Measuring & estimating

Board foot calculator with waste factor

A textbook board foot is just (thickness × width × length) ÷ 144. That's a fine unit of trade, but it's not what to buy. This calculator adds the four losses that bite hobbyists: kerf from the saw, planing allowance if you're working rough lumber, defect allowance for the grade, and a project-type scrap factor based on how many offcuts you'll genuinely have at the end.

Calculator

Finished thickness (in) The thickness of your final piece (e.g. 0.75 = 3/4″) Finished width (in) Finished length per piece (in) Number of pieces Saw kerf (in) Typical 1/8″ combo blade; thin-kerf 3/32″ Number of cuts per piece Rip cuts + cross cuts combined, per piece Planing allowance per side (in) Rough-sawn usually needs 1/8″–3/16″ per face Project scrap factor (%) Defect allowance (%) Price per board foot ($) Optional — leave 0 to skip cost

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What this solves

You sized a project to a board foot total (say, 12 BF for a small tabletop). You went to the lumber yard, bought 12 BF, and came home half a piece short. This calculator tells you the number you actually need to ask for, so the offcut pile at the end is small enough to live with but the project doesn't run out mid-glue-up.

How to read the result

"Net board feet" — the textbook figure. What you'd need if wood were free of waste.
"After kerf + planing" — what you need off the rough board before any human error.
"Buy this many" — the number to order. This includes scrap and defect allowance. Round up to the nearest whole board if you're buying from a rack.
If your lumber yard sells by the linear foot of fixed-width stock, divide by board thickness × width in inches and multiply by 12 to get linear feet.

Worked example: a small walnut serving board

You're building a 7″ × 36″ × 3/4″ walnut serving board — one piece, glued up from two narrower strips. The calculator's defaults (4 pieces, 2 cuts each, 1/8″ kerf, 15% scrap, 10% defect) give you this:

Net board feet = 4 × (0.75 × 7 × 36) / 144 = 5.25 BF.
Add kerf and planing: ~5.95 BF.
Add 15% scrap and 10% defects: ~7.2 BF.
At $6.50/BF, that's about $47 in walnut before you cut anything.

Walnut usually comes in 4/4 (1″) rough, sold by the board foot, so you'll need to buy enough rough to net 7.2 BF after surfacing. Because most yards won't sell you 0.2 BF, you'll round up to the next 1/2 BF and end up with a piece of walnut for a future project. That's the right amount of "waste".

Where the four waste factors come from

Kerf is mechanical — every saw blade removes material. A typical 10″ combo blade is 0.125″ wide. A thin-kerf blade is 0.091″. A track-saw blade is 0.063″. Multiply by the number of cuts and it adds up. For rough dimensioning on the table saw, expect ~3–6% loss to kerf alone on a small project.

Planing allowance is the material you lose when you surface a rough board flat and to thickness. 1/8″ per face (so 1/4″ per board, total) is a reasonable starting point for 4/4 stock. Skip this if you buy S4S lumber.

Defect allowance is the knots, checks and sap pockets you'll work around. FAS-grade hardwood has ~5% unusable volume. #1 Common has more like 10%. #2 Common or rustic can run 20%+.

Project scrap is the trickiest. A simple bookshelf with straight rip cuts has very little scrap (10%). A figured-wood tabletop where you need adjacent boards from the same log to match grain can hit 35% before you even think about kerf.

Limitations

This is a planning tool, not a CNC optimiser. It assumes you can pick through a stack; if you buy online without selecting boards, add 5–10% more.
It assumes straight, square cuts. Angled or compound cuts waste more; bump scrap factor up.
If you're resawing thick stock into thinner pieces, add an extra 1/16″ per resaw for blade drift.
Board-foot price varies wildly by region, season, and whether the lumber is domestic or imported. The price field is just a back-of-envelope cost; verify with your supplier.

Frequently asked questions

Why isn't my net board foot number the same as L×W×T/144?

Because L×W×T/144 is the textbook. It assumes you start with a board that's exactly your finished size. In practice you don't — you start with rough lumber, lose material to kerf, lose material to planing, lose material to knots you have to cut around, and lose material to offcuts that are too small to be useful. We add those layers so the number you get is the number to buy.

I buy S4S lumber — do I still need planing allowance?

Set planing allowance to zero. S4S lumber is already surfaced. You can still expect ~1/32″ of additional material loss if you joint one edge or trim a slight bow, but it's not worth modelling for most projects.

What about plywood and sheet goods?

Plywood is sold by the sheet, not the board foot. A 4×8 sheet of 3/4″ plywood is ~32 sq ft of surface. For sheet goods, drop kerf and defect down (plywood is graded more uniformly) and bump scrap up if you're cutting small parts — offcuts from a 4×8 sheet are usually unusable for furniture.

I always run short. Should I just always add 50%?

That's a popular piece of advice on forums and it's usually a sign that something earlier in the process is off — wrong starting thickness, miscounted pieces, or under-estimating scrap on a complex project. This calculator is designed to expose which factor is biting you, rather than paper over it with a blanket fudge.