Concrete Calculator
Calculate how much concrete you need for a slab, footing, or pour.
Try an example
1.235 cubic yards
About this calculator
How Much Concrete Do You Need?
A concrete calculator estimates the volume of concrete required for a slab, footing, wall, column or other pour, plus how many bags you will need or how much a ready-mix truck must deliver. Ordering the right amount matters: too little leaves you with a cold joint and a ruined pour, and too much wastes money. Because measuring is never perfect, most contractors order a little extra — around 5%–10% more than the estimate.
The Basic Volume Formula
Volume = Length × Width × Thickness. Convert everything to the same unit before multiplying. A patio slab 12 ft long, 10 ft wide and 4 inches (0.333 ft) thick needs 12 × 10 × 0.333 ≈ 40 cubic feet, or about 1.48 cubic yards (divide cubic feet by 27). For a round column or hole, use Volume = π × r² × height. The same shapes can be metric: a 4 m × 3 m slab 10 cm thick needs 1.2 cubic meters.
Worked Example
A 10 ft × 10 ft slab poured 4 inches deep needs about 1.23 cubic yards of concrete — roughly 56 standard 80 lb bags.
Bags or Ready-Mix?
- Bagged mix comes in 60 lb and 80 lb bags. An 80 lb bag yields about 0.6 cubic feet, so a cubic yard needs roughly 45 bags — practical for small jobs like fence posts, steps and a small pad.
- Ready-mix delivery from a mixer truck is the better choice for bigger pours — usually when a project needs more than about 1 cubic yard — and it is mixed to a consistent standard.
What Is Concrete?
Concrete is a composite made of coarse aggregate such as gravel, crushed stone or slag, fine aggregate such as sand, cement as the binder and water. Portland cement is the most common type. The mix hardens through hydration, a chemical reaction rather than mere drying, and it keeps gaining strength for weeks. Proper mixing matters, and since the mix begins to set soon after water is added, it needs to be placed and finished promptly.
Tips for a Successful Pour
- Prepare a compacted, level base, and use forms that are well braced.
- Add reinforcement — rebar or wire mesh — where the design requires it.
- Keep the concrete damp for several days while it cures; curing too fast leads to cracks.
- Avoid pouring in freezing or very hot weather unless you take precautions.
Work out the area of the pour first with the Square Footage Calculator, handle round footings with the Circle Calculator, and convert between cubic feet, yards and meters using the Unit Conversion Calculator.
Worked example
A 10 ft × 10 ft slab poured 4 inches deep needs about 1.23 cubic yards of concrete — roughly 56 standard 80 lb bags.
Frequently asked questions
Should I order extra concrete beyond the calculated amount?
Yes — most contractors add 5-10% to account for spillage, an uneven sub-base, and small measurement inaccuracies, especially for irregularly shaped pours.
When should I use ready-mix instead of bagged concrete?
For anything beyond roughly 1 cubic yard, ready-mix delivery is usually more practical and cost-effective than mixing dozens of bags by hand — bagged concrete is best suited to small jobs like post footings or patch repairs.
