Thickened slab edges

How it is calculated

V = outside length × outside width × main thickness + (outside length × outside width − inner length × inner width) × (total edge thickness − main thickness).

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.

Separate the slab from its thicker edge

Measure the outside rectangle and the inner rectangle where the thicker edge ends. Enter the full thickness of the main slab and the full thickness at the edge. The edge value includes the slab thickness; the calculator adds only the extra depth.

The inner length and width do not have to leave equal-width bands on all four sides. This method subtracts the inner rectangle from the outer footprint, so it can represent different edge widths while keeping the corners counted once.

Break the example into two volumes

For a 20 ft by 10 ft outside rectangle with an 18 ft by 8 ft inner rectangle, a 0.25 ft main slab, and 0.75 ft total edge thickness, the base slab is 200 × 0.25 = 50 ft³. The edge ring covers 200 − 144 = 56 ft² and adds 0.75 − 0.25 = 0.5 ft of depth, or 28 ft³. The total is 78 ft³.

If the main slab thickness is zero, the same relationship calculates an open rectangular ring. The calculator rejects inner dimensions outside the outer rectangle and an edge thickness less than the main slab thickness.

Before you use the result

• The inner rectangle must fit entirely inside the outer rectangle. The main slab thickness must be uniform, and the edge thickness must be uniform.
• Enter the full edge thickness, including the main slab thickness. The calculator adds only the extra edge depth, so each corner is counted once.
• The edge does not need to be the same width on all sides. Set main slab thickness to zero to calculate an open rectangular ring.

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