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Lumber Conversion

Cross-section (W×T) × length × quantity × species → volume in m³, sai (才), board feet (BF), and estimated weight all at once.

Member Dimensions

Wood Species

Result

Enter member dimensions and quantity.

What this tool does

The Lumber Board Feet Calculator converts width, thickness, length, and quantity into total volume (m³), Japanese/Korean Sai (才), North American Board Feet (BF), and weight (kg) by species — all in a single calculation. It is built for mixed-unit job sites where import SPF or Douglas Fir invoices are quoted in board feet while domestic lumber dealers still price in Sai, eliminating the unit conversion errors that inflate material costs.

Who uses this

  • Total cost check for SPF 2×4 or 2×6 rafter stock purchased at a board-foot price
  • Sai-unit quantity and cost for domestic larch or pine sourced from a Korean lumber yard
  • Pre-delivery weight check to stay within truck payload limits
  • Comparing cost-per-volume between Douglas Fir and SPF at current market prices
  • Verifying a supplier's quoted board-foot count against your own dimensions

How to Use (3 Steps)

  1. 1Enter width (mm), thickness (mm), length (mm), and quantity. Note that nominal dimensions differ from actual (dressed) dimensions — a 2×4 is actually 38 mm × 89 mm after drying and planing.
  2. 2Select wood species: SPF (Spruce-Pine-Fir mix), Larch, Pine, Cedar, Douglas Fir, or Oak. The density value for your chosen species (at 12% moisture content) is applied to calculate weight.
  3. 3Read the results: total volume (m³), Sai, Board Feet, and weight (kg). Multiply by your unit price (per Sai or per BF) to get a material cost estimate.

Formulas & References

■ Volume per piece V_per = (width mm × thickness mm × length mm) ÷ 1,000,000,000 [m³] ■ Total volume V_total = V_per × quantity ■ Sai (才) conversion Sai = V_total ÷ 0.003342 (1 才 = 1 Sun × 1 Sun × 12 Shaku ≈ 30.303 mm × 30.303 mm × 3,636.4 mm ≈ 0.003342 m³) ■ Board Feet (BF) conversion BF = V_total ÷ 0.002359737 (1 BF = 1 ft × 1 ft × 1 in = 144 in³ ≈ 2,359.737 cm³) ■ Weight (air-dry, 12% moisture content) W = V_total × species density [kg/m³] SPF: 460 / Pine: 500 / Larch: 540 / Cedar: 380 / Douglas Fir: 530 / Oak: 750

Worked Examples

Example 1: 30 pieces of 2×6 SPF rafter stock (3,600 mm)

Width 140 mm, thickness 38 mm, length 3,600 mm, qty 30, species SPF. Volume per piece ≈ 0.019152 m³. Total volume ≈ 0.5746 m³. Sai ≈ 172 才. Board Feet ≈ 243.5 BF. Weight ≈ 264 kg (460 kg/m³ × 0.5746).

Example 2: 10 pieces of larch 4×4 post (3,000 mm)

Width 89 mm, thickness 89 mm, length 3,000 mm, qty 10, species Larch. Total volume ≈ 0.2376 m³. Sai ≈ 71 才. BF ≈ 100.7. Larch density 540 kg/m³ → weight ≈ 128 kg. Comfortably fits on a 1-ton pickup.

Example 3: 5 oak slabs for a dining table (400 × 50 × 2,000 mm)

Total volume = (400 × 50 × 2000 / 1,000,000,000) × 5 = 0.2 m³. Sai ≈ 60 才. Weight ≈ 150 kg (750 kg/m³). Oak is dense — 50 pieces of this size would weigh 1.5 tonnes. Plan lifting equipment accordingly.

Frequently asked questions

Why is a 2×4 actually 38 mm × 89 mm?

North American dimensional lumber is sold by its nominal (rough-sawn) size of 2 in × 4 in (50.8 × 101.6 mm). After kiln drying and planing (S4S — Surfaced Four Sides), the lumber shrinks to its actual dressed size of 38 mm × 89 mm. Korea's KS F 3020 adopts these same North American dressed dimensions. Always input the actual (dressed) dimension, not the nominal label.

What is the difference between Sai (才) and Board Feet (BF)?

Sai is the traditional Korean and Japanese lumber unit: 1 才 ≈ 0.003342 m³ (one Sun square by 12 Shaku long). Board Feet is the North American unit: 1 BF ≈ 0.002360 m³ (one foot square by one inch thick). For the same volume: 1 m³ ≈ 299 Sai ≈ 424 BF. Korean lumber yards typically quote per Sai; imported lumber invoices use BF.

Does moisture content affect the weight result?

Yes. This tool uses air-dry density at 12% moisture content (MC), which is typical of kiln-dried (KD) structural lumber. Green lumber (fresh-sawn, MC 30–60%) is 30–60% heavier. If you are unsure of the actual MC, weigh a sample board and scale proportionally.

What exactly is SPF?

SPF stands for Spruce-Pine-Fir, a combined species designation for Canadian structural lumber that groups White Spruce, Red Pine, and Balsam Fir. It is the dominant import species for light-frame (stick-frame) construction due to its low density (≈ 460 kg/m³), ease of working, and wide availability. All three species have similar mechanical properties and are grouped for grading and pricing.

How do I enter dimensions in Shaku (尺) or feet?

This tool uses millimetres. Conversion: 1 Shaku (尺) = 303.03 mm. A standard 12-Shaku board (≈ 4.8 m) = 3,636 mm. One foot = 304.8 mm, so 12 ft = 3,658 mm. Enter the converted value in the length field.

Cautions

  • •Weight is calculated at 12% moisture content (air-dry). Green or high-MC lumber can be 30–60% heavier in practice.
  • •Always enter actual (dressed) dimensions, not nominal dimensions. A 2×4 is 38 × 89 mm, not 50 × 100 mm.
  • •This tool outputs quantities only — it does not provide market prices per Sai or per BF.
  • •Volume and weight calculations are for procurement planning only and do not assess structural load capacity or span adequacy.

Last reviewed: 2026-06-17

Lumber Conversion | Workmate