How to Calculate Concrete for a Slab (Formula + Examples)

How Do You Calculate Concrete for a Slab?

To calculate concrete for a slab, multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 to get cubic yards — the unit ready-mix concrete is sold in. A 12 ft × 12 ft slab at 4 inches thick needs 1.78 cubic yards, or about 2 yards once you add waste.

Every slab estimate comes down to four steps:

1. Measure length, width, and thickness.
2. Convert all three to the same unit (feet is easiest in the US).
3. Multiply them together, then divide by 27 for cubic yards.
4. Add 5–10% for waste and round up to a practical order size.

Quick start: Skip the manual math with the Slab Concrete Calculator — enter length, width, and thickness in any unit and it returns volume in cubic yards, cubic feet, and cubic meters, with a waste factor built in.

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Step 1: Measure Length, Width, and Thickness

The biggest source of estimating errors isn't the math — it's mixing units. Length and width are usually in feet, but slab thickness is almost always quoted in inches, and you can't multiply feet by inches.

Convert thickness to feet first by dividing by 12:

Thickness	In feet (÷ 12)
4 inches	0.333 ft
5 inches	0.417 ft
6 inches	0.500 ft
8 inches	0.667 ft

So a slab that is 20 ft long, 10 ft wide, and 4 inches thick becomes 20 × 10 × 0.333 once every dimension is in feet. Measure the actual formed area, not the rough excavation — the concrete only fills inside the forms.

Tip: For an L-shaped or irregular slab, split it into rectangles, calculate each separately, then add the volumes together.

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Step 2: Convert Volume to Cubic Yards (and Cubic Meters)

Concrete volume in the US is ordered by the cubic yard, and there are exactly 27 cubic feet in 1 cubic yard (3 ft × 3 ft × 3 ft). That single conversion does most of the work.

Formula:

Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27

Using the 20 × 10 × 4-inch slab from Step 1:

• Volume = 20 × 10 × 0.333 = 66.6 cubic feet
• Cubic yards = 66.6 ÷ 27 = 2.47 yd³

Working in metric instead? Measure everything in meters and the result is already in cubic meters (m³). To convert cubic feet to cubic meters, divide by 35.31. For a deeper reference on unit conversions, see our concrete volume conversions guide.

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How to Calculate Square Yards for a Slab

Square yards measure area (the flat surface of the slab), while cubic yards measure volume (the concrete that fills it). To get square yards, multiply length × width in feet for square feet, then divide by 9 (there are 9 ft² in 1 yd²). Confusing the two is the most common slab-estimating mistake.

Square yards = (Length ft × Width ft) ÷ 9

For our 20 ft × 10 ft slab:

• Area = 20 × 10 = 200 ft²
• Square yards = 200 ÷ 9 = 22.2 yd²

You'll use square yards for surface finishing, sealing, or formwork coverage — and cubic yards for ordering the concrete itself. Never order concrete by the square yard; it ignores thickness entirely. To convert any measurement quickly, the Concrete Yards Calculator handles square feet, square yards, cubic feet, and cubic yards side by side.

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Worked Example: Concrete for a 12 × 12 Slab

A 12 ft × 12 ft slab at 4 inches thick is one of the most common DIY pours — a shed base, a hot-tub pad, or a small patio. Here's the full calculation, start to finish.

Given:

• Length = 12 ft
• Width = 12 ft
• Thickness = 4 in = 0.333 ft

Step-by-step:

1. Area = 12 × 12 = 144 ft²
2. Volume = 144 × 0.333 = 48 ft³
3. Cubic yards = 48 ÷ 27 = 1.78 yd³
4. Add 10% waste = 1.78 × 1.10 = 1.96 yd³ → order 2 yd³

That waste allowance matters. If you order exactly 1.78 yards and lose a little to spillage or an uneven subgrade, you'll be short — and a second truck trip for a fraction of a yard is expensive. For a different size, the worked 10×10 slab example follows the same method.

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How Thick Should a Concrete Slab Be?

For most residential slabs, 4 inches is the standard minimum thickness, increasing to 5–6 inches where vehicles or heavy loads are involved. Thickness drives both your concrete volume and the slab's load capacity, so it's worth getting right before you calculate quantity.

Slab Use	Typical Thickness	Notes
Patio, walkway, shed floor	4 in	Light foot traffic only
Standard garage floor	4–5 in	Passenger vehicles
Driveway (cars/light trucks)	4–6 in	5–6 in for heavier vehicles
Driveway (RVs, heavy trucks)	6 in+	Often with rebar
Workshop / equipment pad	6 in+	Sized to the point load

These ranges align with ACI 332-20 (Residential Code Requirements for Structural Concrete) from the American Concrete Institute and common International Residential Code (IRC) practice. A thicker slab almost always benefits from reinforcement — check our rebar sizing and spacing guide before you finalize the design. To verify a slab can carry a specific load, use the Slab Load Capacity Calculator.

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How Much Waste Should You Add?

