door casing or an access panel. Tell your supplier which number is which. A half-inch of drywall plus a half-inch of tile on each side is a full two inches across the opening.

Step 2: measure the width three times

Measure the width from the back corner area outward at three heights: just above the floor, at roughly waist height, and as high as you can reach. Walls bow, and old walls bow more. The smallest of the three is your working width. If the difference between the largest and smallest reading is more than about a quarter inch, note it, because the installer will need to shim or plane something to get panels to sit flat.

Step 3: measure the depth at both ends

Depth runs from the back wall to the front edge of the opening. Measure at the left wall, the right wall and the center. For a tub alcove, measure to the front face of where the tub apron sits now, not to the edge of the bathroom floor tile. Again, work from the smallest number.

Step 4: check that the corners are square

Measure diagonally from the back-left corner to the front-right corner, then from back-right to front-left. If the two diagonals match within about an eighth of an inch, the opening is square. If they differ, the room is a parallelogram and a rectangular base will leave a tapering gap along one wall. That is normally solved with shims and framing adjustments rather than a custom base, but the installer needs to know before the day, not on it.

Step 5: check the floor and walls for level and plumb

Lay the level on the floor in both directions where the base will sit, and hold it upright against each wall. Note which way the floor falls and roughly how much. A base designed to arrive pre-leveled still needs a flat, solid surface underneath it; a floor that dips or flexes has to be corrected first, and that work belongs in your budget.

Step 6: locate the drain

This is the measurement that moves the price most. Measure from the back wall to the center of the drain, then from the nearer side wall to the center. Note the pipe size if you can see it, and whether the floor is a concrete slab or wood framing with a crawlspace or basement below. A base whose drain lines up with your existing waste line can be connected without opening the floor. A base whose drain does not line up means cutting into the subfloor, or into a slab, to move the pipe.

In a tub alcove, the drain sits at one end. Stand facing the tub: if the drain is on your left, it is a left-hand drain. Kits are offered with end drains on either side or center drains, so record which you have.

Step 7: locate the valve and shower head

Measure the height of the valve center above the floor and its distance from the nearest side wall, plus the height of the shower-head arm. If you plan to keep the valve where it is, the wall panels must be cut around it. If you plan to move it lower for someone who sits to shower, that is plumbing work behind the wall, and it is the moment to do it.

Step 8: measure the ceiling and anything overhead

Measure floor to ceiling inside the opening and note soffits, sloped ceilings, light fixtures and windows. Wall panels come in set heights; a low or sloped ceiling can rule some systems out or call for trimming. A window inside a shower wall needs a plan for how the panel will be cut and sealed around it.

Step 9: measure the way in

This is the step almost everyone skips. Measure the narrowest point between the delivery truck and the bathroom: the front door, any interior door, the tightest hallway turn and any stair landing. Measure door openings with the door open, from the face of the door to the opposite jamb. A one-piece unit has to pass through all of them whole. Multi-piece kits exist precisely because most finished homes cannot take a one-piece shower through the front door. If you are unsure, send photos of the route.

Step 10: check the space in front of the shower

The 2024 International Residential Code asks for a shower access opening of at least 22 inches of clear finished width, and a compartment with at least 900 square inches of interior area and no dimension under 30 inches, with limited exceptions (IRC P2708.1). Your city or county may have adopted a different edition or a different plumbing code, so treat those as a floor, not a target. If a walker, wheelchair or helper will be in the room, measure the open floor in front of the opening as well, and read our guide to curbed and curbless kits.

The measuring worksheet

Common openings and the kits that go in them

One of the most common openings for a kit is a standard bathtub alcove, usually about 60 inches long. That is why our most common footprints are 60 inches wide, in several depths, and why our shower sizes page lists every stock footprint from 30×30 up to 78×48. Corner stalls, neo-angle stalls and walk-in closets converted to showers are the next most common, and those are the openings most likely to end up as a custom size.

Photos to send with your numbers

• The whole shower or tub area, straight on, from the bathroom door.
• The drain, from directly above, with the tape measure laid from the back wall.
• The valve and shower head wall.
• The floor in front of the shower, and the bathroom door.
• Any tight turn or stair on the way in.

With those and the worksheet, we can tell you which footprint fits, what threshold makes sense, and whether anything about the room needs fixing before the kit goes in. Most of the mistakes on our buying-mistakes list are measuring mistakes.

Sources

• International Code Council, 2024 IRC P2708.1 Shower compartments

Checked October 2026. Codes and published prices change; follow the links for current figures.