Quikrete Calculator
Calculate Quikrete concrete bags for slabs, fence posts, and footings including standard yellow bag and Fast-Setting red bag mixes.
Volume: 36.67 cu ft (1.36 cu yds)
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What Is a Quikrete Calculator?
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.
How to Use This Calculator
- Select project type: Slab/Patio, Post Hole, or Round Footing.
- Enter dimensions (Length/Width/Thickness for slabs, or Hole Diameter/Depth for posts).
- Review the bag count across 80-lb, 60-lb, and Fast-Setting 50-lb sizes.
- Check estimated water requirement and total project volume in cubic yards.
Quikrete Post Hole & Slab 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.
Worked Example
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).
Tips & Key Notes
- For fence posts, Quikrete Fast-Setting concrete can be poured dry directly into the hole, then soaked with water—no tub mixing required.
- Always ensure post holes are dug below the local frost line (often 30 to 48 inches in northern climates) to prevent frost heave.
- Use Sonotube cardboard forms for above-ground footing pours to ensure smooth, uniform cylindrical shapes.
Frequently Asked Questions
How many bags of Quikrete do I need for a 4x4 fence post?
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.
How many 80 lb bags of Quikrete make one cubic yard?
45 bags of 80-lb Quikrete make exactly one cubic yard (27 cubic feet).
Do you need to mix Quikrete Fast-Setting concrete with water before pouring?
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.
How many square feet does an 80 lb bag of Quikrete cover?
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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