If you've ever stood at a lumberyard trying to figure out how much wood to buy, you've run into board feet. It's the standard unit hardwood is priced and sold by, and it trips up even people who've been buying lumber for years. The math itself is simple. Where people actually go wrong is everywhere around the math: rounding, units, and what "thickness" even means for a given board.

Here's how to calculate it correctly, the mistakes that throw estimates off, and a calculator at the end so you don't have to do any of it by hand.

What Is a Board Foot?

A board foot is a unit of volume, not length. One board foot equals a piece of wood that's 12 inches long, 12 inches wide, and 1 inch thick, or 144 cubic inches total. It's the standard measurement hardwood lumber is priced by, which is why getting the calculation right actually affects what you pay.

The Board Foot Formula

The formula is straightforward:

Board feet = (Thickness × Width × Length) ÷ 144

with thickness and width in inches, and length in inches. If you're working with length in feet instead, divide by 12 instead of 144.

A couple of worked examples:

• A board that's 1" thick, 6" wide, and 8 feet long: (1 × 6 × 96) ÷ 144 = 4 board feet
• A board that's 2" thick, 6" wide, and 8 feet long: (2 × 6 × 96) ÷ 144 = 8 board feet

For a full project, calculate each board separately, then add the totals together. That running total is your project's board footage.

Quick Reference: Common Board Sizes

For a fast look-up without doing the math, here's board footage for common sizes at a few standard lengths:

Size (actual, T x W)	6 ft	8 ft	10 ft	12 ft
1" x 4"	2.0 BF	2.67 BF	3.33 BF	4.0 BF
1" x 6"	3.0 BF	4.0 BF	5.0 BF	6.0 BF
1" x 8"	4.0 BF	5.33 BF	6.67 BF	8.0 BF
1.5" x 3.5" (2x4)	2.63 BF	3.5 BF	4.38 BF	5.25 BF
1.5" x 5.5" (2x6)	4.13 BF	5.5 BF	6.88 BF	8.25 BF
2" x 6"	6.0 BF	8.0 BF	10.0 BF	12.0 BF
2" x 8"	8.0 BF	10.67 BF	13.33 BF	16.0 BF

These use actual (milled) dimensions where noted. For anything outside these sizes, or for an exact number on odd dimensions, the calculator below does it instantly.

Where the Math Goes Wrong

The formula is the easy part. Here's where estimates actually get thrown off:

Nominal vs. actual dimensions. A "2x4" isn't really 2 inches by 4 inches. After milling, it's typically closer to 1.5" by 3.5". Using the nominal size instead of the actual measured size will overstate your board footage every time. For dimensional lumber, use actual dimensions if you want an accurate number; for rough-sawn hardwood, measure what's actually in front of you.

Rounding the wrong way. For dimensional lumber (2x4s, 2x6s, etc.), the convention is to round length up to the next whole foot, so an 8'4" board counts as 9'. For hardwood boards, the opposite applies: use the exact length, don't round up. Mixing up which rule applies to which type of lumber is one of the most common ways estimates drift.

Thickness in quarters. Hardwood thickness is often sold and described in quarters of an inch rather than plain inches, so a "4/4" board is 1 inch thick, "8/4" is 2 inches, and so on. If you plug in the quarter-notation number instead of converting it to inches first, the total comes out wrong by a wide margin.

Mixing units mid-calculation. The formula only works if length is consistently in the same unit throughout, either feet or inches, matched to the correct divisor (12 or 144). Switching partway through, or leaving length in feet while dividing by 144, is a quiet but common source of bad totals.

Forgetting waste. Even a perfect calculation only tells you the board footage of clean, usable material. It doesn't account for defects, offcuts, or mistakes during the build. Most experienced buyers add 10-15% on top of their calculated total, and some go higher for lumber with more natural character or visible defects.

Why It's Worth Getting Right

A small error compounds fast. Miscalculate by even 10% on a mid-sized project, and you're either short lumber mid-build (a second trip to the yard, mismatched grain if you're buying more later) or you've tied up money in extra material sitting in your shop. Getting the number right the first time is what lets you walk into a lumberyard, or place an online order, with confidence instead of a guess.

Skip the Math

All of the above is useful to understand, but you don't actually need to do any of it by hand. Our wood calculator does the conversion for you, thickness, width, and length in, an accurate board footage out, so you can spend your time planning the project instead of double-checking arithmetic.

Have a specific project in mind? Try the wood calculator to get your board footage, then browse our urban wood lumber to see what's available.