How do I calculate how much concrete I need?

Multiply length x width x depth (all in feet) to get cubic feet, then divide by 27 for cubic yards. For a 10x10 ft slab at 4 inches deep: 10 x 10 x (4/12) = 33.3 ft3 / 27 = 1.23 cubic yards. Add 10% for waste. The calculator above handles all conversions automatically.

How many bags of concrete do I need?

One 60 lb bag of concrete mix covers approximately 0.45 cubic feet. One 80 lb bag covers approximately 0.60 cubic feet. Divide your total cubic feet by 0.45 (60 lb) or 0.60 (80 lb) to get bag count. For jobs over 1 cubic yard, ready-mix delivery is more economical than bags.

How thick should a concrete slab be?

Residential patios and walkways: 4 inches. Driveways with passenger vehicles: 4-6 inches. Garage slabs: 4-6 inches. Shed foundations: 4 inches. Heavy vehicle or commercial use: 6-8 inches. Always check local building codes for structural applications.

When should I order ready-mix instead of using bags?

Ready-mix concrete is more economical and practical for any project over about 1 cubic yard (roughly 45 x 60 lb bags). Below that, bags are convenient for small repairs, footings, or post holes. Most concrete trucks have a minimum order of 1 cubic yard and charge a short-load fee below that.

How much waste should I add for a concrete slab?

Add 10% waste for most slab projects. This covers uneven subgrade, spillage, and over-excavation. For irregular shapes or projects with many penetrations, use 15%. Simple rectangular slabs with a good subgrade can use 5-8%.

How many cubic yards is a 20x20 concrete slab?

A 20x20 ft slab at 4 inches deep needs 4.94 cubic yards before waste. With 10% waste added, order 5.44 cubic yards. At 6 inches deep: 7.41 cubic yards before waste, or 8.15 with 10% waste.

Concrete Slab Calculator – Cubic Yards, Bags & Volume Estimator

Unit system

Waste allowance

Length (ft)

Width (ft)

Depth (in)

Patio/walkway: 4 in. Driveway: 4–6 in.

Concrete Slab Planning Checklist

Covers subgrade preparation, formwork, reinforcement, ordering, pouring sequence, and curing steps before your pour.

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Concrete Slab Calculator: Volume, Bags & Thickness Guide

The number that matters most before any concrete pour is cubic yards. That's what ready-mix trucks are priced by, and it's what determines whether you're ordering the right amount. Get it wrong by half a yard on a patio and you're either paying for a short-load top-up or left with an incomplete pour. This calculator gives you cubic yards, cubic meters, and bag counts from the same set of inputs.

Concrete volume quick reference

Slab size	4 in deep	5 in deep	6 in deep	Bags (60 lb) at 4 in
10×10 ft	1.23 yd³	1.54 yd³	1.85 yd³	~74 bags
12×16 ft	2.37 yd³	2.96 yd³	3.56 yd³	~143 bags
20×20 ft	4.94 yd³	6.17 yd³	7.41 yd³	~296 bags
24×24 ft	7.11 yd³	8.89 yd³	10.67 yd³	~426 bags
30×30 ft	11.11 yd³	13.89 yd³	16.67 yd³	~667 bags

No waste allowance applied. Add 10% before ordering. 20×20 ft highlighted as a common garage slab size. Bag counts based on 60 lb bags at 0.45 ft³ yield each.

Slab thickness by project type

Project	Recommended depth	Notes
Residential patio	4 in (100 mm)	Foot traffic only
Walkway / path	4 in (100 mm)	No vehicle loads
Driveway (passenger vehicles)	4–6 in (100–150 mm)	Most common residential driveway
Garage slab	4–6 in (100–150 mm)	Thicker edges recommended
Shed / small structure	4 in (100 mm)	Monolithic slab typical
Driveway (trucks, RVs)	6–8 in (150–200 mm)	Thickened edge or full-depth
Commercial / heavy load	6 in+ (150 mm+)	Structural engineer recommended

Bags vs ready-mix: which to use

Bags are convenient for small repairs, fence posts, and footings under about 0.5 cubic yards. Mixing 45 bags of 60 lb concrete by hand is realistic for one person with a rented mixer. Mixing 150 bags is not. The crossover point where ready-mix becomes the better choice is roughly 1 cubic yard, which is about 60 bags of 80 lb mix. Below that, bags. Above that, call a truck.

Ready-mix trucks typically have a minimum order of 1 cubic yard and charge a short-load fee for anything under 3–4 yards. If your job is 1.5 yards and the minimum is 1, you're paying for what you ordered. Plan around that minimum when sizing your project, and add your waste factor before calling for a quote.

How the calculation works

Volume in cubic feet = length × width × (depth in inches ÷ 12). Cubic yards = cubic feet ÷ 27. Bags needed = cubic feet ÷ bag yield (0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb). All results are before waste. The calculator applies your selected waste percentage on top and that's the number you should order.

Frequently Asked Questions

Multiply length × width × depth (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10×10 ft slab at 4 inches deep: 10 × 10 × (4÷12) = 33.3 ft³ ÷ 27 = 1.23 cubic yards. Add 10% for waste = 1.35 yd³ to order. The calculator above handles all of this automatically.

One 60 lb bag yields approximately 0.45 cubic feet. One 80 lb bag yields approximately 0.60 cubic feet. Divide your total cubic feet by 0.45 or 0.60 to get bag count. For the 10×10 ft at 4 in example: 33.3 ÷ 0.45 = 74 bags (60 lb) or 56 bags (80 lb). For anything over 1 cubic yard, ready-mix delivery is far more practical.

Patios and walkways: 4 inches. Residential driveways: 4–6 inches. Garage slabs: 4–6 inches. Heavy vehicle or commercial use: 6–8 inches. See the thickness reference table above for a full breakdown by project type. Always check local building codes for any structural application.

At 4 inches deep: 4.94 cubic yards. At 6 inches deep: 7.41 cubic yards. With 10% waste: 5.43 yards and 8.15 yards respectively. Enter your exact dimensions in the calculator above for a precise figure with your chosen waste allowance.

Ready-mix is more practical for anything over about 1 cubic yard (roughly 45–60 bags depending on bag size). Below that, bags work fine for small slabs, footings, and post holes. Most concrete trucks have a minimum delivery of 1 cubic yard and charge a short-load fee below 3–4 yards, so factor that into your budget when planning.

10% is the standard recommendation for most slab pours. It accounts for uneven subgrade, spillage, and over-excavation in corners. Simple rectangular slabs with a well-prepared base can use 5–8%. Irregular shapes, many penetrations, or sloped sites should use 12–15%.

Accuracy & Review

Reviewed by: Maria Ramirez, PE

Maria is a licensed Professional Engineer with experience in residential and light commercial concrete quantity takeoffs. She reviewed the volume formulas, unit conversions, bag yield assumptions, and waste factor guidance used in this calculator.

Last updated: Feb 12, 2026

See: Methodology · Data Sources · Review Board

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Disclaimer: Results are for general planning only and do not constitute engineering or structural design advice. Actual quantities vary based on site conditions, subgrade preparation, slab thickness tolerances, and local code requirements. Structural applications should be reviewed by a qualified professional.

See Methodology and Data Sources for details.