Concrete Formwork Calculator

1. Choose what you are forming: a wall or footing, a square column or a slab edge.
2. Enter the length in feet (or the slab perimeter), the thickness or column side in inches and the form height in inches.
3. Set the number of elements, whether to form the ends, and a waste allowance.
4. Read the contact area, the 4 by 8 panel count and the concrete the forms hold.

Here is what each result line means:

A concrete formwork calculator works out how much form surface a pour needs. Formwork is usually measured by contact area: the area of form face that touches the fresh concrete. That figure drives how many panels or boards you buy or rent, how much form release you need and how long the forming takes.

The tool handles the common cases: walls, footings and beam sides formed on two faces, square columns formed on four, and slab edges formed around the perimeter. It also reports the concrete the forms will hold. To size that concrete order in more detail use the concrete wall calculator or the concrete slab calculator.

Contact area is simply the area of every formed face. A wall has two long faces, each length times height, and two short end faces, each thickness times height, if the ends are formed. A square column has four faces, each side times height. A slab edge is a strip around the perimeter, perimeter times slab thickness.

1. Convert the height and the thickness or column side from inches to feet by dividing by 12.
2. Add up the formed faces for one element, then multiply by the number of elements.
3. Add the waste allowance for cuts, corners and damage.
4. Divide by 32 square feet for the number of 4 by 8 panels, and work out the concrete the forms hold.

For the reinforcement inside the forms, the concrete rebar calculator counts the bars.

Suppose you are forming a wall 20 feet long, 8 feet (96 inches) high and 8 inches thick, with both ends formed and 10 percent waste.

Calculation: the two faces are 2 x 20 x 8 = 320 sq ft. The two ends are 2 x 0.667 x 8 = 10.7 sq ft. Together that is 330.7 sq ft, or 363.7 sq ft with waste, which is 363.7 / 32 = 11.4, so 12 panels. The forms hold 20 x 0.667 x 8 = 106.7 cubic feet, about 3.95 cu yd of concrete.

The lesson: the two faces of a wall are almost all of the formwork; the ends add only a few percent.

Height

Contact area grows directly with the form height. A taller wall also pushes harder on the forms, so it needs more ties and bracing, not just more panels.

Number of Faces

A wall cast against an excavation or an existing wall may need only one face formed. Use the length on one side only in that case, or halve the result.

Openings and Blockouts

Doors, windows and sleeves need their own blockout forms around the edges, which add form material even though they remove concrete.

Reuse

Forms are often stripped and reused on the next pour. If you pour in stages, you may need only enough panels for the largest single pour.

Form faces are commonly built from panels or from boards, and the choice depends on the element.

Rented engineered form systems are also common on larger jobs; the contact area is the figure suppliers quote against.

Buying or Renting Forms

Turn the pour into a contact area and a panel count before you buy plywood or call a form rental company.

Pricing a Forming Job

Forming is often priced per square foot of contact area, so the total gives you the basis for a labour and material estimate.

Ordering Form Release and Ties

The contact area sizes the form release agent, and the height tells you how heavy the ties and bracing need to be.

1. Counting Only One Face of a Wall

A free-standing wall is formed on both sides. Forgetting the second face halves the estimate.

2. Mixing Up Feet and Inches

Enter the length in feet and the height and thickness in inches, as the fields are labelled.

3. Ignoring Blockouts and Corners

Openings, corners and pilasters use extra form material. Keep a waste allowance, and more for complex walls.

4. Under-bracing Tall Forms

Fresh concrete pushes hard on tall forms. Use the form manufacturer’s tie and bracing requirements, not a guess.

5. Treating the Estimate as a Form Design

Contact area is not capacity. Form design, ties and shoring come from the manufacturer, an engineer and safety regulations.

The geometry is exact for straight walls, square columns and slab edges; real jobs add blockouts and corners.

Multiply the wall length by its height and double it for the two faces, then add the end bulkheads, thickness times height times 2, if they are formed. A 20 by 8 foot wall 8 inches thick is 320 + 10.7 = 330.7 square feet of contact area before waste.

It is the area of form face that touches the concrete. Estimators usually measure and price formwork by contact area, so it is the figure that sizes panels, form release and forming labour.

Divide the total contact area, including waste, by 32 square feet per 4 by 8 sheet and round up. A 20 by 8 foot wall formed on both faces with ends needs about 364 square feet, or 12 sheets.

For a square column, multiply the side by the height and by 4 for the four faces. A 12 inch column 9 feet tall has 36 square feet of contact area; four of them have 144 square feet before waste.

A slab on ground is formed only around its edge, so multiply the perimeter by the slab thickness. An 80 foot perimeter at 4 inches is about 26.7 square feet of edge form, usually built from dimensional lumber.

A free-standing wall is formed on both faces. A wall cast against a stable excavation or an existing wall may need only one face formed; in that case use about half the result.

Around 10 percent covers cutting panels to size and minor damage on a simple job. Walls with many openings, corners or pilasters need more.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• American Concrete Institute (ACI; formwork and cast-in-place concrete practice).
• APA, The Engineered Wood Association (APA; plywood and panel products used for concrete forming).
• American Cement Association (American Cement Association; concrete construction basics).