Concrete Calculator · Guide
Concrete Yards & Bags
Concrete quantity starts with geometry. Convert the measured volume into cubic yards or other units, then use a product-specific yield to estimate whole bags.
From dimensions to cubic yards
Rectangular pours use length × width × thickness/depth/height. Round pours use π × radius² × height/depth. When dimensions are entered in different units, convert them to one consistent unit before multiplying. One cubic yard equals exactly 27 cubic feet.
Net volume versus order volume
Net volume is the geometry of the measured pour. Concrete to order adds the extra allowance you choose: order volume = net volume × (1 + allowance ÷ 100). Keeping those values separate makes the assumption visible rather than hiding overage inside the geometry.
Worked slab example
A 10 ft × 10 ft slab at 4 in thick has 33.3333 ft³ of net volume. Dividing by 27 gives about 1.23457 yd³. With a 10% allowance the order volume becomes about 36.6667 ft³, or 1.35802 yd³.
How bag estimates work
Bag count is order volume ÷ product yield, rounded up to the next whole bag. For the worked example, an 80 lb reference yield of 0.60 ft³ gives 36.6667 ÷ 0.60 = 61.11…, so the estimate is 62 bags.
Reference yields used here
| Bag size | Approximate reference yield |
|---|---|
| 40 lb | 0.30 ft³ |
| 50 lb | 0.375 ft³ |
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
These reference yields follow published QUIKRETE Concrete Mix Product 1101 approximate yields. Concrete products vary. The yield on the product you actually buy should control your final bag calculation; the calculator includes a custom-yield input for that reason.
Metric volume
The calculator also reports cubic meters and liters. One liter is 0.001 m³. A metric slab entered as 3.6 m × 3 m × 10 cm is 1.08 m³ net; with a 10% allowance it becomes 1.188 m³ to order.
What the numbers do not decide
Volume math does not determine whether a slab, footing, wall, pier, or column is structurally adequate. It also does not decide reinforcement, concrete mix strength, building-code compliance, supplier minimums, delivery quantities, or pricing.