Concrete Bag Calculator
Work out how many bags of concrete to buy from your measurements or a volume you already know, using the wet yield printed on the product you are actually buying.
Results
Enter your project measurements to see your results.
What the bag calculator does
Bagged concrete is priced and sold by weight, but it is used by volume. This page bridges that gap: it converts your project into a volume, adds your waste allowance, then divides by the wet yield the manufacturer states for the specific product in your trolley.
How to measure for bagged concrete
- Measure the space you are filling rather than the finished surface you want to see.
- For small repairs, measure the deepest point rather than the shallowest.
- If another tool has already given you a volume, switch the input method and type it straight in.
- Note the yield from the bag before you leave the merchant, or check it online first.
The bag formula in plain language
Take the volume you need after waste. Divide it by the volume one bag makes once mixed. That gives a decimal number of bags, which is then rounded up. If you need 0.17 m³ and a bag yields 17 litres, that is 0.17 ÷ 0.017 = 10 bags exactly. If you need 0.175 m³ the exact figure becomes 10.29, so you buy 11.
Metric example
A path 6 m × 0.6 m × 75 mm is 0.27 m³. With a 5% allowance it becomes 0.2835 m³. A product stating 17 litres per 25 kg bag gives 0.2835 ÷ 0.017 = 16.68 bags, so you buy 17 bags, which is 425 kg of product.
Imperial example
A 3 ft × 3 ft pad at 4 in thick is 3 cubic feet. With 10% waste that is 3.3 cubic feet. A product stating 0.6 cubic feet per 80 lb bag gives 3.3 ÷ 0.6 = 5.5 bags, so you buy 6 bags, which is 480 lb of product.
Common mistakes
- Using a yield figure from a different manufacturer's bag of the same weight.
- Confusing dry bag weight with mixed volume.
- Rounding down because the exact figure is close to a whole number.
- Forgetting that fast-setting post products behave differently from general-purpose mixes.
When bags stop making sense
Past roughly a cubic metre or a cubic yard, mixing by hand becomes hard work and the joins between batches become a real risk. At that point speak to a ready-mixed concrete supplier about a small load, or ask about a mix-on-site service. For structural work, get the specification confirmed by a qualified professional before you buy anything.
Frequently asked questions
- Why do I have to enter the yield instead of just the bag weight?
- Because the finished volume depends on the mix, the aggregate size and how much water the product needs. Two bags of identical weight from different manufacturers can set to different volumes, so the only reliable figure is the wet yield printed on the bag or its data sheet.
- Where do I find the wet yield?
- It is usually on the back or side of the bag, often written as approximate litres of mixed concrete per bag, or as cubic feet per bag in the United States. If it is not on the packaging, the manufacturer's technical data sheet or customer helpline will have it.
- Why does the calculator always round up?
- You cannot buy a fraction of a bag, and a shortfall part-way through a pour causes a cold joint. The exact mathematical figure is still shown so you can see how close you are to the next whole bag.
- Can I enter a volume I already know?
- Yes. Switch the input mode to a direct volume and type the figure in litres, cubic metres, cubic feet or cubic yards. That is useful when another calculator or your supplier has already given you the quantity.
- Is Post Mix the same as bagged concrete?
- Products sold in the UK under names such as Post Mix or Postcrete are fast-setting mixes intended for post holes. They come in bags like any other mix, but their stated yields and setting times differ from general-purpose concrete, so use the figure from the specific product you buy.