A drawer can't be any depth you like. The box runs on a pair of runners, the runners come in set lengths, and the box is made to suit them. So the real question isn't how deep the cabinet is, it's which runner fits in it.
Here's how to work it out, and what changes it.
Runners come in set lengths
Ball-bearing side runners are usually sold in 50 mm steps, commonly from 250 mm up to 600 mm, with longer ones for deep units. Undermount runners come in their own set of lengths. Whichever you use, the drawer box is normally made as long as the runner (check the maker's chart: some undermount runners want the box a few millimetres shorter, or a notch at the back).
Keep the lengths you actually buy in a list, and design to those. A box made between two sizes means a runner you can't get.
The room behind the front
Start with the inside depth of the cabinet: from the inside of the back panel to the front edge of the sides. Then take off:
- A gap at the back, so the drawer never hits the back panel or anything fixed to the wall behind it. Many people allow 20 to 30 mm.
- The front's thickness, if it's inset: an inset front sits inside the opening, so the box starts behind it. An overlay front sits on the face of the cabinet and takes nothing off.
- Anything in the way at the back: pipes, a skirting left in place, a socket box.
What's left is the room for the box. The drawer takes the longest runner that fits in it. For example, a 600 mm deep carcass with a 6 mm back and a 30 mm gap leaves 564 mm, so a 550 mm runner fits and a 600 mm one doesn't.
When to choose a shorter runner
Going for the longest runner gives the most storage, but there are good reasons to step down a size: a pipe or a boxed-in service at the back of one drawer only, matching drawers across units of different depths, or keeping a top drawer light. It's fine for drawers in one unit to have different depths, as long as each is on a runner length you can buy.
Under a sloping ceiling
In a unit under the eaves, the room behind each front changes with height: measure it at the top of each drawer box, where the slope comes down lowest over it. Each drawer then takes the longest runner that fits there, so the drawers get shallower going up. More on that in fitted furniture under a sloping ceiling.
Working it out in JoinerKit
In JoinerKit your runner lengths and the gap at the back live in Settings. Every drawer is made as deep as the longest runner that fits behind its front, and you can pick a shorter length for any one drawer: only the lengths that fit are offered, so a drawer can never go through the back. The cutting list, the fittings (a pair of runners of each length) and the price follow.
Questions
How long should drawer runners be?
As long as fits: take the cabinet's inside depth, less a gap at the back (often 20 to 30 mm) and an inset front's thickness, and use the longest runner length that fits in what's left.
Is the drawer box the same length as the runner?
Usually, yes. The box is made as long as its runners. Some undermount runners want the box a little shorter or notched at the back, so check the maker's chart for the ones you use.
Can drawers in the same unit be different depths?
Yes. Each drawer can take its own runner length, for a pipe behind one drawer, say, or under a sloping ceiling where the higher drawers have less room.