Calculate Quikrete concrete bags for slabs, fence posts, and footings including standard yellow bag and Fast-Setting red bag mixes.
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A Quikrete calculator determines the required bags of Quikrete brand ready-to-use concrete mixes for flatwork slabs, sonotube footings, and fence post installations. It models both standard 80-lb and 60-lb mixes as well as Fast-Setting (red bag) formulas.
V_{\text{post}} = \pi \left(r_{\text{hole}}^2 - r_{\text{post}}^2\right) h, \qquad V_{\text{slab}} = L \times W \times DFor fence posts, the volume of the embedded post is subtracted from the cylindrical auger hole volume to determine exact net concrete needed.
Scenario: Setting six 4x4 fence posts in 10-inch diameter holes dug 36 inches deep.
Hole volume: π × (5/12)² × 3 = 1.636 cu ft per hole.
Subtract 4x4 post displacement (~0.26 cu ft): net 1.38 cu ft per hole.
Total for 6 posts: 6 × 1.38 cu ft = 8.28 cu ft.
Using Fast-Setting 50-lb bags (0.375 cu ft yield): 8.28 ÷ 0.375 = 22.1 bags (23 bags).
For a standard 10-inch wide hole dug 30 to 36 inches deep, you typically need between 3.5 and 4 bags of 50-lb Fast-Setting Quikrete per post.
45 bags of 80-lb Quikrete make exactly one cubic yard (27 cubic feet).
No. Quikrete Fast-Setting (red bag) is specially formulated to be poured dry around the post, followed immediately by pouring clean water directly into the hole. It sets in 20 to 40 minutes.
An 80-lb bag yields 0.60 cubic feet. At a 4-inch depth (1/3 ft), one bag covers approximately 1.8 square feet.
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Volume: 36.67 cu ft (1.36 cu yds)