Comment Calculer le Béton pour n'Importe Quel Projet : Dalles, Fondations et Poteaux

Concrete Math: The Core Formula

Concrete is measured and sold by the cubic yard. One cubic yard = 27 cubic feet. The formula for any rectangular pour:

Volume (cubic feet) = Length (ft) x Width (ft) x Thickness (ft)

Cubic yards = Volume (ft³) ÷ 27

The most common mistake is forgetting to convert thickness from inches to feet before multiplying. A 4-inch slab is 0.333 feet thick, not 4 feet.

Use our concrete calculator to run these numbers automatically for any project shape.

Thickness Conversion Reference

Thickness	In Feet
3 inches	0.25 ft
3.5 inches	0.292 ft
4 inches	0.333 ft
5 inches	0.417 ft
6 inches	0.5 ft
8 inches	0.667 ft

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Slab Calculations

Slabs are the most common DIY concrete project -- patios, driveways, garage floors, sidewalks. Residential slabs typically use 4 inches for patios and sidewalks, 5-6 inches for driveways, and 4 inches for garage floors (6 inches if parking heavy vehicles).

Formula: Cubic yards = (Length x Width x Thickness in ft) ÷ 27

Common Slab Sizes at 4-Inch Thickness

Slab Size	Square Feet	Cubic Yards	80 lb Bags Needed
8 x 8 ft	64 sq ft	0.79 cu yd	18 bags
10 x 10 ft	100 sq ft	1.23 cu yd	28 bags
10 x 20 ft	200 sq ft	2.47 cu yd	56 bags
12 x 12 ft	144 sq ft	1.78 cu yd	40 bags
20 x 20 ft	400 sq ft	4.94 cu yd	111 bags
24 x 24 ft	576 sq ft	7.11 cu yd	160 bags
30 x 30 ft	900 sq ft	11.11 cu yd	250 bags

Bag counts above do not include waste factor. Add 10% to all bag counts for ordering.

Driveway at 5 Inches (2-Car, 24 x 20 ft)

24 x 20 x 0.417 = 200 ft³ ÷ 27 = 7.41 cubic yards + 10% = 8.15 cubic yards

For a project this size, ready-mix concrete (truck delivery) is standard. Bagging 8 yards would require 182 bags of 80 lb concrete and enormous labor.

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Footing Calculations

Footings support structural loads -- deck footings, wall footings, foundation footings. They're typically rectangular or cylindrical, and almost always deeper than they are wide.

Rectangular footing formula: Cubic yards = (Length x Width x Depth) ÷ 27

Cylindrical footing formula: Cubic yards = π x radius² x depth ÷ 27

Common Footing Dimensions and Concrete Needed

Footing Size	Depth	Cubic Feet	Cubic Yards
12 x 12 in	12 in	1.0 ft³	0.037 cu yd
12 x 12 in	24 in	2.0 ft³	0.074 cu yd
16 x 16 in	12 in	1.78 ft³	0.066 cu yd
16 x 16 in	24 in	3.56 ft³	0.132 cu yd
24 x 24 in	12 in	4.0 ft³	0.148 cu yd
24 x 24 in	24 in	8.0 ft³	0.296 cu yd

Deck with 6 footings (12 x 12 in at 36 in deep): Each footing: 1 x 1 x 3 = 3 ft³ 6 footings: 18 ft³ ÷ 27 = 0.67 cubic yards + 10% = 0.74 cubic yards At 0.74 yards, this project calls for bags -- roughly 17 bags of 80 lb mix.

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Post Hole Calculations

Post holes are cylindrical. You're filling the annular space around the post, not the entire cylinder. Subtract the post volume from the hole volume.

Hole volume: π x (hole radius)² x depth ÷ 27 Post volume: post width² x depth ÷ 27 (for square posts) Concrete needed: Hole volume -- Post volume

Standard Post Holes with QUIKRETE 80 lb Bags

Hole Diameter	Post Size	Depth	Concrete Volume	Bags Needed
8 in	4x4	2 ft	0.049 cu yd	~1 bag
10 in	4x4	2 ft	0.074 cu yd	~2 bags
10 in	4x4	3 ft	0.111 cu yd	~3 bags
12 in	6x6	3 ft	0.145 cu yd	~4 bags
12 in	6x6	4 ft	0.193 cu yd	~5 bags

Each 80 lb bag of fast-setting concrete covers approximately 0.022 cubic yards. Each 50 lb bag covers approximately 0.015 cubic yards.

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Waste Factor: How Much Extra to Order

Always order more than your calculated volume. Concrete waste comes from:

• Spillage during pour and finishing
• Slight form irregularities
• Subgrade soft spots requiring more fill
• Mixing inefficiency when using bags

10% waste: Standard slab, clean forms, experienced crew 15% waste: Irregular shapes, curved forms, first-time DIY pour, difficult access

For a 4-cubic-yard slab: add 10% = order 4.4 cubic yards. Ready-mix trucks typically deliver in 0.25-yard increments, so you'd order 4.5 yards.

