How do I choose and size timber for French cleats when I have no plywood?

Start from what the cleat is actually doing, because it is not carrying the load the way people assume.

The pair of angled strips converts the hanging load into two things: a downward force taken by the bearing surface, and a component pulling the wall-side cleat away from the wall. The critical failure modes, in order of how often they occur:

1. The fixings pull out of the wall. This is by far the most common failure and it has nothing to do with your timber choice.
2. The cleat splits along the grain at the fixing line.
3. The bevel crushes or deflects under load.

So timber selection is mostly about resisting the second one. What matters is straight grain with no runout, no knots anywhere near the fixings, and adequate thickness so the screw has material either side of it.

Concretely, for solid stock:

Thickness: a minimum of around 12 mm, and 18 to 20 mm is much better. Below that you are relying on a thin section to resist splitting at the screw, and the bevel becomes a knife edge.

Species: a hardwood if you can, for anything substantial. Something dense and reasonably stable rather than the softwood most people have in the offcut pile. Avoid species prone to splitting when screwed near an end.

Grain: straight, running along the length of the cleat. This is more important than the species. A board with grain running out at an angle across the width is a split waiting to happen, and it will look perfectly fine until the day it does not.

Movement: solid wood moves across the grain with humidity, plywood barely does. On a long cleat that is a real consideration — a two-metre solid cleat will change width slightly through the year, which is fine, and will also want to cup, which is not.

Practical notes from doing this regularly:

• Pilot and countersink everything. Screwing near the edge of a bevelled strip without a pilot hole splits solid timber far more readily than plywood.
• Stagger the fixings rather than putting them in a straight line along the grain. A row of holes on one grain line is a perforation.
• The bevel is conventionally 45 degrees, and anything from 30 to 45 works. Shallower angles pull the cabinet tighter to the wall and are less forgiving to cut accurately.
• Fix into structure, not into plasterboard. For anything heavy this is the whole game. Find the studs, and if you cannot, use fixings genuinely rated for the load rather than the ones in the drawer.
• Run the cleat long and fix into as many studs as you can reach. Spreading the load across three fixing points is worth more than any upgrade in timber.

And add a spacer strip of the same thickness at the bottom of the cabinet, so it hangs plumb instead of leaning in.

One thing worth doing whichever material you use: test the pair before you trust it.

Hang the cleat, hook the mating piece on, and load it with considerably more than the intended weight — sit on it, or hang a few bags of something heavy. Better to find the fixing that missed the stud on the floor than with a cabinet full of glassware attached.

Takes two minutes and I have found a bad fixing this way more than once.