Hardwood Lumber Thickness: 4/4, 5/4, 6/4 and 8/4 Explained
Hardwood lumber thickness is written in quarters of an inch. A 4/4 board (“four quarter”) is 1 inch thick in the rough, 5/4 is 1 1/4 inches, and 8/4 is 2 inches. The number describes the board before it is planed, so the piece you take home is usually thinner. This guide lists each size, its surfaced thickness, and how thickness changes the board feet you pay for.
Hardwood Lumber Thickness Chart
The surfaced figures are the standard thicknesses for lumber planed on two sides (S2S) under the grading rules of the National Hardwood Lumber Association (NHLA).
Quarter size | Rough thickness (in) | Standard surfaced thickness, S2S (in) | Board feet per square foot |
4/4 | 1 | 13/16 | 1 |
5/4 | 1 1/4 | 1 1/16 | 1.25 |
6/4 | 1 1/2 | 1 5/16 | 1.5 |
8/4 | 2 | 1 3/4 | 2 |
10/4 | 2 1/2 | 2 1/4 | 2.5 |
12/4 | 3 | 2 3/4 | 3 |
16/4 | 4 | 3 3/4 | 4 |
Use this hardwood lumber thickness chart when you compare a quote with what arrives.
The last column is the same as the rough thickness in inches. One square foot of 8/4 lumber contains 2 board feet.
How to Read the Quarter System
The top number counts quarters of an inch, and the bottom number is always 4. Divide the first by the second to get inches.
- 4/4 = 4 quarters = 1 inch
- 6/4 = 6 quarters = 1 1/2 inches
- 12/4 = 12 quarters = 3 inches
Yards say the sizes aloud as “four quarter”, “five quarter” and “eight quarter”. Width and length are not part of the name. Hardwood is sold in random widths and lengths, and each board is measured as it is.
Rough, S2S and S4S
Hardwood is sold in different states of finish, and the state decides the thickness you get.
- Rough. The board comes straight from the saw and the kiln. Kiln dried rough lumber can be slightly under its nominal thickness, since the grading rules allow a small shortfall after drying. A 4/4 board may be as thin as 15/16 inch at its thinnest point.
- S2S (surfaced two sides). Both faces are planed. This is where the standard surfaced thicknesses in the chart apply, such as 13/16 inch for 4/4.
- S4S (surfaced four sides). Both faces and both edges are planed, so the board is ready to use. Retail 4/4 stock sold S4S is commonly 3/4 inch thick.
A board planed “hit or miss” has been skimmed lightly, so some rough patches remain and more thickness is left for your own milling.
Thickness and Board Feet
Hardwood lumber thickness also sets the price, since board feet rise with every quarter.
Board feet are charged on the rough thickness, even if the board has been surfaced. A 4/4 board planed to 13/16 inch is still tallied as 1 inch thick.
Thickness multiplies the total directly. The table shows the same face size in five thicknesses.
Thickness | Board, 6 in x 8 ft | Board feet |
4/4 | 6 in x 8 ft | 4.00 |
5/4 | 6 in x 8 ft | 5.00 |
6/4 | 6 in x 8 ft | 6.00 |
8/4 | 6 in x 8 ft | 8.00 |
12/4 | 6 in x 8 ft | 12.00 |
The formula is thickness in inches, times width in inches, times length in feet, divided by 12.
The board foot calculator does the math for any size, and the board foot chart lists 4/4 figures with a multiplier for thicker stock.
Which Thickness to Buy
Choosing the right hardwood lumber thickness starts with the finished size you need.
Flattening and smoothing a rough board at home removes wood, and a cupped or twisted board loses more. Start one size thicker than the finished part.
Finished thickness you need | Buy |
3/4 inch | 4/4 |
1 inch | 5/4 |
1 1/4 inches | 6/4 |
1 1/2 to 1 3/4 inches | 8/4 |
2 to 2 1/4 inches | 10/4 |
2 1/2 to 2 3/4 inches | 12/4 |
Wide or long parts need more allowance than short ones, because any bow or twist has to be planed out across the whole length. For a wide tabletop, many woodworkers choose 5/4 stock to be sure of a 7/8 inch or 1 inch finished top.
Softwood Uses a Different System
Framing lumber and softwood boards are named by nominal size, such as 2×4 or 1×6, and their finished sizes are fixed by the American Softwood Lumber Standard. The 5/4 label also appears on softwood decking, where a 5/4 x 6 board is 1 inch by 5 1/2 inches. The nominal vs actual lumber sizes page covers those dimensions.
Frequently Asked Questions
Sources
NHLA Grading Rules for North American Hardwoods (rules card)
Woodworking Network, Wood Doctor: Correct thickness for 4/4 and 8/4 lumber