How Much Concrete Do I Need? Concrete Calculator for Slabs & Footings

Concrete is calculated in cubic yards. For a 4-inch thick slab, you need approximately 1.23 cubic yards per 100 square feet. A typical 10×10 patio slab requires about 1.3 cubic yards of concrete, or approximately thirty 80-pound bags if mixing by hand.

When I poured a 12x20-foot driveway extension last spring, I calculated 3.3 cubic yards but ordered 3.75 to account for uneven ground. The total came to $565 in ready-mix concrete, and I used every bit of the extra -- the subgrade had low spots that swallowed nearly half a yard more than the formula predicted. That project taught me why rounding up matters more than any waste percentage chart.

Use our Concrete Calculator to get precise quantities for slabs, footings, columns, and more.

Understanding Concrete Measurements

Concrete is sold two ways: by the cubic yard from ready-mix suppliers, or in bags from hardware stores. Knowing which to choose and how much you need prevents costly mistakes.

Cubic Yards vs. Bags

Quick conversions:

• 1 cubic yard = 27 cubic feet
• 1 cubic yard ≈ 45 bags (80 lb each)
• 1 cubic yard ≈ 60 bags (60 lb each)

How to Calculate Concrete for a Slab

The basic formula for any rectangular slab is:

Volume = Length × Width × Thickness

Step 1: Measure in Feet

Convert all measurements to feet:

• 4 inches = 0.33 feet
• 6 inches = 0.5 feet
• 12 inches = 1 foot

Step 2: Calculate Cubic Feet

Example (10×12 foot patio, 4 inches thick):

Volume = 10 × 12 × 0.33 = 39.6 cubic feet

Step 3: Convert to Cubic Yards

Cubic Yards = Cubic Feet ÷ 27

Continuing our example:

39.6 ÷ 27 = 1.47 cubic yards

Step 4: Add Waste Factor

Always add 5-10% for waste, spillage, and slightly uneven ground:

1.47 × 1.10 = 1.62 cubic yards

Order 1.75 cubic yards (round up to nearest quarter yard for ready-mix orders).

Slab Thickness Guide

Different applications require different thicknesses:

Quick Reference: Concrete for Common Slabs

Use this table for quick estimates (includes 10% waste):

Calculating Concrete for Footings

Footings support structures and require careful calculation:

Continuous Footings (Foundation Walls)

Formula:

Volume = Width × Depth × Total Linear Feet

Example (8" wide × 12" deep footing, 60 linear feet for a shed):

Volume = 0.67 × 1.0 × 60 = 40 cubic feet
40 ÷ 27 = 1.48 cubic yards
Add 10%: 1.63 cubic yards → Order 1.75 cubic yards

Square Footings (Deck Posts)

Formula:

Volume = Length × Width × Depth × Number of Footings

Example (four 16"×16"×12" footings for a deck):

Each footing: 1.33 × 1.33 × 1.0 = 1.77 cubic feet
Total: 1.77 × 4 = 7.08 cubic feet
7.08 ÷ 27 = 0.26 cubic yards

For small quantities like this, use bags:

• 0.26 cubic yards ≈ 12 bags (80 lb)

Calculating Concrete for Round Columns and Posts

For round forms like sonotubes:

Formula:

Volume = π × radius² × depth

Example (12" diameter × 48" deep post hole):

Radius = 6 inches = 0.5 feet
Volume = 3.14159 × 0.5² × 4 = 3.14 cubic feet per post

Common Sonotube Sizes

Calculating Concrete for Walls

For poured concrete walls:

Formula:

Volume = Length × Height × Thickness

Example (retaining wall 20' long × 4' tall × 8" thick):

Volume = 20 × 4 × 0.67 = 53.6 cubic feet
53.6 ÷ 27 = 1.98 cubic yards
Add 10%: 2.18 cubic yards → Order 2.25 cubic yards

Concrete Coverage by Bag Size

When using bags instead of ready-mix:

Bags Needed Calculator

Formula:

Bags = (Cubic Feet ÷ Bag Coverage) × 1.10

Example (40 cubic feet with 80-lb bags):

Bags = (40 ÷ 0.60) × 1.10 = 73.3 → Order 74 bags

Ready-Mix Concrete Ordering Tips

What to Tell the Dispatcher

1. Total cubic yards needed (rounded up)
2. Project type (slab, footing, wall)
3. Desired slump (typically 4-5" for residential)
4. Any additives (fiber reinforcement, accelerator, retarder)
5. Access considerations (truck reach, slopes, obstacles)

Standard Concrete Mixes

Timing Your Pour

• Order early — Schedule at least 2-3 days in advance
• Weather check — Avoid pouring in rain, extreme heat, or freezing temperatures
• Have help ready — You'll have 60-90 minutes of working time
• Prep forms completely — The clock starts when the truck arrives

Common Concrete Calculation Mistakes

Mistake 1: Forgetting the Waste Factor

Problem: Ordering exactly the calculated amount leaves you short due to uneven ground, spillage, and form bulging.

