Concrete Bag Calculator
Volume, bag count, and rough cost.
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
Concrete bag planning guide
Concrete bag estimates are driven by volume. Small changes in slab thickness, footing depth, or hole diameter can add a surprising number of bags, especially when the shape is repeated across multiple posts or piers.
Use this calculator for small slabs, paths, post holes, round piers, and trench-style pours where bagged concrete is practical. Measure the finished dimensions, then add waste for uneven excavation, spillage, and minor overbreak in the ground.
Practical checks before ordering
- Confirm whether dimensions are finished concrete size or excavation size.
- Use the yield printed on the exact bag you plan to buy.
- Add more allowance for rough ground, deep holes, or hand-dug excavations.
- Consider ready-mix delivery when bag counts become high.
Important limits
This is a quantity planning tool only. Structural thickness, reinforcement, drainage, curing, and permit requirements should be confirmed separately for the job. For footings, suspended slabs, retaining walls, and load-bearing work, use the project drawings or a qualified professional.
Worked concrete example
A 3 m by 2 m pad at 100 mm thick is 0.6 cubic metres before waste. If a bag yields 0.01 cubic metres, the base count is 60 bags; adding 5% waste brings the purchase estimate to 63 bags.
Concrete estimates are sensitive to depth. A trench or slab that is only 10 mm deeper than expected can add several bags. Measure excavation depth in several places, and allow more margin where the base is uneven or hand-dug.
Concrete calculator FAQ
What if my bag has a different yield?
This calculator estimates standard premix yield from bag size. Check the product label before ordering if the bag lists a different yield or the job is structural.
Why does the result show both volume and bags?
The raw concrete volume helps you sense-check the result before relying on the rounded bag purchase recommendation.