Lumber Waste Factor: How Much Extra Lumber to Buy, and Net vs Gross Board Feet

Lumber math · Waste and yield · Updated 2026-09-13

Every cut list produces two different board-foot numbers, and buying from the wrong one is how a project ends with a second trip to the lumber yard. Net board feet is the volume of your finished parts. Gross board feet is the volume you have to buy so those parts can exist. The gap between them is waste, and on rough hardwood it is routinely a quarter to a third of the order rather than a rounding error you can ignore.

This guide gives you the arithmetic, a worked example with net, waste percent, waste board feet and gross all side by side, an honest account of where the waste physically goes, and the allowances woodworkers commonly use. It also covers the three ways the calculation goes wrong in practice, including the one that quietly costs money: adding waste to the board feet and then forgetting it in the cost.

The allowances below are rules of thumb. No grading rule and no supplier publishes an official waste factor, so the goal is a number you chose on purpose and can defend, not a number you inherited from a default field.

Net board feet and gross board feet are not the same number

Net board feet is the sum of your finished parts. Take each part, run it through the board foot formula - thickness in inches times width in inches times length in feet, divided by 12 - multiply by the number of pieces, and add the column up. Nothing in that number is a guess. It falls straight out of your drawing, and if two people work the same cut list they land on the same answer to the hundredth of a board foot.

Gross board feet is what you buy. It is net plus an allowance for everything that will end up on the floor, in the dust collector or in the offcut bin. This is where a lot of calculators quietly fail their user: they show one figure, label it board feet, and leave you to work out for yourself whether that is the wood in the project or the wood in the truck. Those are different quantities with different jobs. Net is what you check a design against and what you compare against a second design. Gross is what you say out loud at the counter. Both belong on screen at the same time, each with its own label, so you can see the size of the gap you just accepted instead of inferring it later from a receipt.

The arithmetic

Three lines cover all of it. gross = net x (1 + waste percent / 100) turns a cut list into a purchase quantity. waste bf = gross - net tells you how much of that purchase will never be part of the finished piece. And net = gross / (1 + waste percent / 100) runs it backwards, which is the form you want when a board is already in your hand and the question is how much usable stock is inside it. The percentage is always taken of the net, never of the gross. That sounds like a pedantic distinction until you work it through: a 25 percent waste allowance is an 80 percent yield, not a 75 percent one, because the 25 percent gets added to the smaller number and is then measured against the larger.

Worked example: a small cherry bookcase

Two things in that table are worth pointing out. The thickness column reads 1.0000 in, not the 13/16 in the boards will actually measure when they arrive surfaced two sides, because hardwood is billed on the rough thickness: 4/4 is a full inch on the invoice even though an S2S board shows up at 0.8125 in. Working net at the thickness you are charged for is the honest way to do it. Second, the lengths here are given in inches, so every row uses the divide-by-144 form of the formula rather than divide-by-12. Mixing the two forms is an easy slip to make, and since the error is a factor of twelve it is never a small one. The net total, 19.53 bf, is the wood in the bookcase. Now look at what you have to carry out of the yard.

The spread between the top row and the bottom row is 4.88 bf, which is almost exactly what the top and the whole face frame cost you put together: 2.44 + 1.33 + 1.17 = 4.94 bf. That is the entire decision in a single number, and it deserves a minute of judgment rather than a reflex 10 percent. It also deserves writing down afterwards, because the allowance that turned out right on this project is the best starting point you have for the next one. Board Foot Calculator keeps net board feet, waste percent, waste board feet and gross on screen together for exactly that reason, so the allowance you chose stays visible next to what it added.

Where the waste actually goes

• Defects and knots. Every board has something you will cut around: a knot, a bark inclusion, a check, a stained end, a patch of sapwood on a walnut board. Grade tells you roughly how much clear cutting to expect, which is what NHLA hardwood grades actually measure, but no grade tells you where the defect sits relative to the part you need.
• Jointing and planing to thickness. A 4/4 board that measures a full inch rough does not finish at a full inch. You lose material flattening one face, more taking the second face parallel, and more again if the board is cupped or twisted. You paid for the rough inch and you keep roughly three quarters of it.
• Ripping to width. One straight edge costs you a strip. A second parallel edge costs another. On a board with wane or a crooked edge that can be an inch or more off a width you were billed for in full.
• Crosscut offcuts. Parts rarely divide a board evenly. A 96 in board that yields three 30 in shelves leaves 6 in that is not long enough for anything else on this project, and that 6 in was bought and paid for like the rest.
• Snipe and end checks. Planers snipe the first and last few inches of a board, and boards dry with splits running in from the ends. Both mean you trim before the cut list even starts, typically a few inches off each end of every board in the stack.
• Grain and color matching. On a pair of doors or a glued-up panel you will reject sound, defect-free wood simply because it is the wrong color or the grain runs the wrong way next to its neighbor. This waste is invisible in any calculator and completely real at the bench.
• Cutting around figure. Curl, quilt and crotch figure never land where the part needs them. Getting a figured drawer front out of a figured board means positioning the part on the figure you are paying for and letting the rest become offcut.

