Understanding 4/4, 6/4 and 8/4 Lumber

4/4 lumber means a nominal thickness of 1 inch; 6/4 means 1½ inches; 8/4 means 2 inches. These hardwood labels describe quarter-inch thickness categories, rather than a promise about the finished board you will take home. Check the surfacing specification and the actual stock before choosing material for a project. [1]

Read the quarter-inch label

Say 4/4 as “four quarters.” Dividing the first number by four gives the nominal thickness in inches. The same arithmetic explains 6/4 and 8/4. The American Hardwood Export Council’s grading guide uses this quarter-inch system for rough-sawn hardwood. [1]

The millimetre values convert the nominal numbers using 25.4 mm per inch. They do not describe an inspection of a particular board. A 4/4 label also says nothing about that board’s width or length; record those separately.

Check rough versus surfaced stock

Rough stock has not been brought to a smooth finished surface. S2S means surfaced on two sides. Surfacing removes material, so a surfaced board can retain its original quarter label while measuring thinner. Woodworkers Source explains these terms and describes its surfaced 4/4 stock as 13/16 inch. That is a supplier example, not a guarantee for every 4/4 product. [2]

Ask the seller two separate questions: “What thickness category is this sold as?” and “What actual thickness will you supply?” If the project requires more machining, also ask what remains to be done. Do not assume that a surface designation alone guarantees a board will meet every final dimension in your drawing.

Keep the distinction from our nominal versus actual lumber size guide, but use the hardwood supplier’s specification here. A familiar softwood 2×4 rule is not a conversion chart for quarter-inch hardwood labels.

Work back from finished thickness

Start with the dimension the finished part must have, then check whether the available material can produce it. For example, suppose a small furniture component needs to finish at 1 inch thick. Buying something labelled 4/4 does not by itself establish that it will still be 1 inch after the required preparation.

If the stock offered measures 13/16 inch, it is already 3/16 inch below that target:

1 − 13/16 = 3/16 inch

A thicker category may be appropriate, but choose it from the seller’s actual specification and your machining plan. Do not replace one assumption with another by treating 6/4 as an automatic guarantee of a particular finished size.

On your woodworking cut list, write the finished part thickness. On the buying list, add a separate stock category and the supplier’s stated dimensions. This keeps a purchasing label from quietly becoming a joinery dimension.

Compare board feet without mixing dimensions

A board foot is a volume equal to 1 foot × 1 foot × 1 inch. With thickness and width in inches and length in feet, calculate: [1]

Board feet = thickness (in) × width (in) × length (ft) ÷ 12

For an original comparison, assume three rectangular boards are each 6 inches wide and 8 feet long. Use the nominal thicknesses shown below as the stated basis for this estimate.

Holding width and length constant makes the effect of thickness easy to see: the 8/4 example has twice the nominal volume of the 4/4 example. It does not establish twice the price; prices per board foot and processing charges may differ.

Before comparing an estimate with an invoice, ask which dimensions and tally method the supplier uses. The AHEC guide distinguishes tally before and after kiln drying. [1] Label your own calculation as a nominal estimate or a measured-volume calculation, as appropriate, and do not silently mix the two. Our board feet and lumber cost guide walks through the units and cost arithmetic.

Take a clear buying checklist

• Finished requirement: write the part’s target thickness, width and length with units.
• Stock label: record 4/4, 6/4 or 8/4 separately from the finished requirement.
• Supplied condition: confirm whether the offer is rough or surfaced and request the actual supplied thickness.
• Remaining work: account for the material your planned preparation still needs to remove; do not assume a universal allowance.
• Quantity basis: ask how the quoted board feet are tallied and whether processing is included.
• Final check: measure the material before transferring dimensions to a critical fit.

A useful order note separates the final target, the proposed stock category and the seller’s confirmed dimensions. If one of those fields is unknown, resolve it before relying on the order for a dimension-sensitive part.

Keep your lumber calculations together

Sources and scope

1. American Hardwood Export Council — The illustrated guide to American hardwood lumber grades, printed pages 2 and 4 (PDF pages 4 and 6): board-foot definition, quarter-inch labels, nominal metric equivalents and tally terminology. This older educational guide is used for those fundamentals, not as a claim about a current sales contract.
2. Woodworkers Source — Alder 4/4 Lumber specifications: rough/S2S terminology and that supplier’s 13/16-inch example. Confirm the current specification with your own seller.

Sources checked November 7, 2026. Comparisons, calculation examples and buying checklist are AppMaven’s own illustrations. This guide addresses lumber thickness and estimating; it does not specify structural member sizes or guarantee a finished yield.