About This Tool
Buying lumber requires understanding two measurement systems that often confuse DIYers and frustrate contractors. A 2×4 board is not actually 2 inches by 4 inches. Board feet are not the same as linear feet. Pricing varies by the board foot, but you buy by the piece. This lumber calculator eliminates the confusion by converting between nominal sizes, actual dimensions, board feet, and total costs instantly. The board foot is the standard unit for lumber pricing, especially for hardwoods and specialty woods. One board foot equals a piece of wood measuring 1 inch thick by 12 inches wide by 12 inches long, or 144 cubic inches total. The formula is straightforward: multiply thickness (in inches) by width (in inches) by length (in feet), then divide by 12. A standard 2×4 stud that is 8 feet long contains 5.33 board feet, not 8 linear feet worth of pricing. Understanding nominal versus actual dimensions prevents costly measuring mistakes. Mills plane rough lumber smooth, removing about half an inch from each face. A 2×4 starts as a rough-cut 2×4 but finishes at 1.5×3.5 inches. Wider boards lose even more: a 2×10 is actually 1.5×9.25 inches. This matters when calculating coverage for decking, figuring joist spans, or building furniture where precise dimensions determine fit.
Nominal vs Actual Lumber Dimensions Explained
Every piece of dimensional lumber sold in North America has two size systems: nominal (the name) and actual (the reality). This confusing system dates back to when lumber was sold rough-cut from the mill at full dimensions. Modern lumber is kiln-dried and planed smooth, losing material in the process.
Why Lumber Shrinks: Green lumber (freshly cut) contains significant moisture. Kiln-drying removes 15-20% of the moisture content, causing the wood to shrink. Then four-sided planing smooths all surfaces, removing another 1/4 to 1/2 inch from each face. A rough-cut 2×4 that measures 2×4 inches becomes 1.5×3.5 inches after drying and planing.
Common Nominal to Actual Conversions:
- 1× lumber (1" nominal): Actually 3/4" (0.75") thick. Used for trim, paneling, and shelving.
- 2× lumber (2" nominal): Actually 1.5" thick. The workhorse of framing and construction.
- 4× lumber (4" nominal): Actually 3.5" thick. Used for posts, beams, and heavy framing.
Width Conversions (applies to all thicknesses):
- 2" nominal → 1.5" actual
- 4" nominal → 3.5" actual
- 6" nominal → 5.5" actual
- 8" nominal → 7.25" actual (note: not 7.5")
- 10" nominal → 9.25" actual
- 12" nominal → 11.25" actual
Notice that boards 8 inches wide and larger lose 3/4 inch from the nominal width, not just 1/2 inch like narrower boards. This matters when calculating deck board coverage or subfloor sheathing. A 2×10 joist is 1.5×9.25 inches, giving you 9.25 inches of bearing surface, not 10 inches.
When Actual Dimensions Matter Most: Building furniture requires actual dimensions because pieces must fit together precisely. Deck framing calculations use actual widths to determine board coverage per square foot. Door and window rough openings are based on actual lumber dimensions plus framing clearances. Always use actual dimensions for measurements and nominal names for ordering lumber at the yard.
The Board Foot Formula and Why It Matters
Board feet (abbreviated BDFT or BF) is the pricing standard for lumber, particularly hardwoods, exotic woods, and rough-cut lumber. Understanding board feet calculation helps you compare prices between suppliers and estimate project costs accurately.
The Basic Formula: Board Feet = (Thickness in inches × Width in inches × Length in feet) ÷ 12
The division by 12 converts cubic inches to board feet. Remember that thickness and width are in inches, but length is in feet. This is the critical detail that trips up first-time calculators.
Example Calculation for 2×4×8:
A 2×4 stud that is 8 feet long: (2 × 4 × 8) ÷ 12 = 64 ÷ 12 = 5.33 board feet
If the lumber yard charges $2.50 per board foot, that 2×4 costs: 5.33 × $2.50 = $13.33
Why Hardwoods Use Board Feet: Hardwood lumber (oak, maple, walnut, cherry) comes in random widths and lengths because mills cut around defects and maximize yield from each log. A walnut board might be 8 inches wide and 10 feet long, while the next board is 6 inches wide and 12 feet long. Pricing by the board foot accounts for this variation fairly. Both boards cost the same per board foot regardless of width differences.
Linear Feet vs Board Feet: Linear feet measure length only. Ten 2×4s that are 8 feet long equals 80 linear feet. But those same boards contain 53.3 board feet (5.33 BDFT per board × 10 boards). Softwood construction lumber (pine, fir, spruce) is often priced per piece or per linear foot at big box stores, but specialty lumber yards and wholesale suppliers use board foot pricing.
The Thousand Board Foot (MBF): Large orders are quoted in thousands of board feet. "M" is the Roman numeral for 1,000. If a supplier quotes walnut at $8.50 per MBF, that means $8.50 per thousand board feet, or $0.0085 per board foot. This pricing method is common in wholesale and commercial lumber sales.
