For a lot of jobs, you can mix concrete by hand. You can probably tackle up to four yards. Here’s what to do.

First, estimate your needs.

1. Measure in feet to get the volume in cubic feet (surface area x depth). To get surface area for circles multiply pi(3.14) x radius x radius (Ã£R2). And if the overall shape’s too complicated, divide it into sections and calculate each area individually.

Make sure you convert the thickness to fractions of a foot (eg. 6″ = .5 feet)

2. Add 10 to 20% of the volume for waste.

3. Now, convert cubic feet to cubic yards (or yards) by dividing the total volume by 27. Round up to nearest tenth of a yard.

Ordering

For premix, multiply cubic feet by 2 to get the number of 66 lb (1/2 cubic foot) bags.

Mixing your own is less expensive, and it’s not that difficult. Concrete’s a mixture of portland cement, sand, gravel and water. For setting posts, footings for additions, underpinning and concrete slabs, 3/4″ gravel is a good choice. Even if you’re want a smooth surface, there’s no advantage to smaller aggregate, because trowelling brings the fine stuff to the surface.

To make one yard, order:

1. 7 bags of portland (in 88 lb. bags), which is 4.5 cubic feet.

2. A third of a yard of sand.

3. And half a yard of 3/4″ aggregate.

When mixing, the cement/sand/gravel ratio is simply 1:2:3. You can measure it with your shovel.

Then you’ll need about 30 gallons of water for each yard of concrete.