Running short mid-pour is a serious problem. Cold joints (where fresh concrete meets a partially set previous pour) are structural weak points. Always have more than you need.

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Bags vs. Truck Delivery: The Breakeven

Bag Concrete Costs (2026)

Bag Size	Price (Home Depot)	Yield	Cost per Cubic Yard
40 lb	~$6.50	0.011 cu yd	~$591
60 lb	~$7.00	0.017 cu yd	~$412
80 lb	~$9.50	0.022 cu yd	~$432

Ready-Mix Concrete Costs (2026)

• Short-load fee (under 5 yards): $50-$150 per load
• Per cubic yard: $150-$175 depending on mix design and region
• Delivery charge: $75-$150 depending on distance

Breakeven math: At $432/yard for bagged vs. $160/yard for ready-mix + $100 short-load fee:

• 0.5 yards: Bags = $216, Ready-mix = $180. Ready-mix wins even at small volumes.
• At 1+ cubic yard, ready-mix is almost always cheaper and far less labor.

The real cost of bags is labor. Mixing 45 bags of 80 lb concrete by hand for a 1-yard slab takes 3-4 hours of exhausting work. A truck delivers, and you pour.

Use bags when: Project is under 0.5 yards, site access is impossible for a truck, or you're pouring in stages over multiple days (post holes, small footings).

Use ready-mix when: Project is 1 cubic yard or more, you want a consistent mix, or you're on a deadline.

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Quick Reference: Common Projects

Project	Typical Size	Thickness	Cubic Yards	Bags (80 lb)	Use Truck?
Mailbox footing	1x1 ft	12 in	0.04	2	No
Fence post (4 holes)	10 in dia	2 ft	0.30	14	No
Small patio	10x10 ft	4 in	1.23	56 + waste	Yes
Deck footings (6)	12x12 in	36 in	0.74	34 + waste	No
Driveway approach	10x12 ft	5 in	2.31	--	Yes
2-car driveway	24x20 ft	5 in	7.41	--	Yes
Garage floor	24x24 ft	4 in	7.11	--	Yes

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Concrete Mix Design Basics

Not all concrete is the same. Standard ready-mix comes in different PSI ratings:

• 2,500 PSI: Light residential, non-structural
• 3,000 PSI: Standard slabs, driveways, footings -- the most common spec
• 4,000 PSI: High-traffic areas, structural applications
• 5,000 PSI+: Heavy commercial, freeze-thaw exposed surfaces

For most residential work, 3,000 PSI is the default. Your ready-mix supplier will ask for the PSI, aggregate size (usually 3/4 inch), and slump (flowability, typically 4-5 inches for slabs). Prebagged mix (QUIKRETE, Sakrete) is a 3,000 PSI mix at standard water ratio.

Reinforcement and Concrete Quantity

Rebar and wire mesh don't reduce concrete volume in any meaningful way for estimating purposes -- the steel cross-section is negligible. Calculate your full volume, then plan reinforcement separately.

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Frequently Asked Questions

How many bags of concrete do I need for a 10x10 slab?

A 10x10 ft slab at 4 inches thick needs 1.23 cubic yards. That's roughly 56 bags of 80 lb premix. Add 10% waste: 62 bags. At that quantity, a ready-mix truck is worth calling.

When should I order a ready-mix truck?

At roughly 1 cubic yard or more. Bagged concrete for 1 yard costs $432+ in materials and 3-4 hours of mixing. Ready-mix delivered costs $225-$275 total and takes 30 minutes to pour.

How much waste factor should I add?

10% for standard projects, 15% for irregular shapes or difficult access. Never order less than your calculation -- running short mid-pour creates cold joints that compromise structural integrity.

What's the difference between cubic feet and cubic yards?

One cubic yard equals 27 cubic feet. Concrete is priced by the cubic yard. Convert your measurement by dividing cubic feet by 27.

How deep should a concrete footing be?

Below the frost line for your region. That's 12 inches in the deep south and up to 48 inches in northern states. Check local building codes -- most jurisdictions require a permit for structural footings.

TL;DR

• Core formula: Volume (cubic yards) = Length x Width x Thickness (in feet) / 27 -- always convert thickness from inches to feet first (4 inches = 0.333 ft) or the result will be off by a factor of 12.
• Bags vs. truck breakeven: At $432/cubic yard for bagged concrete vs. $160/yard for ready-mix, a truck is cheaper even at 0.5 yards -- for any project over 1 cubic yard, ready-mix also saves 3-4 hours of exhausting mixing labor.
• Always add waste: Order 10% extra for standard pours, 15% for irregular shapes or difficult access -- running short mid-pour creates cold joints that compromise structural integrity.
• 10x10 slab reality check: A 10x10 ft slab at 4 inches thick needs 1.23 cubic yards -- that's 56 bags of 80 lb premix plus waste, enough volume that calling a ready-mix truck is worth it.
• Footing depth: Footings must extend below the local frost line -- 12 inches in the deep south, up to 48 inches in northern states -- check your local building department before digging.