The rules for choosing the right screw length are based on ensuring sufficient penetration into the base material.

1. Measure Material Thickness: Measure the thickness of the top piece of material (the one being fastened) and the bottom piece (the one being fastened to).
2. Determine Required Penetration: As a rule of thumb, the screw should penetrate the bottom material by at least 50-60% of its thickness for medium-duty applications. For heavy loads, aim for 75% or more.
3. Calculate Minimum Screw Length: Add the thickness of the top material to the required penetration depth in the bottom material.
4. Choose a Standard Size: Screws come in standard lengths (e.g., 1", 1.25", 1.5"). Select the next available standard length that is equal to or greater than your calculated minimum length. Ensure it's not so long that it will poke through the bottom piece.

• Penetration: The depth to which the threaded part of the screw enters the bottom (base) piece of material.
• Pilot Hole: A hole drilled before inserting a screw to prevent the wood from splitting and to make driving the screw easier.
• Countersink: A conical hole cut around the top of the screw hole, allowing a flat-head screw to sit flush with or below the surface.
• Shank: The smooth, unthreaded portion of a screw's shaft just below the head.
• Holding Power: The amount of force required to pull a screw out of the material. Deeper penetration and coarser threads generally increase holding power.

"The '1/2 to 2/3rds' rule for penetration is a great starting point, but it's not absolute. The real decider is the material itself. In a soft wood like pine, I always aim for maximum possible penetration, closer to 75% or even 80%, without poking through. The wood fibers are weaker, so you need more thread engagement. In a dense hardwood like hard maple or oak, you can get incredible holding power with just 50% penetration, and trying to drive a screw much deeper can be very difficult and risks snapping the screw head if you're not careful. Always drill a proper pilot hole in hardwoods." - Professional Furniture Maker

Attaching a 3/4" Plywood Back to a 3/4" Cabinet Case:
Top Material: 0.75". Bottom Material: 0.75". Medium load.
Required Penetration: `0.75" * 0.6 = 0.45"`
Minimum Length: `0.75" (top) + 0.45" (penetration) = 1.20"`
A standard 1 1/4" (1.25") screw is the perfect choice.

Fastening a 2x4 (1.5" thick) to another 2x4 for a structural frame:
Top Material: 1.5". Bottom Material: 1.5". Heavy load.
Required Penetration: `1.5" * 0.75 = 1.125"`
Minimum Length: `1.5" (top) + 1.125" (penetration) = 2.625"`
A standard 2 3/4" or 3" construction screw would be appropriate.

• Forgetting Material Thickness: The most common error is choosing a screw based only on required penetration, forgetting to add the thickness of the top piece.
• Ignoring the Screw Head: If you are not using a countersink, the height of the screw head adds to the overall length and can prevent a joint from closing tightly.
• Using Drywall Screws for Woodworking: Drywall screws are brittle and have a different thread design. They are prone to snapping under the high torque of woodworking applications and should not be used for structural joints.
• Not Using a Pilot Hole: Driving a screw into hardwood without a pilot hole is a leading cause of split wood and snapped screws.

• Cabinet Assembly: Fastening face frames, cabinet boxes, and attaching hardware.
• General Furniture Making: Assembling table aprons, attaching stretchers, and general carcase construction.
• Construction & Framing: Joining studs, headers, and other framing members.
• Deck Building: Attaching deck boards to joists and assembling railing components.