Solution: Always add 5-10% to your calculation. For rough terrain, add 15%.

Mistake 2: Measuring in Wrong Units

Problem: Mixing inches and feet in calculations leads to wildly wrong results.

Solution: Convert everything to feet first. Our Concrete Calculator handles unit conversions automatically. This same unit-confusion issue comes up frequently in other home improvement projects -- see our tile calculator guide for how to avoid measurement pitfalls when tiling.

Mistake 3: Underestimating Ground Conditions

Problem: Uneven or rocky soil can significantly increase concrete needs.

Solution: Dig to consistent depth and compact the base thoroughly. Add extra for low spots.

Mistake 4: Ordering Too Little Ready-Mix

Problem: Running short mid-pour creates a cold joint (weak point) when you wait for more concrete.

Solution: Order 10-15% more than calculated. Unused concrete can create stepping stones or other projects.

How to Use Our Concrete Calculator

Our Concrete Calculator simplifies the entire process:

1. Select project type — slab, footing, column, or wall
2. Enter dimensions — length, width, thickness/depth
3. Choose units — feet, inches, or meters
4. Get results — cubic yards and bag counts instantly

The calculator automatically adds a 10% waste factor and provides both ready-mix quantities and bag equivalents.

Cost Comparison: Bags vs. Ready-Mix

Once your slab or foundation is complete, you may be planning the next phase of the project. Our flooring calculator guide walks through material estimation for hardwood, laminate, or LVP that you might install on top of a finished concrete subfloor.

Reinforcement Requirements

Most concrete slabs benefit from reinforcement:

Wire Mesh

• Coverage: Typically sold in 5×10' sheets
• Placement: Position at 1/3 from top of slab
• Overlap: Minimum 6" overlap between sheets

Rebar

• Slab on grade: #3 or #4 rebar on 18-24" centers
• Footings: #4 or #5 rebar, continuous with proper lap splices
• Walls: Horizontal and vertical rebar per engineer specifications

Fiber Reinforcement

• Coverage: One bag per cubic yard of concrete
• Benefit: Reduces cracking, improves durability
• Not a substitute for: Wire mesh or rebar in structural applications

Frequently Asked Questions

How many bags of concrete do I need for a 10×10 slab?

For a 10×10 foot slab at 4 inches thick, you need approximately 1.4 cubic yards or about 63 bags of 80-pound concrete mix. Including 10% waste, order 70 bags or 1.5 cubic yards of ready-mix.

How do you calculate concrete in cubic yards?

Calculate cubic feet first (Length × Width × Thickness in feet), then divide by 27 to convert to cubic yards. For example: 12×12×0.33 = 47.52 cubic feet ÷ 27 = 1.76 cubic yards.

How thick should a concrete driveway be?

A residential driveway for passenger vehicles should be 4 inches thick minimum. For heavy vehicles like trucks or RVs, increase to 6 inches. Always use 3500 PSI concrete or higher for driveways.

How much does a cubic yard of concrete cover?

One cubic yard of concrete covers: 81 square feet at 4 inches thick, 54 square feet at 6 inches thick, or 27 square feet at 12 inches thick.

Should I order extra concrete?

Yes, always order 5-10% more than your calculation. For uneven terrain or first-time pours, add 15%. Leftover concrete can be used for stepping stones or disposed of in forms you've set aside.

How long does concrete need to cure?

Concrete reaches working strength in 24-48 hours, but full cure takes 28 days. Keep the surface moist for the first 7 days by misting with water or covering with plastic sheeting.

Related Calculators

• Bricks Calculator — Calculate bricks and mortar for walls
• Flooring Calculator — Estimate flooring materials
• Paint Calculator — Calculate paint for any room

Conclusion

Calculating concrete accurately prevents costly overages and frustrating shortages. Measure carefully, add your waste factor, and round up to the nearest quarter yard for ready-mix orders. For small projects under a cubic yard, bags are practical. For anything larger, ready-mix saves time, labor, and often money.

Use our Concrete Calculator for instant, accurate estimates. And remember: it's always better to have a little concrete left over than to stop mid-pour.

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Concrete calculations are estimates based on standard conditions. Actual requirements may vary based on ground conditions, form construction, and pour technique. Consult with your concrete supplier for project-specific recommendations.