Monolithic Slab Calculator

1. Enter the slab length, width and thickness in the fields, length and width in feet and thickness in inches.
2. Enter the perimeter footing width and depth in inches; enter 0 for both to pour a flat slab with no footing.
3. Set the number of slabs and a 5 to 10 percent waste allowance.
4. Read the result: total cubic yards, split into the slab and the footing, plus bags and weight.

Here is what each result line means:

A monolithic slab calculator estimates the concrete for a monolithic slab, a foundation where the floor slab and its perimeter footing are poured together as one piece. It adds the deepened edge footing to the flat slab so the order is right.

A monolithic slab-on-grade is common for sheds, additions, garages and houses on ground that does not freeze deep. The footing around the edge carries the walls and reaches below the frost line, and the whole thing is poured in a single continuous pour. The tool sizes the slab and the footing separately and adds them. For the mix itself, use the concrete mix calculator or the cement calculator, and for mix water the water cement ratio calculator.

The field slab is length times width times thickness. The perimeter footing is extra concrete: a deepened strip around the edge, so it is the perimeter length times the footing width times the depth below the slab.

1. Find the slab volume in cubic feet from length, width and slab thickness.
2. Find the perimeter, 2 times length plus width, and multiply by the footing width and the depth below the slab for the footing volume.
3. Add the slab and footing, multiply by the number of slabs, and add the waste allowance.
4. Divide by 27 for cubic yards, and by the bag yield for a bag count.

Suppose you are pouring a 12 by 16 foot monolithic slab at 4 inches, with a 16 inch wide perimeter footing 16 inches deep, and 10 percent waste.

Calculation: the slab is 12 x 16 x 0.333 = 64 cu ft. The perimeter is 56 feet, and the footing adds 56 x 1.333 x (12 / 12) = 74.67 cu ft. Together that is 138.67 cubic feet, or 152.5 with 10 percent waste, which is 5.65 cu yd, about 255 eighty-pound bags and roughly 11 short tons.

The lesson: the footing is more than half the concrete here, so a monolithic slab uses far more than a flat slab of the same floor area.

Footing Size

The perimeter footing is often the larger share of the concrete, so its width and depth move the total more than the slab thickness. It must reach below the frost line, which sets the depth.

Frost Depth

In a cold climate the footing goes deeper to sit below frost, which adds a lot of concrete. In a warm climate a shallower footing may be allowed. Local code sets this.

Slab Thickness

The field slab is usually 4 inches; a thicker slab for heavier loads adds concrete across the whole floor area.

Sub-base and Over-dig

The footing trench is rarely cut clean, and the sub-base is never perfectly even, so the real pour beats the geometry. That is what the waste allowance covers.

A monolithic slab pours the slab and footing in one piece, while a stem wall foundation pours footings, then walls, then a separate slab. They suit different sites and climates.

This tool sizes the monolithic option. Either way, the footing size and reinforcement come from an engineer or your building code.

Ordering Concrete for a Foundation

Size the slab and footing together in cubic yards and add waste before you order, so the continuous pour does not run short.

Checking the Footing Share

See how much the perimeter footing adds, so the deepened edge is in the order and not a surprise.

Comparing to a Flat Slab

Compare the monolithic volume with a flat slab of the same floor area to see what the integral footing costs in concrete.

1. Leaving Out the Footing

The perimeter footing is often more than half the concrete. Sizing only the flat slab under-orders a monolithic pour badly.

2. a Footing Shallower than the Frost Line

The footing must reach below frost. Using a shallow depth under-builds the foundation, whatever the volume says.

3. Mixing Up Feet and Inches

Entering the slab or footing sizes in the wrong unit throws the result off. Keep length and width in feet and thicknesses in inches.

4. Forgetting a Waste Allowance

A footing trench over-digs and the sub-base is uneven, so the exact geometry runs short. Keep 5 to 10 percent.

5. Treating the Estimate as a Foundation Design

Volume is not design. The footing size, reinforcement and depth come from an engineer or code, not from this tool.

The arithmetic is exact; the usefulness depends on the slab and footing sizes and the waste allowance you enter.

It depends on the slab and the footing. A 12 by 16 foot slab at 4 inches with a 16 inch footing needs about 5.65 cubic yards with waste, where the footing is more than half the concrete. Enter your sizes above for the exact figure.

A foundation where the floor slab and the perimeter footing are poured together in one continuous pour, rather than as separate footings, walls and slab. It is common for sheds, additions, garages and houses on level ground with shallow frost.

The perimeter footing is often 12 to 24 inches deep and must reach below the local frost line, with a width around 12 to 16 inches. The exact size and reinforcement come from an engineer or your building code, not from a volume estimate.

Because of the perimeter footing. On a small building the deepened edge can be more than half the total concrete, so a monolithic slab uses far more than a flat slab of the same floor area. The tool shows the slab and footing split.

Usually too many to bag: even a small shed slab with a footing runs into the hundreds of bags, so monolithic slabs are almost always a ready-mix pour. The tool shows the bag count mainly for scale.

A monolithic slab is often faster and cheaper on a level site with shallow frost, because it is one pour. A stem wall suits sloped sites and deep frost. The right choice is a site and code decision, not just a concrete-volume one.

Yes, a 5 to 10 percent waste allowance, and a little more for a rough footing trench. A monolithic pour is continuous, so running short mid-pour risks a cold joint in the foundation.

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

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and ordering for a continuous pour).
• American Cement Association (American Cement Association; concrete and slab-on-grade foundations).