Knowing where and how to vary from your plans keeps tiny fractions from leading to big project problems.

Woodworking pop quiz: When is 34 " plywood not actually 34 " plywood?

Answer: Only always.

As face veneers have become thinner (manufacturers get more mileage out of each log), plywood thicknesses have shrunk. So if you rely on the stated thickness when planning or building your projects, you wind up with loose-fitting joints, part dimension errors, and assembly problems. But don't count on a consistent 132 " difference from the stated thickness, either. Depending on the quality of your stock, you may even find thickness variations between two sheets or on a single sheet.

Use these four strategies to prevent those discrepancies from becoming dimensional disasters on your plywood-based projects.

1. Piston-fit parts between dadoes and rabbets 

PROBLEM: I cut 38 "-deep rabbets in my 34 " plywood cabinet sides to accept the top and bottom, but my planned 26"-wide cabinet came out too narrow. The undersized plywood left less than 38 " of material after the38 " rabbets.

SOLUTION: Start with your project's absolute critical dimension—in this case, the final 26" width of the cabinet. After cutting the rabbets in the sides, hold the pieces together back-to-back and let your tape measure tell you the exact length needed to get to a perfect 26" cabinet width, as shown above. Extend your tape to the cabinet width (the critical dimension) and hold that dimension against one rabbet. The exact length of a part to fit between the rabbets is where the edge of the other rabbet meets the tape.In this case, you would cut the top and bottom 25716 " long.

Use a similar strategy to determine the length of drawer fronts and backs joined with rabbets, dadoes, butt joints, or lock rabbets, below.

Make drawers that fit properly. To determine the correct length of the drawer front and back, place the drawer sides together and line up the critical dimension with the bottom of the dado.

2. No-math method to a perfect drawer fit

PROBLEM: I separated side-by-side drawers in my project with a 34 " plywood divider. But, when I assembled the cabinet, the drawers openings were wider than they should have been.

SOLUTION: Once again, it's best to measure directly from the project rather than rely on calculations. Before cutting the divider dado, dry-fit the sides and shelf first, then lay out the dado location, as shown below. That way, if the dadoes the shelf fits into are shallower or deeper than specified in the plans, your drawer opening width remains correct.

Dry-fit, then mark to fit. For a 6" drawer opening, dry-fit the cabinet parts, then mark the divider dado location 6" in from the assembled side.

3. Lay out real-world rabbets and dadoes

PROBLEM: I cut a 34 "-wide rabbet to accept a 34 " plywood cabinet top, and now the cabinet sides stand proud of the top. It looks like I cut the rabbet too wide.

SOLUTION: Instead of laying out a 34 " rabbet and cutting to that line, use an off-cut from the cabinet top as a gauge to mark the true rabbet width, as shown below. Then, cut to that line using a dado set or straight router bit. Keep that sanded scrap handy for gauging shelf dadoes, too.

Hold a finish-sanded scrap of stock flush with the top edge of the cabinet side and mark the exact thickness of the plywood top with a sharp pencil or marking knife.

4. Know when to measure and when to trace 

PROBLEM: I measured all the dado locations for my 34 " plywood drawer dividers starting from the top of the carcase, accounting for my 4316 "-high drawers. Now the gap increases above each successively lower drawer.

SOLUTION: The difference in plywood thickness, multiplied by each additional divider, created those ever-larger gaps. There are two solutions here, depending on whether the critical dimension in the project is the height of the drawer opening, or the overall height of the carcase.

If the drawer opening is the critical dimension, use a spacer and two strips of scrap plywood to lay out the dadoes. In this example, the drawer-face height, plus 116 " for clearance, gives you a 4 14 " critical dimension. Rip a scrap to this width to use as a spacer. Mark the rabbet for the cabinet top using Tip 3 above, then position a second scrap alongside the spacer to locate the first dado, as shown below. Remove the spacer, and mark alongside the plywood.

Use real-world samples. A spacer the same width as the drawer-opening height (including reveal) shows you the exact location of the divider. A scrap of plywood provides the exact width of the dado for the divider.

Then just leapfrog the spacer and plywood scraps to lay out the remaining dadoes, below. If your cabinet has drawers of different heights, cut a spacer to match the height of each drawer opening.

Gauge, measure, repeat. After marking the first dado location, repeat the process to determine the placement of the remaining dividers.

To create equal-size openings without adjusting the height of the carcase, determine the centerline of each drawer divider and mark that on the edge of the carcase. Then center a plywood scrap on that mark and mark the locations of the dadoes, as shown below.

Align centerlines. Mark a centerline on one end of a scrap of the divider stock. Align the mark with each mark on the carcase side, square the scrap with the edge, then mark the dado location.