Work out how much concrete a slab or pour needs in cubic yards, cubic feet and 40/60/80 lb bags. Enter length, width and thickness.
| Input | Result |
|---|---|
| 10 × 10 ft, 4 in thick (80 lb bags) | 1.23 yd³ · 33.33 ft³ · 56 bags |
| 20 × 30 ft, 6 in thick (80 lb bags) | 11.11 yd³ · 300 ft³ · 500 bags |
| 12 × 12 ft, 5 in thick (80 lb bags) | 2.22 yd³ · 60 ft³ · 100 bags |
Concrete volume is pure geometry, so this part is exact. The bag count is not: it depends on the yield of the mix you buy. The yields used here are the published figures for QUIKRETE® Concrete Mix No. 1101, which is tested to ASTM C387:
| Bag size | Yield (ft³) |
|---|---|
| 40 lb (18.1 kg) | 0.30 |
| 50 lb (22.6 kg) | 0.375 |
| 60 lb (27.2 kg) | 0.45 |
| 80 lb (36.2 kg) | 0.60 |
| 90 lb (40.8 kg) | 0.675 |
Other brands and other product lines differ, lightweight mixes yield more per pound, for example. Check the yield printed on your bag and, if it differs, divide your cubic-foot result by that number instead. The same data sheet lists a unit weight of about 140 lb/ft³ (ASTM C138), which is useful if you need the weight of the pour rather than its volume.
Order a margin on top of the raw number. Sub-grade is never perfectly flat, forms spread, and spillage happens; the usual trade practice is 5–10% over the calculated volume.
A 10 ft × 10 ft slab 4 in thick is 33.33 ft³, or 1.23 yd³. An 80 lb bag yields about 0.60 ft³, so that is 56 bags. For a slab that size most people order ready-mix by the yard instead.
No. Sub-grade is never perfectly flat, forms can spread and there is always some spillage. Ordering 5–10% over the calculated volume is standard practice.
From the QUIKRETE Concrete Mix No. 1101 data sheet, which is tested to ASTM C387: an 80 lb bag yields about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. Other brands differ, so check the yield printed on your bag.
As a rough guide, a pour over about 1 cubic yard is usually cheaper and more consistent as ready-mix, and it avoids the problem of batches mixed minutes apart setting differently. Short-set times also make large bagged pours hard to finish.
Multiply length by width by thickness, all in feet, then divide by 27. A 10 ft by 10 ft slab at 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet, which is 1.23 cubic yards. Order roughly ten percent over that: sub-grade is never perfectly flat, forms bow, and some material is lost in placement.
An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet, so one yard is 45 bags. A 1.23 cubic yard slab therefore takes 56 bags. Below about a cubic yard, bagged mix usually works out cheaper than ready-mix, because small deliveries carry a short-load fee that dominates the cost.
Take the area in square feet and multiply by the thickness in feet, 4 in is 0.333 ft, 6 in is 0.5 ft. That gives cubic feet; divide by 27 for cubic yards. It is the same arithmetic as any other slab, just entered from the area instead of the length and width.
Normal-weight concrete is around 140 lb per cubic foot, or about 3,780 lb per cubic yard, measured by ASTM C138. That is for a standard mix: the figure moves with the aggregate and the water content, and lightweight mixes are well below it. For load limits or structural work use the density on your supplier's mix design rather than any general figure.