All four numbers, labelled, at once. Board Foot Calculator₊ shows net board feet, waste percent, gross board feet and the cost of each together on one screen, with every figure carrying its unit label. Tap any total and it expands the working line by line with your own inputs echoed back, so you can see exactly which number you are about to hand to the yard. Learn more about Board Foot Calculator₊.

Allowances woodworkers commonly use

Three ways a waste factor goes wrong

1. Buying your exact net board feet: Order 19.53 bf for a 19.53 bf bookcase and you have ordered enough only if every board is defect-free, arrives at finished thickness, and divides into your parts with nothing left over. None of those is true of real lumber. Buying net is how you end up back at the yard for one more board, usually from a different flitch, off a different log, in a noticeably different color from the rest of the case.
2. Adding the waste board by board: The percentage itself distributes: 25 percent applied to each part and then summed gives exactly the same total as 25 percent applied to the sum. What does not distribute is the rounding. Round each of the five bookcase parts up on its own after grossing and you buy 9 + 10 + 4 + 2 + 2 = 27 bf. Gross the project once and round once and you buy 25 bf. Same allowance, 2 bf apart, and you also gave up sharing offcuts between parts. Add waste once, at the project level.
3. Waste in the board feet but not in the cost: This is the expensive one. You correctly gross 19.53 up to 24.41 bf, then cost the job off 19.53 because that is the number still sitting in your notes. You pay for what you carry out, so the estimate is short by the price of 4.88 bf, and at a 25 percent allowance that is a fifth of your true lumber cost missing. The allowance has to be applied to the money as well as the volume, which is why net cost and gross cost belong side by side. See lumber price per board foot for the rest of that calculation.

Random width, random length, and why that argues for a waste factor

Hardwood is not sold in the sizes on your cut list. It is sold random width and random length: a lift of boards in whatever widths and lengths the logs gave up, and you take what is on the pile. You cannot order two 10 in by 48 in boards and four 9-1/2 in by 30 in boards. You order a quantity of board feet, and the yard hands you a stack that was never sized for your parts. Every part then has to be found inside a board it does not fit, and the strips and ends left over are the price of that mismatch. This is the real argument for a waste factor: exact planning is not possible in a random-width random-length market, so you buy a margin instead and accept that some of it goes in the bin. Softwood construction lumber, sold in fixed nominal sizes you can order by the piece, is the exception, and it is exactly the case where 15 percent is usually plenty.

Frequently asked questions

How much waste should I add when buying lumber?

Most woodworkers add 15 to 20 percent for straightforward parts in clear surfaced stock, and 30 percent or more for rough lumber they have to joint, plane and rip themselves. Figured or knotty boards and projects with a lot of short parts also sit at 30 percent or more. These are rules of thumb rather than a published standard, so treat them as a starting point and adjust from what your own last few projects actually consumed.

What is the difference between net and gross board feet?

Net board feet is the volume of the finished parts in your cut list; gross board feet is the volume you buy so those parts can be cut out of real boards. Net comes straight out of the board-foot formula and is not a guess. Gross is net plus a waste allowance, and it is the number you quote at the counter.

How do I calculate gross board feet from net?

Multiply net by 1 plus the waste percent divided by 100, so gross = net x (1 + waste percent / 100). For 19.53 net board feet at a 25 percent allowance that is 19.53 x 1.25 = 24.41 gross board feet, with 4.88 board feet of waste. Work it once on the project total rather than board by board, so you round up once instead of once per part.

Is a 10 percent waste factor enough for hardwood?

For rough hardwood, almost never. Ten percent barely covers crosscut offcuts, let alone the thickness you lose flattening and planing, the width you lose straightening two edges, and the material you cut around knots and end checks. Fifteen to twenty percent is reasonable for surfaced stock in fixed sizes with simple rectangular parts, which is a very different job.

Should the waste factor apply to rough or surfaced lumber?

Both, but at very different sizes. Surfaced stock has already had the milling waste taken out of it by the supplier and billed to you in the price, so what remains is mostly crosscut offcuts and defect cutting. Rough lumber still has all of the jointing and planing losses ahead of it, which is why the same project needs a much larger allowance when you buy it rough.

Do I add the waste factor to the lumber cost too?

Yes, and forgetting to is the most expensive mistake in this whole subject. You pay for gross board feet, not net, so a cost worked out on net understates the job by the price of the waste. At a 25 percent allowance that is a fifth of your true lumber cost missing from the estimate.

Why can I not just buy the exact board feet I need?

Because hardwood is sold random width and random length, so the boards you are handed were never sized to your parts. You order a quantity of board feet and take whatever mix of widths and lengths came off the logs, then find each part inside a board that does not match it. That mismatch, plus defects and the milling you still have to do, is what the waste factor is buying.

Keep reading

• How to Calculate Board Feet: The Formula, Worked Examples and the Traps
• What Does 4/4 Lumber Mean? Quarter Thickness, Rough and Surfaced
• Lumber Price Per Board Foot: How Hardwood Is Quoted and How to Compare Two Quotes