Formwork is the temporary mold that holds fresh concrete in shape until it hardens. Its cost depends mostly on the contact area, the surface of the forms that touches the concrete. This calculator works out that area for a wall, a column or a strip footing, the number of form panels to provide and the concrete volume.
The formulas
- Wall: 2 faces × length × height + 2 ends × thickness × height
- Rectangular column: perimeter × height = 2 × (length + width) × height
- Strip footing: 2 sides × length × depth (the bottom is the ground)
- Panels = contact area × (1 + extra) ÷ panel area, rounded up
- Concrete volume = length × thickness × height
If several identical elements are cast one after the other, the same forms can be reused, so you only need panels for one element.
Example
A wall 20 ft long, 8 in thick and 4 ft high has a contact area of 2 × 20 × 4 + 2 × 0.67 × 4 = 165 sq ft. With 10% extra for cuts and overlaps, that takes 6 sheets of 4 × 8 ft plywood. The wall holds 1.98 cu yd of concrete.
Form materials
| Material | Typical use |
|---|---|
| 3/4 in plywood or form-grade plywood (HDO, MDO) | walls, columns, smooth finishes |
| Dimensional lumber (2 × 8, 2 × 12) | footings, slab edges, small walls |
| Modular steel or aluminum panels | repeated walls, large projects |
| Cardboard tubes | round columns and piers |
| Insulated concrete forms (ICF) | permanent forms that stay in place |
Good practice
- Coat the forms with a release agent so they strip cleanly and can be reused.
- Brace walls every 2 to 3 ft along their length and tie the faces together with form ties sized for the pour height.
- Fresh concrete pushes hard on forms: the pressure rises with height and pour speed, so pour tall walls in lifts.
- Check that the forms are plumb, square and at the right level before the pour.
- Leave forms on walls and columns for at least 1 to 2 days, longer in cold weather, before stripping.
Limits
The calculator gives quantities, not a structural design. Tall walls, heavy pours and elevated slabs need formwork designed for the concrete pressure, with shoring checked by an engineer.