How Many Bags of Concrete Do I Need? The Definitive Quikrete Guide

Whether you're setting fence posts, pouring a patio slab, or building a shed foundation, the first question is always: "How many bags of concrete do I need?" Buy too few and you'll make an emergency hardware store run with half-set concrete. Buy too many and you've got 80-pound paperweights. This guide gives you the exact formulas and real-world examples.

The Basic Formula

Quikrete Bag Sizes: Which One Should You Buy?

Real-World Examples

Fence Post Holes (Most Common DIY Project)

A standard 4×4 fence post needs a hole about 10 inches in diameter and 24 inches deep:

• Volume per hole: π × (5/12)² × 2 = 0.91 cu ft
• Bags per post (80lb): 0.91 ÷ 0.60 = 1.5 → 2 bags
• 10-post fence: 20 bags of 80lb Quikrete (~$130)

For 6×6 posts (gates, corners), use a 12-inch hole, 36 inches deep: ~2.36 cu ft = 4 bags per post.

Patio Slab (10×10 ft, 4 inches thick)

• Volume: 10 × 10 × (4/12) = 33.3 cu ft
• Add 10% waste: 36.7 cu ft
• 80lb bags: 36.7 ÷ 0.60 = 62 bags
• Total weight: 4,960 lbs (2.5 tons!)
• Cost: ~$400 in bags

Sidewalk (3 ft wide × 20 ft long × 4 inches)

• Volume: 3 × 20 × (4/12) = 20 cu ft
• 80lb bags: 22 ÷ 0.60 = 37 bags (with 10% waste)
• This is borderline for ready-mix — get quotes from both options

Essential Tips for Working with Bagged Concrete

1. Always add water to the mixer first — Then add concrete. Never the reverse. It prevents clumping.
2. Use the right water ratio — Too much water weakens concrete. Follow bag instructions: ~3 quarts per 80lb bag.
3. Work in batches — Concrete starts setting in 20–40 minutes. Only mix what you can place in that window.
4. Prep the base — 4–6 inches of compacted gravel under slabs. Gravel in the bottom of post holes for drainage.
5. Buy 10% extra — Some concrete sticks to the mixer, holes are rarely perfectly round, and the ground absorbs some. Always order 10% more than calculated.
6. Check the weather — Don't pour if it's going to rain within 24 hours or if temperatures will drop below 40°F.

Concrete Types: Which One Do You Need?

• Standard Mix (Quikrete Original) — 4,000 PSI. Good for most projects: slabs, footings, post holes.
• Fast-Setting (Quikrete Fast-Set) — Sets in 20–40 minutes. Perfect for fence posts (you don't even need to mix — just pour dry and add water).
• High-Strength (5000 PSI) — For structural applications: footings, load-bearing columns, garage slabs.
• Crack-Resistant — Fiber-reinforced. Good for thin slabs and applications where cracking is a concern.

How Thick Should My Concrete Be?

Conclusion

Calculating concrete isn't hard once you know the formula: length × width × depth (in feet) gives you cubic feet, then divide by the bag yield. Remember the 80lb bags are the best value, always add 10% for waste, and consider ready-mix delivery for anything over ~40 bags. Use our free calculator to get exact counts for any project shape — slabs, post holes, columns, footings, and steps.