Door Handle Spindle Sizes 2026: 8mm and 9mm Squares, Split Spindles, and How to Calculate the Length

A spindle is a square bar, usually under 100mm long, that costs a fraction of the handle it drives. It is also the single component most likely to arrive in the wrong specification, because it is the part that connects two things a buyer often orders separately — the handle and the lock.

Three numbers decide whether a handle set will work: the square size, the length, and the type.

THE SQUARE SIZE

The spindle square is the cross-section of the bar, quoted as a single number because it is square. The two sizes that matter in commercial door hardware are 8mm and 9mm.

8mm (about 5/16") is the European convention and the dominant size worldwide for lever handles on mortise and tubular locks. If a specification says nothing, 8mm is the safe assumption in most markets.

9mm (about 3/8") appears on heavier-duty and some regional ranges, where the larger section gives more torsional strength. It is not interchangeable with 8mm in either direction: a 9mm spindle will not enter an 8mm follower, and an 8mm spindle in a 9mm follower has a millimetre of play that becomes a rattle and then a rounded corner.

Some ranges are supplied to accept either, and where that is the case a product page states both — for example "square spindle: 8×8mm or 9×9mm". That is a supply option, not a tolerance. The handle is built for one of them and shipped with it.

The matching hole in the lock is the follower. The follower and the spindle have to be the same square size, and the follower belongs to the lock rather than to the handle. This is why a buyer who orders handles from one supplier and locks from another has to state the square size to both.

SOLID VERSUS SPLIT

A solid spindle is one bar passing through the lock, so both handles are mechanically linked. Turn one and the other turns.

A split spindle is two half-bars that meet inside the lock without engaging each other, so each handle operates independently. This is what makes a one-sided function possible: the inside handle always retracts the latch while the outside handle is locked out, which is the arrangement behind most entrance functions.

Ordering a solid spindle for a lock designed around a split one defeats the lock's function entirely — the outside handle will operate the latch whether or not it is supposed to. It is a failure that passes every visual inspection and is only found by trying the door.

HOW TO CALCULATE THE LENGTH

The spindle has to pass through the whole assembly and leave enough engagement in both handles. The working formula is the door thickness, plus the depth of the two roses or plates, plus the engagement each handle needs inside its hub.

For a common case — a 40mm (1-9/16") door with shallow roses — that lands near the 60mm (2-3/8") that we publish as the spindle length on the models that state one.

A thicker door needs a longer spindle, and this is the most common reason a shipment of handles cannot be fitted. A 45mm or 50mm door with the spindle cut for 40mm leaves too little bar in the outside handle; the handle wobbles, the grub screw bites on the taper rather than on the flat, and the joint eventually rounds off.

A spindle that is too long is a smaller problem but still a problem: it stands proud of the handle, it can bind against the rose, and it cannot be shortened neatly on site without a saw and a file.

Some ranges are supplied with an adjustable spindle length based on door thickness. Where a product page says that, the spindle is cut or set to the door thickness you specify, which means the door thickness has to appear on the order.

THE SPINDLE CENTRE

Separately from the spindle itself, some products publish a spindle centre dimension, written as P= on drawings. It is the distance from a datum — commonly the fixing centre or the edge of a plate — to the axis of the spindle.

Our catalogue publishes both P=25mm (about 1") and P=34mm (about 1-5/16") on different models. The number matters because it positions the handle relative to the fixing holes, and a plate whose P dimension disagrees with the lock's is a plate that cannot be screwed down with the handle centred on the follower.

This is one of the dimensions that has to come from a drawing rather than from a description. Two handles that look identical in a photograph can have different P dimensions, and the difference is only visible once both are held against the same door.

MATERIAL, AND WHY IT IS NOT A DETAIL

A spindle carries the whole torque of a lever handle at the smallest section in the assembly. A soft or undersized bar twists before the lock does, which is a cheap failure in a place that is awkward to reach.

Our pages state the spindle material where a model publishes it — on one model, iron. Where a spindle is supplied in a different material or a different temper, the product page says so rather than leaving the reader to assume.

WHAT WE PUBLISH

Across the catalogue, spindle square sizes are published as 8mm on most lever models and 9mm on one, with some ranges stated as accepting 8×8mm or 9×9mm. Spindle length is published where a model states one — 60mm on the model that does. Spindle centre appears as P=25mm or P=34mm on the models whose drawings give it.

Where a model does not publish a spindle figure, the row is absent rather than filled with a typical value. A spindle is the cheapest part to get right on an order and one of the most expensive to get wrong, because a container of handles that will not fit the locks is not a partial problem.

THE THREE LINES TO PUT ON AN ORDER

Square size, in millimetres. Spindle type, solid or split. Door thickness, in millimetres, so the length can be confirmed against it.

Those three lines cost nothing to write and remove the entire class of failure this page describes. If you are ordering handles and locks from different suppliers, send all three to both.