How much concrete do I need?
Treat the pour as a rectangular box. Convert thickness to feet or metres before you multiply so every dimension shares one unit system.
That is the neat mathematical volume. Real jobs need forms that sit true, a compacted sub-base, and a little extra mix for low spots — covered below. Open the Concrete Calculator when you want the arithmetic done for slabs, footings, tubes or stairs; this article stays with field decisions.
US example: 10×12 ft shed base
Worked example (US)
Garden shed pad, 10 ft × 12 ft, 4 in thick (0.333 ft), poured on compacted gravel (gravel ordered separately).
Volume = 10 × 12 × 0.333 = 40 cu ft → 40 ÷ 27 = 1.48 cubic yards. Add 10% waste → about 1.63 yd³. Most yards will round a small residential delivery upward; ask their minimum. If you bag it instead, jump to our bags of concrete guide after you lock volume.
Answer: Order roughly 1.6–1.75 cubic yards ready-mix, or calculate bags from 1.63 yd³ — not from the footprint alone.
Why 4 inches? It is a common light-duty slab thickness for sheds and patios on stable ground. Soft clay, vehicle loads or frost depth may need a thicker section and steel — that is a design call, not a calculator default.
UK example: garden path
Worked example (UK)
Path 8 m long × 0.9 m wide × 75 mm thick (0.075 m), a typical footpath depth on Blue Circle’s multipurpose guidance for pedestrian paths.
Volume = 8 × 0.9 × 0.075 = 0.54 m³. With 10% waste → 0.59 m³. At ~0.01 m³ per 20 kg multipurpose bag (Blue Circle), that is about 59 bags before you round for unbroken bags — heavy hand-mixing for a weekend. Many homeowners switch to a mini-mix truck around this size.
Answer: About 0.6 m³ including waste; bag only if you accept the labour, otherwise mini-mix.
Driveway apron and ready-mix
A short apron — say 12 ft × 8 ft × 6 in in the US (12 × 8 × 0.5 = 48 cu ft ≈ 1.78 yd³ before waste) — sits in the awkward middle: too big to enjoy bagging, small for some plants’ full truck minimums. Call two local yards: quote volume with waste, ask about short-load fees, and whether they can place with a chute or need a barrow crew.
In the UK, a 3.5 m × 2.5 m apron at 150 mm is 1.31 m³ before waste. Same phone call: minimum load, plasticiser for a driveway mix, and whether you need a pump.
Waste and order rounding
- 5% for tidy formed slabs on flat ground.
- 10% default for DIY forms, uneven digs and first-time pours.
- 15% when holes, steps or sonotubes waste mix on the walls.
Never order the exact mathematical volume for a continuous pour. Running short mid-slab leaves a cold joint. Leftover bags for a future post hole are cheaper than a patch line across a shed floor.
Bags or truck?
Use bags when you can mix and place in small batches (fence posts, stepping pads). Use ready-mix when the slab must be monolithic and the volume climbs past comfortable wheelbarrow maths. Either way, measure twice, convert thickness correctly (4 in is not 4 ft), and keep gravel base out of the concrete total. For bag counts by 40/60/80 lb or 20/25 kg, continue to how many bags of concrete.
Before the truck arrives
Forms should be staked, levelled and braced; the sub-base compacted and damp (not flooded). Decide where the chute sits and where surplus will go. If the pour spans a doorway or gate, measure access width the day before — a truck that cannot reach the forms turns a 1.6 yd³ order into a barrow marathon.
In hot weather, plan shade and curing covers so the surface does not dry out in the first day. In cold weather, check the mix can be placed above the temperature limits on your bag or ticket. Volume maths does not change with weather; placement risk does.
Split irregular footprints into rectangles, total each volume, then add. L-shaped shed bases and paths with landings are where people under-order because they measure only the long arm. The Concrete Calculator’s multi-section total is built for that; this article’s examples stay rectangular so the arithmetic stays visible.
Frequently Asked Questions
How do I calculate how much concrete I need?
Measure length, width and thickness in the same units, multiply for volume, convert to cubic yards (÷27 from cubic feet) or keep cubic metres in metric, then add 5–10% waste and round up to the next bag or truck increment.
How much concrete for a 10×10 shed base?
At 4 inches thick: 10×10×(4/12)=33.3 cu ft ≈ 1.23 cubic yards before waste. With 10% waste, plan about 1.35 yards. In metric, a 3×3 m base at 100 mm is 0.9 m³ before waste.
Should I order bags or ready-mix?
Bags suit small pads and post holes. Once you approach half a cubic yard (about 0.4 m³) or more of continuous slab, ready-mix usually saves labour — confirm with your supplier’s minimum load.
Do I include the gravel base in the concrete volume?
No. Compacted stone under the slab is a separate gravel order. Concrete volume is only the finished concrete thickness inside the forms.
Get local quotes
Compare ready-mix yards or landscapers for larger pours.
USA: Angi / HomeAdvisor UK: CheckatradeSources
- QUIKRETE Concrete Mix No. 1101 data sheet — Published bag yields used when converting volume to bags later.
- Blue Circle Multi-Purpose Concrete tech info — UK 20 kg bag ≈ 0.01 m³ fresh concrete.
When you are ready to estimate materials or cost for your project, open the Concrete Calculator and enter your measurements.