Sizu You are building a bookcase. The plan calls for eight shelves, each 16 inches deep and 30 inches long, and a sheet of plywood is 48 inches wide. Three strips of 16 inches make exactly 48. The sum looks perfect, and it is wrong.
The saw blade turns a strip of wood into sawdust. That lost strip is called the kerf, and a typical table-saw or circular-saw blade makes a cut about 1/8 inch (0.125 in) wide. It is small, but it adds up, and it changes how many shelves you can get from one sheet.
Step 1: count the cuts, not just the strips
To split a sheet into three strips you make two cuts, between strip one and two, and between strip two and three. Each cut takes 1/8 inch.
- Three strips of 16 in: 3 × 16 = 48 in
- Two cuts: 2 × 0.125 = 0.25 in
- Total needed: 48 + 0.25 = 48.25 in
The sheet is only 48 in wide. You are a quarter inch short, so the third strip comes out at 15¾ in, not 16. (Plywood is often a hair under its nominal size too, which makes it worse. Measure your own.)
Step 2: see what it costs you
Suppose the shelves must be a strict 16 in. Then only two strips fit across the sheet (16 + 0.125 + 16 = 32.125 in, and what is left over, 15¾ in, is too narrow).
Along the 96 in length, 30 in pieces are fine: 3 × 30 = 90 in, plus 2 cuts of 0.125 in is 90.25 in, which is under 96.
- Strict 16 in: 2 strips across × 3 pieces along = 6 shelves per sheet
- Eight shelves needed, so you must buy 2 sheets
At an example price of $60 a sheet, that is $120.
Step 3: shave the strips instead
Work backwards. Take the 48 in width, take away the two cuts (0.25 in), and divide what is left by three:
(48 − 0.25) ÷ 3 = 47.75 ÷ 3 = 15.92 in.
Round down to a measurement you can read on a tape: 15⅞ in (15.875). Check it: 3 × 15.875 = 47.625, plus 0.25 for the cuts is 47.875 in, which leaves ⅛ in spare. Now all three strips fit.
- 3 strips across × 3 pieces along = 9 shelves per sheet
- Eight shelves needed, so 1 sheet is enough, with one shelf to spare for a mistake
That is $60 instead of $120. An eighth of an inch of depth on a shelf is something nobody will ever notice, but a second sheet is something you will notice on the receipt.
Step 4: the general rule
For any number of equal strips across a sheet:
- Count the cuts: strips minus one.
- Multiply cuts by the kerf width and add that to the strip widths.
- If the total is over the sheet width, trim every strip a little, using (sheet − cuts × kerf) ÷ strips and rounding down.
Two more things catch people. A blade’s real kerf varies: thin-kerf blades are around 3/32 in, so read the blade or measure a test cut. And keep your offcuts in mind: the leftover strip in Step 2 is 15¾ in wide, which would make a fine shelf somewhere else, or the back stretchers.
One more check: four strips
Say you want four equal 12 in strips for drawer fronts. Four strips mean three cuts, so the total is 4 × 12 + 3 × 0.125 = 48 + 0.375 = 48.375 in. Over again. Trim: (48 − 0.375) ÷ 4 = 47.625 ÷ 4 = 11.91 in, so cut at 11⅞ in (11.875). Check: 4 × 11.875 = 47.5, plus 0.375 is 47.875 in, which fits.
The pattern is the same every time: the nice round number from the plan is the first thing to question, and a tape-friendly eighth or sixteenth of an inch is usually all it takes to make the sheet work.
Rule of thumb
Strips across a sheet = cuts to count, and every cut costs a blade width. Before you buy sheets, add the kerf for every cut to the total, and if you are over, trim each part slightly rather than buying more material.
Try it with your own numbers
The free Sheet Yield & Kerf calculator gives how many strips of one width fit once every saw cut is counted. It is a tool from Woodworking Math: Cut It Once, whose book page describes how to nest a whole cabinet into one sheet.
This is arithmetic for planning, not machine technique. Follow your tool manuals and use the safety guards, push sticks and eye and ear protection they call for.