Quick answer: Cubic yards = length × width × thickness (all in feet) ÷ 27. A 10 × 10 ft slab 4 in thick is 33.3 cu ft, or 1.23 cubic yards. Add 10% overage, and above about one yard order ready-mix rather than bags.
How concrete volume is calculated
cubic yards = (length × width × thickness) ÷ 27All in feet. 27 cu ft in a cubic yard (3 × 3 × 3).Divide by 27, not 3, because volume scales in three directions. And convert thickness from inches to feet first: 4 in is 4 ÷ 12 = 0.333 ft.
A 10 × 10 ft patio slab, 4 in thick
- Thickness: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.3 cu ft
- Yards: 33.3 ÷ 27 = 1.23
- With 10% overage: 1.36 cubic yards
Result: order 1.5 cubic yards
Footings and columns
- Strip footing: run × width × depth ÷ 27. A 40 ft run 16 in wide and 8 in deep is 35.6 cu ft, or 1.32 yd³.
- Round column: π × radius² × height ÷ 27. A 10 in tube has a 5 in (0.417 ft) radius.
- Repeated pours: work out one and multiply. Fifty 10 in × 4 ft tubes is about 4 yd³.
Bags or ready-mix?
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | about 0.60 cu ft | 45 |
| 60 lb | about 0.45 cu ft | 60 |
| 40 lb | about 0.30 cu ft | 90 |
- Under about 0.5 yd³: bags are fine, around 23 eighty-pound bags.
- 0.5 to 1 yd³: a judgement call depending on helpers and a mixer.
- Over about 1 yd³: order ready-mix. Even with a short-load fee it is usually cheaper than a day of hand-mixing, and one uniform batch looks and performs better.
Why you always order extra
Order about 10% more than you calculate. Subgrade is never level, forms bow under roughly 4,000 lb per yard of wet concrete, and some is always lost to spillage.
Cold joints: If you run short, concrete already placed starts to set before the next batch arrives. The seam is a cold joint, a permanent weak plane that often cracks later. Extra concrete is cheap; a cold joint through a driveway is not.
How thick should the slab be?
| Application | Typical thickness | Notes |
|---|---|---|
| Patio, walkway, shed base | 4 in | Foot traffic and light loads |
| Driveway, car parking | 5 – 6 in | Over a compacted base |
| Heavy vehicles, RV pad | 6 in + | Reinforcement usually required |
| Garage floor | 4 – 6 in | Depends on vehicle weight |
Going from 4 to 6 in adds 50% more concrete. The base matters as much as thickness: 4 in over compacted gravel outperforms 6 in on soft ground. Local codes take precedence, and structural work needs an engineer.
Common mistakes when estimating concrete
- Forgetting to divide by 27.
- Leaving thickness in inches. Divide by 12 first.
- Using diameter instead of radius. That gives four times too much.
- Ordering the exact amount. Always add overage.
- Underestimating bags. An 80 lb bag makes only about 0.6 cu ft.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in feet, then divide by 27 for cubic yards. A 10 × 10 ft slab at 4 in is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cubic yards.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb mix, 60 of 60 lb or 90 of 40 lb. An 80 lb bag yields roughly 0.6 cu ft; check the yield printed on your bag.
Should I use bags or order ready-mix?
Bags make sense below about half a cubic yard. Above roughly one cubic yard, ready-mix wins: a yard is about 45 eighty-pound bags, near 1.8 tons to mix by hand.
How much extra concrete should I order?
About 10%. Uneven subgrade, bowing forms and spillage all use more than the formula says, and running short creates a cold joint.
How thick should a concrete slab be?
4 in for patios, walkways and shed bases, 5 to 6 in for driveways and 6 in or more for heavy vehicles. Local codes take precedence.
How much does a cubic yard of concrete weigh?
Roughly 4,000 lb, about two tons.
Related tools
Accuracy note: This is for estimating only and is not engineering advice. Bag yields vary by brand, so check the printed yield. Confirm thickness, reinforcement and footing sizes against local building codes.
Content Lead, Anchor AI Tools, California, USA. Spotted an error? Email hello@anchoraitools.com and it will be checked and corrected. See our editorial standards.