Should I use ready-mix or bagged concrete?

For small projects under 1 cubic yard, bagged concrete is convenient and cost-effective. For larger pours, ready-mix is almost always cheaper per yard and produces more consistent results. The crossover point is typically around 1–1.5 cubic yards.

How many bags of concrete per cubic yard?

One cubic yard equals 27 cubic feet. You need about 60 bags of 60 lb mix or 45 bags of 80 lb mix per cubic yard, before adding a waste factor.

Concrete Slab Calculator — Bags, Cost & Ready-Mix Estimate

Enter Slab Dimensions

Quick Presets

Length

Width

Thickness

Unit System

Waste Factor

Currency

Price per 60 lb bag (your local price)

Price per 80 lb bag (your local price)

Ready-mix cost per cubic yard (your local price)

Prices vary by location. Enter your local supplier rates.

Your Concrete Estimate

You Need

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cubic yards of concrete

Total Volume

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cubic feet

60 lb Bags

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bags (inc. waste)

80 lb Bags

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bags (inc. waste)

Cost Breakdown

Prices shown in your local currency ($)

60 lb Bags

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80 lb Bags

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Ready-Mix

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Note your quantities before purchasing — prices and availability vary by supplier.

Bags vs. Ready-Mix

Factor	Bags	Ready-Mix
Estimated Cost	—	—
Practicality	—	—
Best suited for	Pours under 1 cu yd	Pours over 1 cu yd
Verdict	—	—

Example Calculation

Not sure how the math works? Here's a step-by-step walkthrough for a standard 10 × 10 ft patio at 4 inches thick — the most common residential slab size.

10 ft × 10 ft slab, 4 in thick — step by step

Step 1 — Volume 10 × 10 × (4 ÷ 12) ft = 33.33 cu ft
Step 2 — Cubic Yards 33.33 ÷ 27 = 1.23 cu yd
Step 3 — Add 10% Waste 33.33 × 1.10 = 36.67 cu ft
Step 4 — 60 lb Bags 36.67 ÷ 0.45 = 82 bags
Step 5 — 80 lb Bags 36.67 ÷ 0.60 = 62 bags
Recommendation At 1.23 cu yd, this sits right at the crossover — ready-mix saves time, bags work for a patient DIYer.

How This Calculator Works

The calculator uses a straightforward formula: Volume = Length × Width × Thickness. It converts that volume into cubic yards (for ready-mix orders) and into bag counts for both 60 lb and 80 lb bags.

Each 60 lb bag of concrete mix yields approximately 0.45 cubic feet. Each 80 lb bag yields about 0.60 cubic feet. The calculator applies your chosen waste factor and multiplies by your local prices to give you a total cost estimate for each option.

Pro tip: For slabs larger than about 1 cubic yard (~10×10 at 4" deep), get ready-mix delivered instead of buying bags. Bags are more expensive per yard and physically exhausting to mix in large quantities.

Practical Tips for a Concrete Slab

Standard patio/walkway: Use 4 inches thick minimum. Going thinner risks cracking under normal foot traffic.
Driveway: Use 5–6 inches for passenger vehicles. Add steel rebar or wire mesh for extra strength.
Shed floor: 4 inches is usually fine. 6 inches if storing heavy equipment.
Sub-base matters: Compact 4–6 inches of gravel under the slab before pouring. This prevents settling and cracking.
Water-to-cement ratio: Don't add extra water to make mixing easier — it weakens the concrete significantly.
Curing: Keep the slab damp for at least 7 days after pouring. This dramatically increases final strength.
Always round up: You cannot pour half a slab and come back later — you'll get a visible cold joint.

Common Mistakes to Avoid

Forgetting depth. People calculate length × width but forget the thickness. A 10×10 slab looks small until you realize 4 inches deep means over 1 cubic yard.

Unit confusion. Mixing feet and inches without converting. Always convert thickness to feet before calculating (4 inches = 0.333 ft). This calculator handles it automatically.

Skipping the waste factor. Uneven ground, spillage, and slight over-excavation always consume extra material. 10% is the industry standard minimum.

Buying bags for a large pour. Over 1 cubic yard, ready-mix is cheaper per yard and produces stronger concrete (better controlled mix ratio, less physical fatigue).

Assumptions

◦ Concrete yield: 60 lb bags = 0.45 cu ft; 80 lb bags = 0.60 cu ft. These are standard manufacturer figures used across the industry.
◦ Waste factor: The default 10% accounts for spillage, uneven subgrade, and slight over-excavation. Increase to 15% for rough terrain or irregular shapes.
◦ Prices vary by region: Ready-mix costs vary by region and supplier. Bag prices vary by brand. Update the inputs to reflect your local prices for accurate estimates.
◦ Ready-mix delivery fees: Most suppliers charge a delivery fee and have a minimum order of 1 cubic yard. Factor these into your cost before comparing options.
◦ Results are estimates only: This tool is a planning aid. Always confirm quantities with your concrete supplier or contractor before ordering materials.

Frequently Asked Questions

A 10×10 ft slab at 4 inches thick requires about 33.33 cubic feet (1.23 cubic yards) of concrete. With 10% waste, you need roughly 82 bags of 60 lb mix or 62 bags of 80 lb mix. Use the calculator above to adjust for different thickness or waste values.

A 12×12 ft slab at 4 inches thick requires about 48.00 cubic feet (1.78 cubic yards). With 10% waste, that's roughly 118 bags of 60 lb mix or 88 bags of 80 lb mix. At this size, ordering ready-mix is worth considering — it'll be cheaper and easier than mixing that many bags by hand.

For residential patios and walkways, 4 inches is the standard minimum. For driveways with regular vehicle traffic, use 5–6 inches. For areas with heavy trucks or commercial equipment, 6–8 inches is recommended. Thicker slabs require more concrete but crack significantly less over time.

For small projects under 1 cubic yard, bagged concrete is convenient and cost-effective — no delivery minimums, no scheduling. For larger pours, ready-mix delivered by truck is almost always cheaper per yard, more consistent, and far less physically demanding. The crossover point is typically around 1–1.5 cubic yards. Use this calculator's recommendation section to see which makes sense for your specific pour.

One cubic yard equals 27 cubic feet. At standard yields (60 lb bag = 0.45 cu ft, 80 lb bag = 0.60 cu ft), you need about 60 bags of 60 lb mix and 45 bags of 80 lb mix per cubic yard — before adding any waste factor.

For basic patios and walkways, rebar is optional but recommended. Use #3 rebar on a 12-inch grid for patios and #4 rebar for driveways. Wire mesh (6×6 welded wire) is a cheaper alternative that still adds meaningful crack resistance. For structural slabs — like floors inside a building — rebar is required by code.

Concrete reaches initial set in 24–48 hours (safe to walk on lightly). You can drive on a driveway slab after about 7 days. Full structural strength takes 28 days. Keep the slab moist for at least 7 days after pouring — this curing step dramatically increases long-term strength.

Concrete Slab Calculator