What Is the Lumber Calculator?
The Lumber Calculator converts board dimensions into board feet, the unit used to price hardwood and much rough-sawn and structural lumber. You enter the thickness and width in inches, the length in feet and the number of boards. It returns board feet per board, total board feet, total linear feet and, if you enter a price, the cost.
A board foot is a measure of volume. It lets lumber of different sizes be priced on the same basis, so a wide thick board and a narrow thin one can be compared fairly. Sawmills, hardwood dealers and timber framers use it every day. Home centres usually price softwood by the piece or by the linear foot, but converting to board feet is the easiest way to compare prices between suppliers.
How the Calculator Works
One board foot is a piece of wood 1 inch thick, 12 inches wide and 12 inches long, which is 144 cubic inches. The calculator multiplies the thickness in inches by the width in inches by the length in feet, then divides by 12.
The division by 12 comes from the mixed units. Thickness and width are in inches, and length is in feet. Converting the length to inches and dividing by 144 gives the same result.
It multiplies the board feet for one board by the quantity. Linear feet are simply the length multiplied by the quantity. If a price per board foot is entered, the calculator multiplies it by the total.
The Board Foot Formula
Board feet = Thickness (in) × Width (in) × Length (ft) ÷ 12
Total board feet = Board feet per board × Quantity
Cost = Total board feet × Price per board foot
All dimensions in inches
Board feet = Thickness × Width × Length (all in inches) ÷ 144
Step-by-Step Explanation
- Write down the thickness and width in inches.
- Write down the length in feet.
- Multiply thickness by width by length.
- Divide by 12 to get board feet for one board.
- Multiply by the number of boards.
- Multiply by the price per board foot for the cost.
Worked Example
A deck frame needs ten 2×6 joists, each 8 feet long.
- Board feet per board: 2 × 6 × 8 ÷ 12 = 8.
- Total: 8 × 10 = 80 board feet.
- Linear feet: 8 × 10 = 80.
At 1.25 dollars per board foot, the order costs 100 dollars, or 10 dollars per board.
For a hardwood example, a tabletop needs five boards of 4/4 walnut, each 7 inches wide and 6 feet long. Four-quarter lumber is 1 inch thick in the rough.
- Per board: 1 × 7 × 6 ÷ 12 = 3.5 board feet.
- Total: 3.5 × 5 = 17.5 board feet.
At 12 dollars per board foot, that is 210 dollars. Add 20 percent for defects and milling, and you would buy about 21 board feet.
How to Use the Calculator
- Enter the thickness in inches. Use the nominal size for softwood framing lumber.
- Enter the width in inches.
- Enter the length and choose its unit.
- Enter how many boards of that size you need.
- Optionally enter the price per board foot.
- Press Calculate. Repeat for each size on your cut list and add the totals.
The quantities often come from another tool. The Decking Calculator gives the number of deck boards and joists. The Fence Calculator lists posts and rails. The Stair Calculator gives stringer length, and rafters follow from the Roof Pitch Calculator. For sheet goods used alongside framing, see the Drywall Calculator.
Practical Tips
- Know whether your supplier prices by the board foot, linear foot or piece, and convert before comparing.
- Softwood framing lumber is sold in even lengths: 8, 10, 12, 14, 16 feet and longer. Plan cuts to suit.
- Sight down each board to check for bow, twist and crook before buying.
- Hardwood is sold in random widths and lengths. Bring your cut list and a tape measure and pick boards that suit your parts.
- Add 10 to 15 percent for framing lumber and 20 to 30 percent for hardwood projects to allow for defects and offcuts.
- Store lumber flat, off the ground and with spacers between layers so air can circulate.
- Let wood acclimate to the space where it will be used before cutting to final size.
Common Mistakes
Confusing board feet with linear feet. A linear foot of 2×4 is 0.67 board feet. A linear foot of 2×12 is 2 board feet.
Using actual dimensions for softwood. Softwood board footage is based on nominal size. A 2×4 is counted as 2 by 4 inches even though it measures 1.5 by 3.5.
Forgetting the division by 12. Multiplying inches by inches by feet without dividing gives a figure twelve times too large.
No allowance for waste. Knots, splits, end checks and saw kerfs all reduce the usable yield.
Ignoring thickness fractions in hardwood. Hardwood thickness is quoted in quarters of an inch. 5/4 is 1.25 inches and 8/4 is 2 inches.
Assuming surfaced size for rough lumber. Hardwood sold rough is measured before planing. A 4/4 board finishes at about 13/16 inch.
Nominal and Actual Sizes
Softwood lumber is named by its size before drying and planing. The finished size is smaller.
- 1×4: 3/4 × 3 1/2 inches
- 1×6: 3/4 × 5 1/2 inches
- 2×4: 1 1/2 × 3 1/2 inches
- 2×6: 1 1/2 × 5 1/2 inches
- 2×8: 1 1/2 × 7 1/4 inches
- 2×10: 1 1/2 × 9 1/4 inches
- 2×12: 1 1/2 × 11 1/4 inches
- 4×4: 3 1/2 × 3 1/2 inches
Use nominal sizes for board feet and actual sizes when designing joints and spans.
Board Feet Per Linear Foot
- 1×4: 0.33
- 1×6: 0.5
- 2×4: 0.67
- 2×6: 1.0
- 2×8: 1.33
- 2×10: 1.67
- 2×12: 2.0
- 4×4: 1.33
- 6×6: 3.0
Multiply by the length in feet to get board feet for a single piece.
Hardwood Thickness Notation
Hardwood dealers quote thickness in quarters. 4/4 is 1 inch, 5/4 is 1.25 inches, 6/4 is 1.5 inches, 8/4 is 2 inches and 12/4 is 3 inches. These are rough-sawn measurements. After surfacing two sides, expect to lose about 3/16 inch.
Sheet Goods
Plywood, OSB and MDF are sold by the sheet, not by the board foot. A standard sheet is 4 by 8 feet, or 32 square feet. To estimate sheets, divide the area to be covered by 32 and add 10 percent.
Lumber Grades
Framing lumber is graded for strength. Number 2 and better is the standard grade for joists, studs and rafters. Select Structural is stronger and used for long spans. Stud grade is intended for vertical use in walls. Appearance boards are graded differently, with Select and Common grades describing how clear the face is. Hardwood uses its own system, where FAS is the top grade with long, clear cuttings, followed by Select and Number 1 Common. Lower hardwood grades cost less per board foot but yield less clear material, so the real cost per usable foot can be similar.
Moisture Content
Green lumber shrinks as it dries, mostly across its width. Kiln-dried framing lumber is marked KD or S-DRY and has a moisture content of 19 percent or less. Hardwood for furniture should be dried to 6 to 8 percent. Pressure-treated lumber is often sold wet and will shrink after installation, which is why deck boards are laid tight when wet. Building with wood at the right moisture content prevents gaps, twisting and popped fasteners later.
Building a Cut List
Before you price an order, list every part with its finished thickness, width and length, then group parts by stock size. Nest short parts into standard board lengths on paper so you can see how many boards of each length to buy. A written cut list reduces waste and makes the board foot total far more reliable.