How many 80-lb bags of concrete for a 12x12 slab?

A 12 × 12 ft slab at 4 inches thick needs 80 eighty-pound bags (48 cu ft ÷ 0.6 cu ft per bag). Add 5–10% extra for waste, which brings it to roughly 84–88 bags.

How many 60-lb bags of concrete for a 12x12 slab?

About 107 sixty-pound bags for a 12 × 12 ft slab at 4 inches thick (48 cu ft ÷ 0.45 cu ft per bag), before a 5–10% waste allowance.

How many cubic yards is a 12x12 slab at 4 inches?

A 12 × 12 ft slab at 4 inches thick is 48 cubic feet, which equals about 1.78 cubic yards (48 ÷ 27).

Should I use bags or ready-mix for a 12x12 slab?

At 4 inches thick (1.78 cubic yards), bagged concrete is still practical. At 6 inches thick the slab reaches 2.67 cubic yards, where ready-mix delivery is usually cheaper and far less labor.

How much does an 80-lb bag of concrete yield?

A standard 80-lb bag yields about 0.6 cubic feet of mixed concrete, and a 60-lb bag yields about 0.45 cubic feet. Yields vary by product, so confirm on the bag before calculating.

Guide

How Many Bags of Concrete Do I Need for a 12x12 Slab?

A 12 × 12 foot slab is a popular size for patios and shed floors — a bit larger than the classic 10 × 10, but still very much a bagged-concrete project. At the standard 4-inch thickness it takes 48 cubic feet of concrete, which works out to 80 eighty-pound bags or 107 sixty-pound bags. This guide shows the exact math, a thickness chart, and when bags stop making sense versus ready-mix.

Key Takeaways

A 12 × 12 ft slab at 4 inches thick needs 48 cubic feet (1.78 cubic yards) of concrete.
That equals 80 eighty-pound bags or 107 sixty-pound bags — before waste.
An 80-lb bag yields about 0.6 cu ft; a 60-lb bag yields about 0.45 cu ft.
Add 5–10% extra for spillage, uneven subgrade, and broken bags.
For pours over roughly 2 cubic yards, ready-mix delivery is usually cheaper than bags.

Calculate Your Concrete

How to Calculate Bags for a 12x12 Slab

Bag count comes from the slab volume divided by the yield of one bag. Work in cubic feet throughout, because bag yields are stated in cubic feet.

1Measure the slab: 12 ft long × 12 ft wide, and decide the thickness (4 inches is standard for patios).
2Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
3Calculate cubic feet: 12 × 12 × 0.333 = 48 cu ft.
4Convert to cubic yards: 48 ÷ 27 ≈ 1.78 cu yd (useful when comparing against ready-mix).
5Divide cubic feet by the yield per bag: 48 ÷ 0.6 = 80 eighty-pound bags.

The Bag Count Formula

Bagged concrete is sold by weight, but what matters is yield — the volume of mixed concrete each bag produces. A standard 80-lb bag yields about 0.6 cubic feet and a 60-lb bag yields about 0.45 cubic feet. Always check the yield printed on the specific product, since it varies by manufacturer.

Bags needed = Volume (cu ft) ÷ Yield per bag (cu ft)

Worked Example: 12 ft × 12 ft Slab at 4 Inches

Here is the full calculation for the classic 12 × 12 patio slab at 4 inches thick, plus the numbers for a thicker 6-inch pour.

Step 1

Slab dimensions: 12 ft × 12 ft, 4 inches thick.

Step 2

Thickness in feet: 4 ÷ 12 = 0.333 ft.

Step 3

Volume: 12 × 12 × 0.333 = 48 cubic feet.

Step 4

Cubic yards: 48 ÷ 27 ≈ 1.78 cubic yards.

Step 5

Eighty-pound bags: 48 ÷ 0.6 = 80 bags exactly.

Step 6

Sixty-pound bags: 48 ÷ 0.45 = 106.7 → round up to 107 bags.

Slab thickness	Cubic yards	80-lb bags	60-lb bags
4 in	1.78	80	107
6 in	2.67	120	160

Common Mistakes

Using 80-lb bag math for 60-lb bags (or vice versa) — the yields differ by about 33%.
Forgetting the waste allowance — order 5–10% extra for spillage and uneven ground.
Assuming every bag yields the same volume — check the yield printed on the bag.
Ordering the exact calculated count with no buffer for broken or short bags.
Skipping the ready-mix comparison — at 6 inches thick this slab reaches 2.67 cubic yards, where delivery can be cheaper than bags.

Practical Tips

Round up, never down — coming up half a bag short mid-pour is worse than having one bag left over.
Stage the bags near the pour site before you start mixing to keep the work flowing.
At 6 inches thick this slab needs 2.67 cubic yards — price ready-mix delivery before buying 120 bags.
Add control joints to a 12 × 12 slab to manage cracking as the concrete cures.
Use the NOVORY Concrete Calculator to double-check your bag count before you buy.