Concrete Bag Calculator

Concrete bag calculator for slabs and volume estimates

When you are working with premix concrete, the practical question is never “how many cubic metres is this?” It is “how many bags do I need to buy so I do not run short halfway through the pour?” This concrete bag calculator converts either a known concrete volume (in cubic metres) or a simple slab size into a bag count, with a waste allowance so your estimate is realistic for normal DIY and light construction work.

If you already have a volume estimate from a drawing, a site engineer, or a previous calculation, select the volume mode and enter the number in m³. If you are pouring a basic slab like a small patio, walkway, shed base, or equipment pad, select slab mode and enter the length and width in metres plus the thickness in millimetres. The calculator converts thickness from millimetres to metres, multiplies the dimensions, and produces a slab volume in m³.

Next, choose the bag option. Many premix bags list an approximate yield on the packaging, usually in litres or cubic metres. Yields vary by brand, aggregate size, moisture, compaction, and how “wet” you mix it. This calculator includes typical yield estimates for common bag weights as a convenience. If you have the yield printed on your bag, switch to the custom option and enter your yield per bag in cubic metres. That is the most accurate way to use the tool because it matches your actual product.

Finally, add a waste allowance. Real pours lose volume because of spillage, over-excavation, uneven ground, voids, and mix left in the wheelbarrow or mixer. The calculator applies your waste percentage to the base volume and then divides by the chosen yield per bag. The result is rounded up to a whole number of bags because you cannot buy a fraction of a bag. You also get the total bag weight and an estimate of how much concrete volume you will have left over after buying whole bags.

Use this as a planning tool, not a structural design tool. If you are pouring anything load-bearing or you are unsure about slab thickness, reinforcement, sub-base preparation, or concrete strength, follow local building guidance and professional advice. For ordinary projects, though, a clean volume estimate plus a reasonable waste percentage gets you close enough to buy materials confidently and avoid multiple trips to the store.

Assumptions and how to use this calculator

• Bag yield estimates are “typical” and can differ from your specific brand; use the custom yield option if your bag lists a yield.
• Thickness is entered in millimetres and converted to metres for the slab volume calculation.
• Waste allowance is applied to the volume before converting to bags; 5% to 15% is common for small pours.
• The bag count is rounded up to the next whole bag so you do not run short mid-pour.
• This tool estimates quantity only and does not verify structural requirements, reinforcement, or concrete strength.

Common questions

What is a good waste percentage for premix concrete?

For clean slab pours on prepared ground, 5% to 10% is usually enough. If your base is uneven, you expect spillages, or you are filling around stones or irregular edges, 10% to 15% is safer. Very small jobs can have proportionally higher waste because a bit left in the mixer is a bigger percentage of the total.

Why does the calculator ask for “yield per bag” instead of only bag weight?

Bag weight does not directly equal volume because the mix density and aggregate content vary by product. The yield printed on the bag is the manufacturer’s best estimate of the volume you will get from that bag when mixed correctly. If you enter your actual yield, the calculation becomes straightforward and more accurate than guessing from weight alone.

How do I find the yield per bag on my product?

Look for a line on the packaging that mentions “yield” or “covers” and references litres or cubic metres. Some bags list coverage like “covers X m² at 50 mm thickness” instead. If that is what you have, convert it to volume by multiplying area by thickness in metres. When in doubt, use the custom field and base it on the most relevant number the bag provides.

Does this calculator work for post holes or footings?

It can, as long as you enter a correct concrete volume. If you know the hole diameter and depth, you can calculate the hole volume separately and then use the “I know the concrete volume” mode. For irregular holes, estimate conservatively and increase the waste percentage because shape and collapse can increase how much concrete you need.

Why does it round up bags and show leftover volume?

Buying a fraction of a bag is not possible, so rounding up is the only practical purchasing plan. The leftover estimate helps you decide whether to reduce waste percentage, change bag size, or plan a small extra task using any remaining mix. In real life, leftover volume is often consumed by site irregularities, which is one reason the waste allowance exists.