Reading quarter notation
Hardwood thickness is traditionally expressed in quarters of an inch. Four-quarter, written 4/4, is nominally four quarters or 1 inch. Five-quarter is 1.25 inches, six-quarter is 1.5 inches, eight-quarter is 2 inches, and twelve-quarter is 3 inches. The spoken form is “four quarter,” not “four fourths.”
The notation normally describes rough stock before final milling. A board can vary in thickness and still be sold within a recognized category under a supplier's grading and tally rules. Always read the seller's definitions rather than treating this guide as a contract specification.
Common starting thicknesses
- 4/4: nominal 1 inch rough; often selected for parts finishing near 3/4 inch.
- 5/4: nominal 1.25 inches rough; useful when a full inch or extra milling margin is needed.
- 6/4: nominal 1.5 inches rough; common for thicker frames, legs, and parts finishing above one inch.
- 8/4: nominal 2 inches rough; used for legs, benches, turning blanks, and resawn components.
- 10/4 and 12/4: heavy stock for substantial legs, posts, sculpted work, or specialized milling.
These examples are purchasing categories, not guaranteed finished sizes. Flat, straight stock may retain more thickness than a warped board. A wide part can require more flattening loss than a narrow strip cut from the same board.
Why finished yield varies
To produce a flat, parallel board, high spots must be removed. Cup, twist, bow, surface checks, and saw variation determine how much. Machine setup also matters: jointer technique, planer snipe, feed pressure, and whether the board is rough-cut before flattening can change the outcome.
For a project that requires an exact finished thickness, buy enough margin and test the worst boards before committing the cut list. If a 7/8-inch finished part is essential, ordinary 4/4 may be risky even though a particularly good board could yield it. Moving to 5/4 can reduce risk but increases volume and cost.
How thickness affects board feet
Board-foot volume increases directly with thickness. An 8-inch-wide, 8-foot-long board contains 5.333 BF at 4/4, 6.667 BF at 5/4, 8 BF at 6/4, and 10.667 BF at 8/4. If price per board foot is identical, the 8/4 board costs twice as much as the 4/4 board because it contains twice the volume.
Prices per board foot may also differ by thickness because thicker stock can be less available or take longer to dry. Enter each thickness as a separate row in the Board Foot Calculator, and use the price quoted for that exact species, grade, and thickness.
Surfaced thickness labels
A board marked S2S has two faces surfaced, while S3S usually adds one straight edge and S4S includes both faces and edges. A yard may advertise 4/4 S2S stock at a target delivered thickness such as 13/16 inch, but practices vary. Confirm whether the price tally remains based on the original rough thickness.
Surfaced lumber can make selection easier because grain and defects are visible, and it reduces shop preparation. It also removes some flexibility to orient the board during flattening. Compare the delivered usable dimension and the cost of milling services, not only the headline board-foot price.
Resawing thick stock
Resawing 8/4 stock does not normally produce two perfect 4/4 boards. The saw kerf consumes material, the fresh faces require cleanup, and internal stress can cause the halves to move. If two finished 3/4-inch pieces are required, calculate the total thickness needed for both parts, kerf, flattening, and movement before choosing stock.
Resawing can be valuable for bookmatched panels and grain continuity. Allow the split pieces to rest and equalize before final milling when the process and schedule permit. Follow safe machine capacity and blade setup; this article explains planning math, not machinery operation.
Matching thickness to the project
Choose stock from the required finished dimension backward. Consider whether the part will be flattened as one wide piece, ripped into narrow components, bent, turned, carved, or resawn. A leg may justify thicker stock, while rails cut from the same board could waste volume if they need only 3/4 inch.
Mixed-thickness purchasing can improve yield. Group the cut list by final thickness, then select the lowest rough category that provides a realistic milling allowance for each group. Keep visual matching in mind: parts that must share color and grain may need to come from the same thicker board even when that is not the minimum-volume solution.
Storage and moisture
Thicker lumber takes longer to dry and equalize than thin stock. Ask about moisture content and drying history, especially for heavy blanks. Store boards with support and airflow appropriate to the material, and measure moisture rather than relying only on time in the shop.
Do not mill every part to final thickness immediately if the stock is still adjusting or if removing a large amount of material may release stress. A staged milling process can leave room for a final flattening pass.
Before ordering
- Write the required finished thickness for every part group.
- Add realistic flattening, kerf, and defect allowance.
- Confirm the supplier's rough and surfaced thickness definitions.
- Use the quoted price for each thickness, not a generic species price.
- Inspect width, length, moisture, and straightness as well as thickness.
For a broader explanation of measurement systems, read Rough vs. Nominal vs. Actual Lumber Dimensions.