How much concrete do I need?
Concrete is ordered by volume but sold by the bag or the truckload. Here is the arithmetic that connects your tape measure to the number on the invoice.
The three-step calculation
- Cubic feet = length (ft) × width (ft) × thickness (ft). If thickness is in inches, divide by 12 first.
- Cubic yards = cubic feet ÷ 27, because a yard is 3 × 3 × 3 ft.
- Bags = cubic feet ÷ the yield printed on the bag.
That is the whole method. The only inputs that vary are the dimensions and the bag yield, and the yield is always on the label.
Worked example: a 10 × 12 patio
| Step | Calculation | Result |
|---|---|---|
| Area | 10 ft × 12 ft | 120 sq ft |
| Thickness in feet | 4 in ÷ 12 | 0.333 ft |
| Volume | 120 × 0.333 | 40 cu ft |
| Volume in yards | 40 ÷ 27 | 1.48 cu yd |
| 80 lb bags | 40 ÷ 0.60 | 67 bags |
| With 5% spillage | 67 × 1.05 | 70 bags |
Bag yields and bags per yard
| Bag | Yield | Bags per cubic yard | Weight per yard |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 3,600 lb |
| 60 lb | 0.45 cu ft | 60 | 3,600 lb |
| 80 lb | 0.60 cu ft | 45 | 3,600 lb |
Every size adds up to about 3,600 lb per yard, because it is the same mix in different bags. Choose the bag you can carry and mix in five minutes, not the one that minimises the count.
Slab thickness by project
| Project | Thickness | Reinforcement |
|---|---|---|
| Stepping stones, garden path | 3–4 in | None |
| Patio, shed floor | 4 in | Mesh optional |
| Hot tub pad | 5–6 in | Mesh or rebar |
| Driveway, cars | 5 in | Rebar or mesh |
| Driveway, trucks or RV | 6 in | #4 rebar grid |
Thickness is the cheapest insurance against cracking. Going from 4 to 5 inches adds 25% to the material cost of a small slab and roughly doubles its load capacity.
Bags or ready-mix?
The break-even is usually somewhere between 1 and 2 cubic yards, and it depends almost entirely on delivery fees.
| Volume | Practical choice | Why |
|---|---|---|
| Under 0.5 cu yd | Bags | 45 bags or fewer — mixable by hand in an afternoon |
| 0.5–1.5 cu yd | Bags or a mixer rental | 45–68 bags; rent a mixer and two people |
| 1.5–3 cu yd | Ready-mix, watch short-load fees | Many suppliers charge extra below 3 yards |
| Over 3 cu yd | Ready-mix | Bags stop being physically practical |
Always ask for the delivered price, including the short-load fee and any Saturday surcharge, before comparing it to bags. The per-yard price on a quote is rarely the number you pay.
How much extra should I order?
Five percent for a formed slab on a flat, compacted base. Ten percent if the sub-base is uneven, the forms are informal, or you are pouring against soil. Concrete that runs short mid-pour leaves a cold joint you cannot hide — and a second delivery is far more expensive than the extra half yard.
Common mistakes
- Measuring thickness in inches and forgetting to convert. A 4 inch slab is 0.333 ft, not 4 ft. This is the single most common error and it is off by a factor of 12.
- Forgetting the base. Concrete needs 4 inches of compacted gravel underneath, which is a separate order entirely.
- Ordering bags for a driveway. A 20 × 20 driveway at 5 inches is 6.2 cubic yards — 278 bags. Rent a truck, not a cart.
- Mixing in the sun. Concrete sets faster in heat; keep bags shaded and mix in smaller batches.