Slab Pouring Made Simple: How Many Concrete Bags You’ll Need

If you’re planning a small slab — a patio, shed floor, or even a garage base — one of the first things you’ll ask is, “How many bags of concrete do I need?”

Getting this number right means saving both money and time. Buy too few bags, and you’ll run short mid-pour. Order too many, and you’ve spent extra on materials you don’t need.

This guide walks you through the correct formula, field-proven examples, and quick reference tables to estimate bag counts for concrete slabs accurately. Whether you’re a DIY homeowner or a professional crew member, you’ll be able to size your pour confidently.

Understanding Slab Volume and Bag Yield

Every concrete slab calculation starts with volume — how much space your slab will fill once poured. You’ll express this in cubic feet or cubic yards, then match it to how much concrete each bag yields.

1 cubic yard = 27 cubic feet
1 cubic meter = 35.3147 cubic feet

The basic formula to find slab volume is:

Volume = Length × Width × Thickness

Thickness is often the wild card — 4 inches is standard for patios and floors, while 5–6 inches may be used for garages or driveways.

Once you know the volume, convert it into bag counts using manufacturer yield data. Here’s the quick chart most contractors carry in their trucks:

Bag Size	Yield (ft³)	Bags per Cubic Yard
40 lb	0.30	90
50 lb	0.375	72
60 lb	0.45	60
80 lb	0.60	45

That means if your slab totals 1 cubic yard of concrete, you’ll need roughly 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags.

Step-by-Step: From Dimensions to Bag Count

Let’s go step by step with a realistic slab example.

Step 1 — Measure the Slab

Say you’re pouring a 10 ft × 10 ft slab that’s 4 inches thick.

Convert thickness to feet:
4 in ÷ 12 = 0.333 ft

Now calculate the slab volume:
Volume = 10 × 10 × 0.333 = 33.3 ft³

Step 2 — Convert to Cubic Yards

33.3 ÷ 27 = 1.23 yd³

That means your slab will need about 1.23 cubic yards of concrete.

Step 3 — Determine Bags per Yard

Using the 80 lb bag yield (0.60 ft³ per bag or 45 bags/yd³):

Bags = 1.23 × 45 = 55.4 → round up to 56 bags

So, a 10×10×4" slab requires about 56 bags of 80 lb concrete mix.

Example Calculations for Common Slab Sizes

To make things practical, here’s a quick guide to common slab sizes and approximate bag requirements:

Slab Size (ft)	Thickness (in)	Volume (yd³)	40 lb Bags	60 lb Bags	80 lb Bags
8×8	4	0.79	71	48	36
10×10	4	1.23	111	74	56
12×12	4	1.78	160	107	80
10×12	5	1.85	166	111	83
12×16	4	2.37	213	142	106
20×20	4	4.93	444	296	222
20×20	6	7.39	666	444	333

Field Tip: Always round bag counts up to the nearest whole bag and add 5–10% extra for waste, overfill, or subgrade variation.

Using the Formula Directly

If you’d like to do your own math on site, this is the working formula that gives you bag counts directly:

Bags = (Length × Width × Thickness × 0.037037) × (Bags per Yard)

The constant 0.037037 converts cubic feet to cubic yards.

For example:
A 12 ft × 10 ft × 4 in slab using 60 lb bags (60 bags per yard):

Bags = (12 × 10 × 0.333 × 0.037037) × 60 = 53.3 → 54 bags

Choosing the Right Mix

Not all bagged concrete mixes are the same. The correct mix depends on what the slab will support.

Mix Type	Strength	Typical Use
General Purpose (3000 psi)	Patios, paths, walkways	Smooth finish, moderate load
High Strength (4000–5000 psi)	Garages, driveways	Better durability, heavier loads
Crack Resistant	Same as above + fibers	Reduces shrinkage cracking
Fast Setting	2500–3000 psi	Fence posts, small repairs

For a typical slab or patio, 3000–4000 psi general-purpose concrete is enough.
If you’re building a garage floor or driveway, choose a high-strength mix with proper rebar or wire mesh reinforcement.

Pro Tips from the Field

Subgrade prep is everything. Compact gravel or sand base well to avoid settling and cracking later.
Add control joints. For large slabs, cut 10–12 ft intervals to control shrinkage cracks.
Avoid overwatering. Adding too much water weakens concrete and increases shrinkage.
Batch evenly. Mix full bags at consistent water ratios for a uniform slab finish.
Plan for weather. Pour early morning in summer to avoid rapid drying; cover with plastic for curing.
Use mechanical mixers when using 80 lb bags — hand mixing more than 30 bags is tough work.
Reinforce where needed. For heavy-duty slabs, wire mesh or #3 rebar at 12" spacing is standard.

Professionals often use the Concrete Bags Calculator before every pour to cross-check their math and avoid overbuying or delays.

Quick Reference — Bag Count by Slab Thickness

To estimate faster on site, use this rough chart for 1 square foot of slab area:

Thickness (in)	Cubic Feet per sq. ft	80 lb Bags per 100 sq. ft
3"	0.25	42
4"	0.33	56
5"	0.42	70
6"	0.50	84

That means a 100 sq. ft patio at 4" thick will need about 56 bags of 80 lb mix, which aligns with our earlier full example.

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Visit Concrete Calculator Max for complete concrete tools, calculators, and expert advice.

Conclusion

Estimating bags for a slab doesn’t need to be guesswork. Once you know your slab’s length, width, and thickness, use the yield chart or formula to determine total bags. Round up, add 5–10% for safety, and always check the mix strength rating to match your application.

You can double-check your math anytime with the Concrete Bags Calculator — it saves time, errors, and guesswork on site.