When estimating concrete, calculating concrete volume is only half the battle. You must also calculate the contact surface area of the formwork required to hold the concrete while it cures.
Formwork contact area is measured in Square Feet of Contact Area (SFCA). Calculating SFCA is essential for estimating form rentals, stripping labor, and form release oils.
The Basic Rule of SFCA
SFCA only measures the surfaces where the formwork material touches the wet concrete. You do not calculate formwork for horizontal surfaces that are left exposed (such as the top of a slab-on-grade) or surfaces poured directly against neat soil (unless excavation plans require forming).
1. How to Calculate Footing Formwork
For a simple rectangular footing pad, formwork is placed around the four vertical sides. The top is left open for finishing, and the bottom rests on the ground.
`Footing SFCA = Perimeter of Footing x Height of Footing`
`Footing SFCA = (2 x (Length + Width)) x Height`
For example, a footing measuring 6 feet long by 4 feet wide by 18 inches (1.5 feet) high has a perimeter of:
`2 x (6 + 4) = 20 linear feet`.
The formwork contact area is:
`20 linear feet x 1.5 feet height = 30 SFCA`.
2. How to Calculate Concrete Column Formwork
For vertical columns, formwork wraps around the perimeter of the column for its entire height.
`Column SFCA = Perimeter of Column x Height`
- Square Columns: `(4 x Width) x Height`
- Round Columns: `(3.1416 x Diameter) x Height`
For example, a 12-inch (1-foot) square column that stands 10 feet tall has a perimeter of 4 linear feet. The formwork contact area is:
`4 feet x 10 feet height = 40 SFCA`.
For a 12-inch diameter round column, the perimeter is:
`3.1416 x 1 foot = 3.1416 linear feet`.
The formwork contact area is:
`3.1416 feet x 10 feet height = 31.42 SFCA`.
3. How to Calculate Slab Edge Formwork
Slabs-on-grade require formwork only around the outer perimeter to hold the edges of the pour. The main body of the slab is poured directly onto the vapor barrier and subgrade.
`Slab Edge SFCA = Perimeter of Slab x Slab Thickness`
For a slab measuring 100 feet by 50 feet with a thickness of 6 inches (0.5 feet), the perimeter is 300 linear feet. The edge formwork required is:
`300 feet x 0.5 feet thickness = 150 SFCA`.
Final Thoughts
Accurate SFCA calculations are vital because formwork labor is often more expensive than concrete placement. If you fail to separate your SFCA estimates by element type (e.g. footings vs. tall columns), you will apply incorrect labor rates and underprice your bid.