How Many Bags of Concrete Do I Need?

By Emiliano · Last updated 2026-11-13

The quick answer: a 10×10 ft slab at 4 inches thick needs 75 bags of 60 lb concrete or 56 bags of 80 lb concrete. That's about 1.23 cubic yards. But bag counts depend heavily on slab size, thickness, and bag weight — so here's the full reference table.

Quick Reference Table: Slab Size × Thickness → Bags

Most common slab sizes at 4, 5, and 6 inch thicknesses:

Table assumes standard 4:1 sand-cement ratio with no waste. Add 5–10% for spillage and uneven subgrade.

Quick Reference Table: Post Holes → Bags

For fence posts, deck posts, and mailbox posts — the standard "1 bag per post" rule depends on hole size:

How to Calculate Bags for Any Slab

The math is straightforward. Follow these steps in order:

1. Find slab volume in cubic feet: Length (ft) × Width (ft) × Thickness (in ÷ 12)
2. Convert to cubic yards: Divide cubic feet by 27
3. Add 5–10% for waste: Multiply by 1.05 or 1.10
4. Convert to bags: Multiply cubic yards by 60 (for 60 lb bags) or by 45 (for 80 lb bags)

Worked Example: 12×14 ft Patio at 4 Inches

Length = 12 ft, width = 14 ft, thickness = 4 in (0.333 ft).

• Cubic feet: 12 × 14 × 0.333 = 56 cu ft
• Cubic yards: 56 ÷ 27 = 2.07 cu yd
• With 10% waste: 2.07 × 1.10 = 2.28 cu yd
• 60 lb bags: 2.28 × 60 = 137 bags
• 80 lb bags: 2.28 × 45 = 103 bags

60 lb vs 80 lb Bags — Which Is Better?

Both are sold at most home centers. The choice matters for two reasons: hand-mixing effort and cost per cubic yard.

60 lb bags are easier to carry and mix in a wheelbarrow or 5-gallon bucket. You'll need more bags (60 per cubic yard vs 45), but the physical effort per bag is 25% lower. Best for small projects under 1 cubic yard, or when mixing by hand.

80 lb bags are more efficient — you'll use 25% fewer bags for the same volume. Best for larger pours where you're using a drum mixer or have two people working. Total cost is usually 5–15% lower per cubic yard, but the bags are heavier to handle.

When Bags Beat Ready-Mix

Bagged concrete is the right choice when:

• Your total volume is under 1 cubic yard (about 60–80 bags)
• Site access is limited — no truck can reach the pour area
• You're doing a one-off small project like a mailbox post or a small pad
• You don't have a helper for the pour

Ready-mix becomes cheaper once you cross about 1.5 cubic yards. A typical short-load delivery charges $150–$250 in fees, which is only worth it if you're avoiding 60+ bag handling. For anything over 2 cubic yards, ready-mix is usually 20–40% cheaper by volume.

Frequently Asked Questions

How many 80 lb bags of concrete for a 10x10 slab?

A 10×10 ft slab at 4 inches thick requires 56 bags of 80 lb concrete. At 6 inches thick, it needs 84 bags. Add 5–10% for waste depending on slab complexity.

How many bags of concrete in a cubic yard?

One cubic yard of concrete requires 60 bags of 60 lb or 45 bags of 80 lb. This assumes standard Portland cement mix at approximately 4,050 lbs per cubic yard.

Is it cheaper to mix my own concrete or buy ready-mix?

For volumes under 1 cubic yard, bagged concrete is usually cheaper. Above 1.5 cubic yards, ready-mix wins on cost per cubic yard, even after short-load fees. The crossover point depends on local pricing — get quotes both ways for any pour over 1 cubic yard.

How many bags of concrete for a fence post?

Most fence post holes (8–10 inch diameter, 24–30 inches deep) require 2–3 bags of 60 lb or 1–2 bags of 80 lb. Set posts at least 1/3 of their total length below grade and use gravel at the bottom for drainage.

Related Calculators

• Concrete Slab Cost Calculator — volume and dollar cost for any slab
• Concrete Mix Ratio Calculator — cement, sand, and gravel ratios by PSI target
• Concrete Footing Calculator — volume for continuous footings
• Concrete Steps Calculator — bag counts for straight-run step sets
• Concrete Stairs Calculator — volume and bag counts for full staircases
• Fence Calculator — posts, rails, pickets, and concrete per post