Concrete calculator for slabs, footings and post holes

How to calculate concrete in cubic yards

Multiply length × width × thickness in feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab 4 in thick is 10 × 10 × 4/12 = 33.33 cubic feet, or 1.23 cubic yards. For a round column or a post hole the volume is 3.14 × radius × radius × depth. The calculator adds up every slab and column and puts 10% on top unless you change it: the National Ready Mixed Concrete Association (NRMCA) advises ordering 4% to 10% more than the volume worked out from the plan dimensions. The extra covers an uneven subgrade, forms that bow a little and the concrete left in the mixer.

Bags are counted from their yield, the volume of fresh concrete one bag makes once mixed with water. The QUIKRETE Concrete Mix data sheet gives 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.30 for a 40 lb bag. You can change the bag weight and yield in the advanced options.

A worked example

In feet and inches (US). The calculator opens with a 10 × 10 ft slab, 4 in thick, 10% extra, 80 lb bags.

• Volume: 33.33 cu ft, or 1.23 cu yd
• With 10% extra: 36.67 cu ft, or 1.36 cu yd
• In 80 lb bags: 36.67 ÷ 0.60 = 61.1, so 62 bags, 4,960 lb to haul and mix

Past 40 bags the calculator suggests ready-mix instead: 1.5 cubic yards, which is 1.36 rounded up to the next quarter yard.

In metres (UK and elsewhere). A 3 × 3 m base, 10 cm thick, is 0.9 m³, or 0.99 m³ with 10% extra. With 25 kg bags yielding 11.5 litres each (the low end of the Sakret data sheet), that is 87 bags, so the calculator suggests 1 m³ of ready-mix, rounded up to the next half cubic metre.

Concrete slab calculator table

Concrete for 100 square feet of slab (10 × 10 ft) with 10% extra, in 80 lb bags yielding 0.60 cu ft. The figures come from the calculator.

Thickness	Cubic feet	Cubic yards	With 10% extra	80 lb bags	Ready-mix
3 in	25	0.93	1.02 cu yd	46	1.25 cu yd
4 in	33.33	1.23	1.36 cu yd	62	1.5 cu yd
5 in	41.67	1.54	1.7 cu yd	77	1.75 cu yd
6 in	50	1.85	2.04 cu yd	92	2.25 cu yd
8 in	66.67	2.47	2.72 cu yd	123	2.75 cu yd

For post holes and round footings 4 ft deep, with 10% extra: 3 bags of 80 lb for an 8 in hole, 4 for a 10 in hole, 6 for a 12 in hole and 11 for a 16 in hole.

Bags of concrete mix or ready-mix

Bagged concrete mix needs only water and suits small pours: post holes, a stoop, a short walk, the pad for a shed. QUIKRETE sells its Concrete Mix in 40, 50, 60, 80 and 90 lb bags; the 80 lb bag is the one the calculator starts with. A whole slab soon runs to dozens of bags, 62 for the 10 × 10 ft example, close to two and a half tons of mix to lift and blend one bag at a time.

Ready-mix comes mixed in a truck and is sold by the cubic yard. Most suppliers take orders in quarter-yard steps, and the NRMCA puts truck mixers at 8 to 12 cubic yards. Ask your supplier about the minimum load and any short-load fee before you order, since a small load can cost more per yard. The calculator rounds ready-mix up to the next quarter yard.

Common mistakes

• Entering the thickness in feet. Thickness goes in inches: 4, not 0.33.
• Counting a bag as a cubic foot. An 80 lb bag makes 0.60 cubic feet, so a cubic yard takes 45 bags, not 27.
• Buying cement instead of concrete mix. A bag of portland cement is only the binder: concrete also needs sand and gravel, and a concrete mix already contains them.
• Ordering ready-mix with no extra. If the truck runs out halfway, a second load means another delivery and a cold joint in the slab.
• Adding water beyond what the bag says. A wetter mix spreads more easily but ends up weaker.