Concrete Bag Calculator

Estimate concrete volume and bag quantities for slabs, trenches, post holes, and round piers. Enter the bag size you plan to buy, such as a 20 kg bag, and the calculator estimates the bag count.

Concrete Inputs

Price field
Use the price for one bag of the selected size.
Slab and trench modes use length x width x depth. Post-hole and round-pier modes use diameter x depth x count. Bag counts are estimates based on a standard premix yield, then rounded up to a whole bag.

Concrete volume is sensitive to thickness

Concrete quantity comes from a three-dimensional measurement. For a slab or trench, length × width × depth becomes volume. For holes and piers, diameter, depth, and number of holes determine the volume. The result then uses the bag yield you enter and rounds up to whole bags.

Concrete calculation

slab volume = length × width × depthbags = allowance-adjusted volume ÷ bag yield, rounded up

A small thickness change is not small in volume

A 100 mm slab made 125 mm thick needs 25% more concrete across the same area. Measure excavation depth in several places and enter the manufacturer’s stated yield where it is available.

What the breakdown separates

For post holes, the result identifies raw hole volume, optional post displacement, net volume, allowance, bag yield, and final whole-bag rounding. That makes the purchase figure easier to audit rather than treating every bag as interchangeable.

Learn before ordering concrete