How Many Bags of Concrete Do I Need?

Every bag of pre-mixed concrete prints its yield: the volume of concrete you get once it’s mixed with water. That’s the figure to use, not the weight. Typical general-purpose mixes in the US land around these numbers:

Treat these as a guide only. Fast-setting post mixes, high-strength mixes and different brands can all yield a bit more or less. In the UK, Canada and Australia, bags are usually 20, 25 or 30 kg and the yield is given in litres.

For a square or rectangular pad, multiply length × width × depth, with everything in feet (or metres).

For a round hole or tube form, use π × radius² × depth. The radius is half the diameter.

Then divide by the bag yield and round up. You can’t buy 8.9 bags, and you don’t want to find out you’re a bag short with the last hole half full.

Bags = Volume ÷ Yield per bag, rounded up

Say you’re setting eight 4×4 posts in holes 10 inches across and 30 inches deep, with 4 inches of gravel in the bottom for drainage. That leaves 26 inches of concrete around each post.

• Hole area: π × 5² = 78.5 sq in
• A 4×4 post is actually 3½ × 3½ inches, so it takes up 12.25 sq in. That leaves 66.3 sq in for concrete.
• Concrete per hole: 66.3 × 26 = 1,724 cubic inches, or 1.0 ft³ (divide by 1,728)
• Eight holes: 8.0 ft³

Now the bags:

• 80 lb bags at 0.6 ft³: 8.0 ÷ 0.6 = 13.3, so 14 bags (1,120 lb to carry)
• 60 lb bags at 0.45 ft³: 17.7, so 18 bags
• 50 lb bags at 0.375 ft³: 21.3, so 22 bags

If you forget to subtract the post, you get 9.5 ft³ and 16 bags of 80 lb. That’s not a disaster, but it’s two bags you’ll carry home again.

Metric version: holes 250 mm across, with 650 mm of concrete around a 90 × 90 mm post, hold about 26.6 litres each, or 213 litres for eight. At an assumed 9.5 L per 20 kg bag, that’s 23 bags.

A 4 × 4 ft landing pad 4 inches thick is 4 × 4 × 0.333 = 5.3 ft³. With 80 lb bags that’s 8.9, so 9 bags. Add 10% for an uneven base and you’re at 10.

Steps are easiest to handle as blocks. Work out each step as a rectangle (its width × depth × rise), add them up, and treat any landing as its own slab.

One cubic yard takes 45 bags of 80 lb, which is 3,600 lb of material to lift, mix and place before it starts to stiffen. Most people find that somewhere around half a yard to a yard, mixing by hand turns from a workout into a problem. Above that, price a ready-mix delivery, including any short-load charge, and compare it with the bag total.

A few other things push the decision. A slab poured in one go needs the concrete to arrive fast enough that you aren’t joining fresh concrete to stiffening concrete. Post holes, on the other hand, are separate small pours, so bags suit them well even when the total is large.

• Buy one or two bags more than the count. Unopened bags keep if you store them somewhere dry.
• Check your vehicle’s payload before loading 14 bags of 80 lb. That’s over half a ton.
• Use the water quantity on the bag. Extra water makes it easier to place but weaker once cured.
• Fast-setting post mixes are poured dry and then watered in place. Their yield is on the bag like any other.

The concrete bag calculator takes the volume of one hole and the number of holes, and lets you type the yield from your bag. If you need the volume first, the concrete calculator handles round holes and rectangular pads. For the posts themselves, the fence calculator counts them from your fence length and spacing.