Concrete Calculator
This concrete calculator works out how much concrete a slab, footing, post hole or tube form needs – in cubic yards, cubic feet and cubic metres, with a waste allowance and the number of premix bags. Enter the size in feet and inches and the result updates as you type.
Source: SAKRET Premium-Beton C30/37 – Technisches Merkblatt (Stand 02.12.2025): Ergiebigkeit ca. 0,46–0,47 l/kg. Updated: .
How it is calculated
Concrete formulas
- Slab or footing: length × width × thickness. A 10 ft × 10 ft slab, 4 in thick: 10 × 10 × (4 ÷ 12) = 33.3 ft³ = 1.23 yd³.
- Round post hole or column: π × (diameter ÷ 2)² × depth. A 10 in hole, 36 in deep: 3.1416 × 5² × 36 = 2,827 in³ = 1.64 ft³.
- Tube or ring: π × ((outside ÷ 2)² − (inside ÷ 2)²) × height.
Divide cubic feet by 27 to get cubic yards – the unit ready-mix suppliers sell in. One cubic yard is 0.7646 m³.
How many bags of concrete?
Premix bags list their yield on the label. A common US data sheet gives about 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb – roughly 0.47 litres per kg of dry mix, which is the default here.
| Volume | 60 lb bags | 80 lb bags |
|---|---|---|
| 1 ft³ | 3 | 2 |
| 10 ft³ | 23 | 17 |
| 1 yd³ (27 ft³) | 60 | 45 |
At around a cubic yard, bagged concrete means 45 or more 80-lb bags – ordering ready-mix is usually easier, and the calculator says so.
Allow for waste
Uneven subgrade, form bulges and spills eat concrete. Plan 5–10% extra, more for footings dug into soil. The extra is added to the bag count, not to the net volume.
Related tools
Turn a volume into weight with the density calculator, scale quantities with the proportion calculator, and check a surface with the spirit level.
Limits
This is a quantity estimate, not a structural design: footing depth below the frost line, reinforcement and concrete strength follow local code and the product instructions. Outside the US, switch the page language to use metres and kilograms.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab, 4 in thick, needs 33.3 ft³ or 1.23 yd³.
How many 80 lb bags of concrete make a yard?
At 0.60 ft³ per 80 lb bag, one cubic yard (27 ft³) needs 45 bags. With 60 lb bags (0.45 ft³) it takes 60.
How much concrete for a post hole?
A 10 in diameter hole, 36 in deep, holds 1.64 ft³ – four 60 lb bags or three 80 lb bags. A post in the hole takes up some of that volume, so a little is usually left over.
How many square feet does a yard of concrete cover?
At 4 in thick, one cubic yard covers 81 ft²; at 6 in it covers 54 ft². Divide 324 by the thickness in inches.
How much extra concrete should I order?
Typically 5–10% for slabs on a level base, more for footings in soil or uneven ground. Set it in the "Waste / extra" field.
Sources and legal basis
- SAKRET Premium-Beton C30/37 – Technisches Merkblatt (Stand 02.12.2025): Ergiebigkeit ca. 0,46–0,47 l/kg
- QUIKRETE Concrete Mix No. 1101 – Product Data Sheet: yield 0.30 ft³ (40 lb) … 0.60 ft³ (80 lb), unit weight ≈ 2,242 kg/m³
- NIST SP 811, Appendix B – conversion factors (foot, inch, cubic yard, pound)
As of:
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