Concrete Pad Calculator

HomeMath multiplies the pad's length × width × thickness in feet for the concrete, adds the waste allowance, and divides by 27 for cubic yards. The gravel base is the same length × width × its compacted depth. Loose gravel shrinks when it is compacted, so HomeMath adds your compaction allowance to that volume for the loose gravel to order. Bags are rounded up to whole bags and the ready-mix order to the next quarter yard.

The published rules it uses: US customary units; metric conversion; premixed bag yields. Their sources, and how HomeMath tests every result, are on the about page.

A 10 × 12 ft shed pad 4 inches thick takes 1.63 cu yd with 10% waste: 74 bags of 80 lb mix or a 1.75 yd ready-mix order. Shed kits come in a handful of standard footprints; the table gives the concrete for each at the three preset thicknesses, the 80 lb bags at 4 inches, and the compacted gravel for a 4 in base.

From the 12 × 16 size up, a 4 in pad takes more than 100 bags of 80 lb, and a ready-mix truck is worth pricing; the concrete cost calculator compares the two at your prices.

Thickness moves the total most, so set it from what the pad carries. Your plans, the product's installation guide and your local code come first where they say more.

Going from 4 to 6 inches takes 50% more concrete: for a 10 × 12 ft pad that is 0.81 cu yd more, or 36 more bags of 80 lb.

A hot tub pad is the one where the maker's guide sets thickness, reinforcement and levelness. Water alone weighs about 8.3 lb a gallon, so a filled 400-gallon tub carries over 3,300 lb before anyone gets in.

Usually about 4 inches, compacted: it drains water from under the pad and gives it an even bed, though your plans and local code set the depth. Set the depth to 0 if you pour on an existing base.

Loose gravel settles when you compact it, so ordering the compacted volume leaves you short. Ask your supplier how much extra they suggest for your stone and enter it as the compaction allowance; the result shows the loose gravel to order, in cubic yards and in tons at 1.55 tons per cubic yard, the middle of Inch Calculator's range for crushed stone and gravel.

For gravel on its own, such as a path or a drain, use the gravel calculator; to turn a supplier's tons into yards, the cubic yards to tons calculator.

At least the footprint of what sits on it; for a shed, measure the base frame, not the roof overhang. Many builders leave the pad a few inches wider on every side, so the walls sit fully on concrete and water drips clear of the frame.

Adding 6 in on each side of an 8 × 10 ft shed makes a 9 × 11 ft pad: 1.34 cu yd instead of 1.09 cu yd at 4 in. The calculator takes inches too, so you can type the frame's measurements as they are.

Switch the units to metric and the pad calculator takes the length and width in meters or centimeters, the thickness and the gravel depth in centimeters, and gives the concrete and the gravel in cubic meters, with liters for the smaller amounts. The ready-mix order rounds up to the next quarter cubic meter.

The bag counts are for the US 80 lb bag (0.60 cu ft, about 17 L). For a bag sold by the kilogram, type its weight and the yield in liters on its label, and the calculator counts your bags as well.

1. Mark out the pad and check it is square by measuring both diagonals: they are equal on a rectangle.
2. Dig out the soil to the depth of the gravel plus the slab, a little wider than the pad.
3. Spread and compact the gravel base in layers, and set level forms around it.
4. Add wire mesh or rebar if your plans call for it.
5. Pour, level the concrete with a straight board across the forms, and finish the surface.
6. Keep the pad damp while it cures, as the bag or your supplier recommends.

This calculator covers the concrete and the gravel. For the cost of the pad, bags against a truck, use the concrete cost calculator; for round footings under a deck or a carport, the column tab of the concrete calculator.