Steel and plywood systems have taken over many large sites, but timber formwork is still everywhere on small and medium projects: columns, beams, footings, staircases and slab edges. Ordering too little timber stops the pour; ordering too much ties up money. Here is a step-by-step method for calculating the timber you actually need.
Step 1: Find the contact area
Formwork is measured by the area of concrete it touches. For each element:
- Column: perimeter × height
- Beam: (2 × depth + width) × length, for the sides and soffit
- Footing: perimeter × depth (sides only)
- Slab: length × width for the soffit, plus perimeter × thickness for the edges
Example: a 300 × 300 mm column, 3 m high. Perimeter = 4 × 0.3 = 1.2 m. Contact area = 1.2 × 3 = 3.6 m².
Step 2: Convert area to sheathing volume
Multiply the contact area by the board (sheathing) thickness. With 25 mm boards:
3.6 m² × 0.025 m = 0.09 m³ of sheathing.
Step 3: Add the supporting members
Sheathing alone does not hold wet concrete. Yokes, battens, cleats and props add a significant amount of timber. A common rule of thumb for an early estimate is to add 40–60% of the sheathing volume for these members; for a detailed estimate, list them piece by piece. In our example, adding 50% gives:
0.09 + 0.045 = 0.135 m³
Step 4: Allow for wastage
Cutting to size, damaged ends and offcuts typically account for 10–15%. Adding 10%:
0.135 × 1.10 ≈ 0.149 m³
Step 5: Account for reuse
Good timber formwork can be reused several times — often 4 to 6 uses for sheathing, more for props. If the same column form will be used for five identical columns, you do not need five sets of timber; one set plus a replacement allowance for damaged boards is enough. Divide the total requirement by the number of uses, then add 10–20% for repairs.
Step 6: Convert to board feet if your supplier needs it
Many timber yards, especially those working with imported or North American stock, quote in board feet rather than cubic metres. The conversion is simple:
- 1 m³ ≈ 424 board feet
- 1 board foot ≈ 0.00236 m³
Our 0.149 m³ is therefore about 63 board feet. For individual pieces, board feet = thickness (in) × width (in) × length (ft) ÷ 12.
A quick checklist before ordering
- Contact areas calculated element by element
- Correct board thickness used (25 mm and 32 mm give very different volumes)
- Supporting members included, not just sheathing
- Wastage and reuse both applied
- Units agreed with the supplier — m³, board feet or running metres
Tools that save time
Spreadsheets work well for repetitive elements, but for converting lumber sizes and volumes quickly, free online tools are faster. Sawmill Planner offers a set of free timber calculators — board feet for any section in inches or millimetres, log volume and cutting patterns — which is handy when a supplier quotes in one unit and your drawings use another.
Conclusion
Formwork timber is easy to underestimate because the visible boards are only part of the story. Work from contact area to sheathing volume, add the supports, apply wastage and reuse, and convert to whatever unit your timber yard uses. The result is an order that arrives once and is enough.