What it works out
This works out how much concrete you need for a size you already have: a shed base, a small pad, post holes, or a footing whose size is on a drawing from your engineer or building control. It gives the volume and turns it into bags of dry mix, ready-mix, or cement, sand and aggregate for mixing on site.
It also works out sand and cement, or bagged screed, for a floor screed. It never says how deep, wide or strong a slab or footing should be: that's a design decision for a professional.
How it works
The volume is the area times the depth, or for post holes, a cylinder per hole less the post. The waste allowance goes on top, for an uneven dig and spills.
Bagged dry mix is worked out from the weight of dry mix it takes to make a cubic metre of concrete. Mixing on site, the dry materials take up more room than the concrete they make, because the sand fills the gaps in the aggregate and the cement fills the gaps in the sand. That volume is shared out by your mix ratio and turned into bags of cement and bulk bags of sand and aggregate.
Ready-mix is the volume to order. Suppliers sell in set steps and may have a minimum load, so ask when you book.
A worked example
A shed base 2.4 m by 1.8 m and 10 cm deep, made from bags of dry concrete mix.
- Area 1: 2.4 m × 1.8 m = 4.32 m²
- Concrete: 4.32 m² × 10 cm = 0.432 m³
- 0.432 m³ + 10% for waste = 0.475 m³
- Dry mix, about 2,200 kg/m³ of concrete: 0.475 m³ × 2,200 kg/m³ = 1,045.44 kg → 42 bags of 25 kg
The result
- Concrete0.475 m³
- Bagged concrete mix1,045.44 kgBuy 42 bags of 25 kg (1,050 kg)
These are the calculator's own figures for these measurements, with 10% for waste. Change any of them above and it works it out again.
Common mistakes
- Measuring the depth in one place. Dug ground is never flat. Measure the depth at several points and use the average.
- Mixing more by hand than you can place. Concrete starts to set within a couple of hours. For more than a mixer load or two, ready-mix is usually easier.
- Guessing the size of a footing. Foundations depend on the ground and the load. Use the sizes on your engineer's or building control's drawing, never a figure from a website.
- Forgetting the formwork. A slab needs edges to hold the concrete while it sets. Measure inside the formwork.
Questions people ask
How do I work out how much concrete I need?
Multiply the length by the width by the depth, all in metres, to get cubic metres. Add a margin for waste. The calculator does this and turns it into bags or a ready-mix order.
How many bags of concrete mix do I need?
It depends on the volume and the bag size. The calculator works out the weight of dry mix for your volume and rounds up to whole bags in your region's sizes.
How deep should my slab or footing be?
That's not something we calculate. For a garden shed base, follow the shed maker's guidance. For a footing, a floor slab or anything carrying a wall, the size comes from a design: see our foundations guidance.
What mix should I use?
Use the mix your drawing, your supplier or the product's instructions give. The calculator shares the materials out in the ratio you choose; it doesn't decide the strength a job needs.
Should I use ready-mix or bags?
Bags suit small jobs you can mix and place in one go. For larger volumes, ready-mix delivered to site is usually cheaper, quicker and more consistent.
How much sand and cement do I need for a screed?
Choose Screed, enter the area and the thickness your flooring or underfloor heating supplier gives, and the calculator works out the sand and cement for a 1:4 mix, or bags of ready-to-mix screed.