About Concrete Slab Form Calculator
With the default setup (8' x 32" panels, four 2x4 horizontals and two end studs, 3/4" form ply, a brace every 4' with a 32" upright, a 4' ground piece and a diagonal) the 320 ft perimeter takes 42 panels (38 full and 4 shorter fillers at the corners), 82 braces, about 327 eight-foot 2x4s and 29 sheets of 4x8 form ply, both including 5%…
By the ToolFluency team · Updated October 2026
Draw any slab shape and get the 2x4s, form plywood and bracing for job-built form panels, with a true-scale 3D model and a printable plan sheet. Free.
How to use
- Set the slab size. Type the width and length in the toolbar (75' x 85' is filled in) and click Set rectangle, drag a rectangle, or use Shape to click the corners of an odd-shaped slab. Lengths can be typed as 75', 12'-6" or 3.5 m.
- Enter the slab build-up: concrete thickness and foam under the slab. The form panels should be at least as high as both together.
- Describe your form panels: panel length and height, framing size, how many horizontals (top and bottom included) and whether the horizontals or the studs run between.
- Pick the form ply thickness and sheet size. The calculator rips sheets into strips the height of your panels and packs them across the whole job.
- Set the bracing: spacing along the edge, the upright on the panel, the flat ground piece, and how high the diagonal meets the upright.
- Read the takeoff on the right: panels, braces, how many boards of each length to buy, ply sheets, concrete and foam. Download the cut list as a CSV.
- Open the 3D tab to check the forms at true scale, and the Plan sheet tab to print or download an 11 x 17 layout sheet.
Frequently asked questions
How much lumber do I need to form a 75 x 85 ft slab?
With the default setup (8' x 32" panels, four 2x4 horizontals and two end studs, 3/4" form ply, a brace every 4' with a 32" upright, a 4' ground piece and a diagonal) the 320 ft perimeter takes 42 panels (38 full and 4 shorter fillers at the corners), 82 braces, about 327 eight-foot 2x4s and 29 sheets of 4x8 form ply, both including 5% extra. Change any setting and the takeoff updates.
How are the form panels and plywood cut?
Each panel is a 2x4 frame: horizontals run between two end studs (or plates run full length with studs between, if you prefer). Form ply is ripped into strips the height of the panel. A 4x8 sheet cut into three 32" x 48" pieces gives two pieces per 8' panel. The calculator packs every strip and offcut across the whole job, so leftover pieces from one panel are used on fillers before a new sheet is opened.
How long is the brace diagonal?
The diagonal runs from the far end of the flat ground piece up to the upright at the height you set, and it sits against the sides of the upright and ground piece so it can be nailed to both. With 2x4 framing, 3/4" ply, a 4' ground piece and the diagonal meeting the upright 30" up, the diagonal cuts at 4'-9 15/16". Every length in the cut list is worked out from the real board sizes (1 1/2" x 3 1/2" for a 2x4, 23/32" for 3/4" ply).
Is the 3D model to scale?
Yes. Every 2x4, ply piece and brace is drawn at its actual size and position, from the same numbers as the cut list. Zoom into a corner to see the end studs, the ply laps and how the braces sit against the panels. Inside corners where braces would hit each other are left without a brace and flagged so you can brace them by hand.
Can I draw an odd-shaped slab?
Yes. Use Shape to click each corner, type exact edge lengths like 40' or 12'-6" as you go, and click the first corner to close it. Drag corners to adjust, double-click an edge to add a corner, or type exact X and Y positions in the Corners list. Each edge gets its own run of panels with a filler panel where it does not divide evenly.
What does the plan sheet show?
A tabloid (11 x 17 in) sheet at a standard scale: the slab plan with form panels, joints, braces and edge dimensions, a typical panel elevation, a brace section through the slab and foam, an outside corner detail, the material schedule, cut list and a title block. Print it at 100% or download the SVG. It is a layout aid, not an engineered drawing.
Does the calculator check the forms will hold the concrete?
No. It counts and lays out the material for the panel and brace setup you enter. Form pressure depends on pour height, rate and mix, so for deep slabs check your bracing and stakes with the form supplier or an engineer.
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