Smart Toilet Rough-In: Choosing 300 mm or 400 mm

Rough-in is the distance from the finished wall behind the toilet to the centre of the drain outlet in the floor. It is a single number, it is measured in about thirty seconds, and getting it wrong is one of the few smart toilet mistakes that cannot be corrected on site without new plumbing.

For North American work the familiar figure is 12 inches, which is 305 mm. Most of the world specifies the same dimension metrically at 300 mm or 400 mm. This guide covers how to take the measurement, which of our models fit which dimension, and the two secondary measurements that decide the outcome just as often as the drain does.

Taking the measurement correctly

Measure from the finished wall surface — the tile face, not the studs and not the substrate — to the centre of the drain. Three habits prevent the common errors:

• Measure to the drain centre, not to the edge of the flange or the waste pipe.
• If the toilet is still in place, measure to the centreline between the two closet bolts, which sits over the drain centre.
• Where tiling has not yet been installed, note the planned finish thickness. A 12 mm tile and adhesive build-up moves the finished wall forward and is enough to push a borderline site from one option into the other.

Then measure the two dimensions that matter next: the wall-to-wall width available, and the distance from the bowl centreline to the nearest side obstruction. Most North American codes require at least 380 mm (15 in) from the centreline to each side obstruction and 530–610 mm (21–24 in) of clear floor space in front — a published example is the Washington plumbing code, which sets 15 in to the side and 24 in (21 in in dwelling and sleeping units) in front (WAC 51-56-0400). The exact figure depends on the code in force, so confirm it with the authority having jurisdiction rather than with a catalogue.

What each model supports

Model	Rough-in	Notes
A17	305 mm	Fixed
A18	305 mm	Fixed
6293	300 mm	Fixed
K1	300 / 400 mm options	Selected per order
W1	300 / 400 mm options	Selected per order
S21G, S31G, S61B, S61B-B	Confirm by configuration	Not published until the source pack is reviewed
T1, B04 bidet seats	Not applicable	Fit an existing pan; see the seat compatibility checklist

Two points follow from that table. A fixed 300–305 mm model cannot serve a 400 mm site; the answer is a different model, not an offset adapter. And the models with two options need the option stated on the order line, because it is a manufacturing selection rather than a site adjustment.

The mounting-hole rule for K1 and W1

The K1 and W1 dimension sheets publish a rule that resolves borderline sites directly, based on the mounting-hole spacing:

• 290–380 mm — choose the 300 mm option.
• Above 390 mm — choose the 400 mm option.

If your measurement lands in the 380–390 mm gap, send us the figure with a photograph rather than rounding it yourself. That band is where a rounded number turns into a unit that sits proud of the wall or crowds the drain.

Trap type comes with the drain

Rough-in and trap type are separate questions asked at the same moment. Our floor-mounted configurations are supplied in two primary S-trap dimensions of 300 mm and 400 mm. For European projects, P-trap wall-hung units can be specified for compatibility with common concealed mounting-frame systems.

For retrofits in older housing, expect surprises: a 254 mm (10 in) rough-in appears in pre-war North American stock, and a skirted base on the outgoing fixture can hide the actual trap arrangement until it is lifted. Neither is a reason to abandon a smart toilet — both are reasons to measure before the order rather than after delivery.

Water pressure can decide the model

This is where a rough-in conversation frequently changes direction. Our records document two different supply windows depending on whether the unit has a tank:

• With a water tank: 0.06–0.75 MPa static.
• Direct supply, no tank: 0.15–0.75 MPa static.

That difference matters on upper floors of tall buildings, at the end of long horizontal runs, and on rural supplies. A site that measures below 0.15 MPa at the fixture during peak draw will disappoint on a direct-supply unit no matter how well the drain lines up. The A17 and A18 records state a 0.15–0.75 MPa static window; the S21G is documented from 0.15 MPa dynamic to 0.75 MPa static; the B04 seat works from 0.07 MPa dynamic.

Supply water temperature is documented at 4–35 °C across the range, which is worth checking on projects with unusually cold winter mains or a recirculating hot feed.

What to send with the enquiry

For a same-day model recommendation, send:

1. Rough-in measurement, stating whether the wall finish is installed.
2. Mounting-hole centre spacing where an existing fixture is in place.
3. Trap type and drain orientation — floor or wall.
4. Measured pressure at the fixture, and whether a tank version is acceptable.
5. A photograph of the space with a tape measure in frame.

Full parameter tables are published on the K1 and W1 records, and the A17 and A18 records give the fixed-dimension alternatives. The electrical side of the same site survey is covered in the voltage and grounding guide, and the compliance side in how cUPC, WaterSense, UL and FCC listings differ. For retrofit seats, the seat category covers the non-applicable cases, and construction projects can work through the engineering solutions page. If the site is unusual, send the measurements to our technical team and we will confirm the option before it reaches a quotation.