Quick Mental Math Shortcut: For standard 2× lumber, multiply length in feet by 2/3 to estimate board feet. An 8-foot 2×4 is about 8 × 2/3 = 5.33 board feet. A 12-foot 2×6 is about 12 × 1 = 12 board feet. This works because (2 × 4) ÷ 12 simplifies to approximately 2/3, and (2 × 6) ÷ 12 equals exactly 1. For 2×8s, multiply length by 1.33. These shortcuts help you estimate costs quickly at the lumber yard.
Common Lumber Lengths and When to Use Each
Lumber is sold in standard lengths, typically in 2-foot increments. Choosing the right length minimizes waste and reduces costs since you pay for every foot you buy, whether you use it or not.
Standard Lengths Available: 6, 8, 10, 12, 14, 16, 18, 20, and 24 feet. Not all sizes are stocked in all lengths. Specialty lengths like 14, 18, 20, and 24 feet often require special order and cost a premium.
8-Foot Boards (Most Common): The default length for studs, deck boards, and general framing. Standard wall height is 8 feet, making 8-foot studs perfect for residential framing. One 8-foot board can be cut into two 4-foot pieces with no waste. Easiest length to transport in a pickup truck. Most DIY stores stock the widest selection of sizes in 8-foot lengths.
10-Foot Boards: Common for deck joists, small beam spans, and trim work. Reduces the number of butt joints in longer runs. Two 10-foot boards can cover 20 linear feet with one splice instead of two splices using 8-foot boards. Still manageable for one person to carry, though awkward in tight spaces.
12-Foot Boards: Ideal for deck joists spanning 11-12 feet or longer trim pieces. Flooring joists for rooms wider than 10 feet typically use 12-foot stock. Allows for slight overhang when spanning 11 feet, giving you room to square cuts. Requires two people to carry safely and won't fit in most vehicles without a lumber rack.
16-Foot Boards: The longest length commonly stocked at big box stores. Used for long deck joists, garage framing, and minimizing seams in fascia boards. Cost per linear foot is usually the same as shorter lengths, making 16-footers a good value when you can use the full length. Transportation requires a truck with lumber rack or trailer. Difficult to maneuver on-site without help.
How to Choose the Right Length: Measure your longest span and add 6 inches for squaring cuts. If you need multiple shorter pieces, calculate whether buying one long board or several short boards generates less waste. For example, if you need five 4-foot pieces, buy three 8-foot boards (yields six 4-foot pieces, one spare) rather than five separate 4-foot pieces that cost more and leave no backup for mistakes. Price breaks sometimes occur at certain lengths, so ask your supplier if buying 12-foot boards is cheaper per foot than 10-footers.
Waste Factor: Always add 10-15% waste for cuts, mistakes, knots, and defects. If you calculate needing exactly 400 linear feet, buy 440-460 linear feet. Running short mid-project costs more in time and fuel than buying an extra board upfront. Leftover lumber stores well in a dry location and gets used on future projects.
Lumber Grades and How They Affect Price
Not all lumber is created equal. Mills grade boards based on appearance, strength, and defects. Understanding grades prevents overpaying for quality you don't need or buying cheap lumber that won't work.
Softwood Grades (Pine, Fir, Spruce):
Select Grade: The cleanest boards with minimal knots, straight grain, and few defects. Used for visible applications like trim, cabinets, and furniture. Costs 30-50% more than construction grade. Easier to work with and less likely to warp.
Construction Grade (No. 2 and Better): The standard for framing lumber. Contains some knots but remains structurally sound. Perfectly adequate for framing, sheathing, and any application that will be covered. Most cost-effective choice for structural work.
Utility Grade (No. 3): Contains more knots, possible warping, and lower structural values. Acceptable for non-structural uses like furring strips, temporary bracing, or blocking. Cheaper but inspect carefully since defects can be significant.
Hardwood Grades (Oak, Maple, Walnut, etc.):
Hardwood grading is more complex, measuring the percentage of clear wood (free from knots and defects) that can be cut from each board:
FAS (Firsts and Seconds): The highest grade with 83% minimum clear wood. One face must be almost completely clear. Used for fine furniture and visible surfaces. Most expensive but worth it for heirloom projects.
Select: Slightly lower than FAS with 83% clear on the best face. Perfect for projects where one side shows and the other is hidden. Good value compared to FAS.
No. 1 Common: 66% clear wood minimum with smaller clear sections. Ideal for cabinets, smaller furniture pieces, and flooring where shorter clear sections work well. Significantly cheaper than FAS and perfectly suitable for most woodworking.
No. 2 Common: 50% clear wood minimum. Contains more knots and color variation. Great for rustic projects, shop furniture, or applications where character matters more than perfection. Budget-friendly option for practicing new techniques.
When to Upgrade Grade: Use select or better for any visible surface like window trim, baseboards, stair treads, or furniture. Pay for higher grades when working with difficult joints that require straight, stable wood. Construction grade is perfect for rough framing, subfloors, blocking, and structural work that gets covered. Mixing grades can save money: use select grade for the face frame of cabinets and construction grade for the carcass.