Most bathrooms that feel wrong measure fine. The fixtures fit; the space between them does not. Clearance decides whether a room works, and it is the first thing absorbed when a layout is drawn to satisfy a wish list. A 1500mm vanity on a 1600mm wall is a fit. Whether its drawer opens with someone standing at it is a separate question, asked in millimetres.
The governing numbers are short and unforgiving. A toilet wants clear space either side of its centreline — 380mm to a wall, vanity or tub as the usual planning minimum, 450mm before it stops feeling like a squeeze — and around 600mm clear in front to stand and turn. A basin needs roughly 550 to 600mm of counter per person and 750mm of floor to use it, 900mm where a drawer opens into it. A run between a vanity front and a tub edge is tight under 750mm. A shower at 900 x 900 works; at 760 x 760 an adult touches two walls at once.
Door swings are where plans quietly break. A 760mm leaf sweeps most of a square metre, and it sweeps it exactly where the towel rail, the light switch and the vanity drawer all want to be. Swinging outward hands the problem to the hallway. A pocket door deletes the arc but claims a stud bay that can then carry no vent stack, no switch box and no recessed cabinet — a layout decision, not a hardware substitution.
The last check is the route rather than the plan. Trace the walk in: to the switch, past the vanity, to the pan, to the shower, and back out with wet feet reaching for a towel. Clearances may overlap where they are used at different moments — the side space beside a pan and the standing space at a basin can share floor, which is what makes a 5' x 8' room plannable at all. A door arc is the exception, because a door moves.