Monolithic Slab Calculator
Answer: a 24 × 20 ft slab, 4 in thick, with a 12-in-wide × 8-in-deep thickened edge below the slab takes 8.00 cubic yards net — 8.80 cubic yards with 10% waste.
A monolithic slab is a flat slab body and a thickened perimeter footing poured in one piece — one pour, no cold joint between slab and footing. This calculator adds the two volumes and counts each corner once, using the footing's centerline length.
Calculate your monolithic slab
What a monolithic slab looks like
The thickened edge runs around the whole perimeter. Its width and depth follow your plans and local code — the calculator takes them as inputs and does not choose them for you.
How the volume is calculated
The pour is two volumes added together:
- Slab body: length × width × slab thickness.
- Thickened-edge footing: perimeter length × footing width × footing depth below the slab. The perimeter length is measured on the footing's centerline — 2 × length + 2 × width − 4 × footing width — so the four corners are counted once, not twice.
Then the total is divided by 27 for cubic yards, and the waste allowance is applied to the order amount. Bag counts use the order volume and QUIKRETE published yields.
| Worked example | Calculation | Result |
|---|---|---|
| Slab body (24 × 20 ft, 4 in) | 24 × 20 × (4 ÷ 12) | 160.0 cu ft |
| Footing centerline length (12-in-wide footing) | 2 × 24 + 2 × 20 − 4 × 1 | 84 ft |
| Thickened-edge footing (8 in below the slab) | 84 × 1 × (8 ÷ 12) | 56.0 cu ft |
| Total, net | 160.0 + 56.0 | 216.0 cu ft = 8.00 yd³ |
| Order amount, 10% waste | 8.00 × 1.10 | 8.80 yd³ (396 × 80 lb bags) |
If you added the footing as a plain 88 ft perimeter run, you would double-count the four corner squares and over-order. The centerline method removes exactly that overlap.
When a monolithic slab is used
Monolithic slabs are common for garages, sheds, workshops, patios with a load-bearing edge, and small additions — anywhere a thickened edge can carry the wall load without a separate footing trench. Because slab and footing are poured together, there is no cold joint between them, and the job needs only one concrete order.
A separate footing is the usual choice where frost depth, soil conditions or the building design require the footing to sit deeper than a thickened edge can reach. Footing depth, width and reinforcement are structural decisions: follow your plans, your local building code, and a licensed engineer where one is required.
How to use this calculator
- Measure the slab length and width, and the slab thickness.
- Enter the footing width and the footing depth measured from the bottom of the slab down.
- Keep the 10% waste allowance unless your edge forms are exact and the subgrade is dead level.
- Read the cubic yards, bag counts and truckloads, then check the result against the worked example above.
Frequently asked questions
How much concrete do I need for a 24 × 20 ft monolithic slab?
With a 4-inch slab and a 12-inch-wide, 8-inch-deep thickened edge below the slab, the net volume is 8.00 cubic yards. With 10% waste, order 8.80 cubic yards — 396 bags of 80 lb mix from the order volume.
How are the corners counted so they are not doubled?
The footing runs around the perimeter on its centerline: 2 × length + 2 × width, minus 4 × footing width. For a 24 × 20 ft slab with a 12-inch-wide footing, that is 84 ft of footing, not 88 ft — each corner is counted once.
Is the footing depth measured from the top or the bottom of the slab?
From the bottom of the slab down. The footing depth you enter is the extra depth below the slab body, so the total edge depth is the slab thickness plus the footing depth — 12 inches in the example above.
Does this calculator design the footing or the reinforcement?
No. It calculates volume only. Footing width, depth and reinforcement follow your plans and local building code — have a licensed engineer specify them for structural work.