Cut Lists, Kerf and Waste: How to Estimate Lumber
Turning a parts list into the number of boards and sheets you need to buy
Most lumber purchases go wrong in the same way: the total length or area added up correctly, but the parts could not be cut from the boards that were bought. A 30-inch rail does not fit into the last 28 inches of an 8-foot board, no matter how good the totals look. A cut list solves this by assigning each part to a specific board before you shop. This guide covers the thinking behind the Cut List Optimizer, the Sheet Goods and the Room / Area calculators on the Woodworking Calculator.
Start with a parts list
Write down every part by name, length, width, thickness and quantity. Work from your drawing, not from memory, and add any allowance you need for trimming and joinery, such as the tenon length on a rail. Group parts by the stock they will come from: all the 3/4-inch solid parts together, the 1/4-inch back panels separately, and so on. The calculator handles one board length at a time, so run it once for each size of stock.
Kerf: the width of the cut
Every saw cut removes a strip of wood equal to the blade's thickness, called the kerf. A full-kerf table saw blade is commonly about 1/8 inch (0.125) wide; thin-kerf blades are about 3/32 inch (0.094). Cut a board into six pieces and you have lost five kerfs, about 5/8 inch, which is enough to make the last piece too short. The calculator's saw kerf field defaults to 0.125 inch, and it counts one kerf between each pair of cuts on a board.
How the optimizer works
You enter the board length in inches, the lengths you need cut separated by commas, and the kerf. The optimizer does the following:
- It sorts your cut lengths from longest to shortest.
- It starts a new board and places the longest remaining piece, then works down the list adding every piece that still fits, allowing for kerf.
- When nothing else fits, it starts another board and repeats until every cut is placed.
It then reports the number of boards, the cuts on each board, the waste on each, and the overall efficiency, which is the share of purchased length that ended up in parts.
Example
Suppose you need two 30-inch, two 24-inch and two 18-inch pieces from 96-inch (8-foot) boards, with a 0.125-inch kerf. Sorted, the list is 30, 30, 24, 24, 18, 18.
- Board 1: 30 + 30 + 24 = 84 inches of parts. With two kerfs the board has used 84.25 inches, leaving about 11.75 inches, which is too short for another 18 or 24.
- Board 2: 24 + 18 + 18 = 60 inches of parts, used about 60.25 with kerfs, leaving about 35.75 inches.
That is two boards, with roughly 47.5 inches of waste out of 192 purchased, or about 75 percent efficiency. The second board's offcut is a useful length for other parts or for test cuts.
The limits of the method
Filling from the longest piece down is quick and often gets a good answer, but it is a simple rule and not a mathematically guaranteed minimum. For a long list of awkward lengths, try juggling the order by hand to see if you can save a board. It also does not know about grain direction, defects or the width of parts. Treat the result as a solid starting plan.
Sheet goods: plywood and panels
Plywood, MDF and similar panels are sold in sheets, with 4 × 8 feet (32 square feet) the common size and 4 × 10 and 4 × 12 also stocked in many places. The Sheet Goods calculator is an area estimate: it takes the area you need, adds your waste percentage and divides by the area of one sheet, rounding up.
For a project needing 100 square feet of parts with 10 percent waste, you need 110 square feet. Divide by 32 and you get 3.44, so you need 4 sheets, which give 128 square feet and leave 18 square feet of surplus.
That figure assumes the parts nest efficiently. Big parts, such as a 30-inch-wide side that must be cut from a 48-inch-wide sheet, can waste more than the percentage suggests. Sketch the sheet and lay out the large pieces first. Allow extra if you need parts to match grain direction.
Floor and surface area
For flooring, paneling or finishing you often start with room dimensions. The Room / Area calculator on the Square Feet tab multiplies length by width, multiplies by the number of identical rooms or areas, adds waste and displays the result in both square feet and square meters (1 square foot is 0.092903 square meters). A typical allowance for straight-lay flooring is around 10 percent, while diagonal layouts and patterned boards need more. Check the installer's or manufacturer's advice for your product.
A buying checklist
- List every part with dimensions and quantity.
- Group by stock thickness and species.
- Run the cut list for each board length you can buy.
- Convert the boards to board feet with the board foot formula and add 10 to 15 percent for defects.
- Round up to full boards and sheets, and buy a little extra for test cuts.
Measure twice, and verify the final numbers against your own drawings before you cut.