Swapping a tub for a tub sounds like the simplest job in a bathroom, and it is the one most often derailed by a measurement. Nominal lengths of 1,370, 1,525, 1,675 and 1,725mm describe the tub. The alcove was framed around whatever is currently in it, and in older houses it measures anything from a few millimetres under to a few over. A 1,525mm tub does not enter a 1,515mm opening, and the recovery is framing work rather than force.
Depth to the overflow is the number that matters, not overall depth. A 400mm deep tub with its overflow at 300mm holds 300mm of water, and the whole difference between two tubs of identical footprint often lives in that one dimension. Alongside it sits the drain hand: apron tubs have a moulded finished front and are made left- or right-hand, and ordering the wrong one is not correctable on site. An undermounted tub is a different proposition, having no finished face and depending on a deck built for it first.
Getting it into the room is a genuine constraint, checked before ordering rather than on delivery day. A 1,725mm tub has to travel a hallway, turn a corner and pass a 760mm door. Cast iron heading the other way weighs 130 to 160kg and is often broken up in place rather than carried out whole.
Support and levelling decide how the tub feels for the rest of its life. Acrylic and fibreglass shells flex, so they sit on a full bed of mortar or structural foam with the flange bearing on a ledger screwed level to the studs. The rim is set dead level in both directions, because the fall to the waste is moulded into the base. Waste and overflow go in before the tub does. The backer board is then held a few millimetres clear of the flange so the tile face lands proud and sheds water onto the tub, and the rim joint is flexible sealant, cut in with the tub full so it is not stretched the first time somebody climbs in.