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Ribbed slab volume
Calculate concrete for a slab with ribs beneath it. The ribs run the full length or width and cross at right angles.
Enter your measurements
How it is calculated
V = L × W × top thickness + rib depth × (lengthwise count × lengthwise width × L + crosswise count × crosswise width × W − lengthwise count × crosswise count × lengthwise width × crosswise width).
Keep the full result when using it in another calculation. Results show up to three decimal places; very small positive results use scientific notation (for example, 1e-6). No extra material for waste is added.
Distinguish top thickness from rib depth
Enter the top slab thickness first. Rib depth is measured below the underside of that slab, so the two dimensions describe separate vertical layers. Count the straight ribs in each direction and enter each direction’s width.
Lengthwise ribs run the full slab length; crosswise ribs run the full slab width. The combined lengthwise rib width must fit within the slab width, and the combined crosswise rib width must fit within its length.
Count each rib crossing once
For the example dimensions, the top slab contains 20 × 10 × 0.25 = 50 ft³. Viewed from above, the two lengthwise ribs cover 20 ft² and the two crosswise ribs cover 10 ft². Their four 0.5 ft by 0.5 ft crossings total 1 ft² and must be subtracted once. At 1 ft rib depth, the ribs add 29 ft³, for a total of 79 ft³.
The model requires straight, full-length ribs with uniform dimensions. It does not select a rib layout or a structural slab system. Use the project’s specified configuration and do not treat this quantity result as a design check.
Before you use the result
- Ribs must be straight, run the full length or width, and have uniform width and depth. Ribs running in the same direction must not overlap.
- The combined width of the lengthwise ribs must fit across the slab width. The combined width of the crosswise ribs must fit along the slab length. Each rib must also fit within the slab’s edges.
- The volume at each rib crossing is counted once. Interrupted ribs and widened rib sections are not included. Enter project dimensions; the calculator does not select a slab system or use product-specific dimensions.