Always add 5–10% to your calculated volume to cover spillage, an uneven subgrade, form deflection, and the concrete left clinging to the chute and wheelbarrow. Most contractors use 10% on small DIY pours and 5% on large, well-prepared sites.

Why the cushion is non-negotiable:

• Subgrade variation — a base that dips even half an inch over a large slab adds real volume.
• Spillage and stuck mix — every transfer point loses a little.
• No partial re-orders — running short means paying for a second short-load delivery, often with a minimum-load surcharge.

Rule of thumb: It is far cheaper to have a little extra concrete (use it for a step or a small pad) than to come up short mid-pour and let a cold joint form while you wait for more.

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Common Slab Sizes — Concrete Volume Cheat Sheet

The table below gives the concrete volume for popular slab sizes at 4 inches thick, before waste. Add 5–10% to each figure for your order.

Slab Size (ft)	Area (ft²)	Volume at 4″ (yd³)	Volume at 6″ (yd³)
8 × 8	64	0.79	1.19
10 × 10	100	1.23	1.85
12 × 12	144	1.78	2.67
12 × 20	240	2.96	4.44
16 × 20	320	3.95	5.93
20 × 24	480	5.93	8.89
24 × 24	576	7.11	10.67
30 × 40	1,200	14.81	22.22

Notice that going from 4 inches to 6 inches adds 50% more concrete — a major cost jump on a large slab, which is why thickness should match the actual load, not guesswork. For instant figures at any size and thickness, the Slab Concrete Calculator does the rounding for you.

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Bags vs. Ready-Mix: Which Should You Order?

The dividing line is roughly 1 cubic yard. Below that, bagged concrete mixed on-site is usually cheaper and more convenient; above it, a ready-mix truck saves enormous labor. One cubic yard is about 45 bags of 80-lb mix — a lot of bags to haul, open, and mix by hand.

Bag yields for the common sizes:

Bag Size	Yield per Bag	Bags per yd³ (27 ft³)
40 lb	0.30 ft³	~90
50 lb	0.375 ft³	~72
60 lb	0.45 ft³	~60
80 lb	0.60 ft³	~45

So our 12 × 12 slab (48 ft³ before waste) would need roughly 80 bags of 80-lb mix — well past the point where a ready-mix truck makes sense. For a full bag breakdown by slab size, see how many bags of concrete for a slab.

Decision shortcut: Under ~½ yard, use bags. Between ½ and 1 yard, weigh convenience against price. Over 1 yard, order ready-mix.

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Don't Forget Rebar, Cost, and Weight

Volume is only the first number you need. A complete slab estimate also accounts for reinforcement, budget, and the dead load the ground below must carry.

• Cost — material plus labor for a poured slab typically runs several dollars per square foot installed; price your specific job with the Concrete Slab Cost Calculator and read how much concrete costs.
• Weight — a 4-inch slab weighs about 50 lb per square foot — that's 150 lb/ft³ (the standard concrete density used in design, per the Portland Cement Association) × 0.333 ft. That matters for subgrade bearing and transport; see how much concrete weighs or use the Slab Weight Calculator.
• Monolithic pours — if your slab includes a thickened edge or footing, the volume math changes. Use the Monolithic Slab Calculator so the integral footing isn't left out.

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Frequently Asked Questions

How much concrete do I need for a 10x10 slab?

A 10 ft × 10 ft slab at 4 inches thick needs 1.23 cubic yards of concrete (100 ft² × 0.333 ft = 33.3 ft³ ÷ 27). Add 10% for waste and order about 1.4 yards. At 6 inches thick the same slab needs 1.85 yards before waste.

How do you calculate cubic yards of concrete for a slab?

Multiply length × width × thickness in feet to get cubic feet, then divide by 27. For example, a 15 × 15 slab at 4 inches: 15 × 15 × 0.333 = 74.9 ft³ ÷ 27 = 2.77 cubic yards. Convert inches to feet first by dividing by 12.

How many cubic feet are in a cubic yard of concrete?

There are exactly 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft = 27 ft³). This is the key conversion for ordering, because ready-mix concrete is sold by the cubic yard while slabs are usually measured in feet.

How thick should a concrete slab be?

The standard minimum for residential slabs is 4 inches for patios, walkways, and garage floors. Increase to 5–6 inches for driveways carrying heavier vehicles, and 6 inches or more for RV pads and equipment pads, generally with rebar per ACI 332.

How much extra concrete should I order for waste?

Add 5–10% to your calculated volume. Use 10% for small DIY pours and uneven subgrades, and 5% for large, well-prepared sites. Running short mid-pour risks a cold joint and an expensive short-load second delivery, so erring high is the safer choice.

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

It depends on slab volume and bag size. One cubic yard equals about 45 bags of 80-lb mix, 60 bags of 60-lb, or 90 bags of 40-lb. A 12 × 12 slab at 4 inches (48 ft³) needs roughly 80 bags of 80-lb mix — usually the point to switch to ready-mix.

Should I use bagged concrete or ready-mix for a slab?

Use bagged mix under about 1 cubic yard and ready-mix above it. One yard is around 45 80-lb bags to mix by hand. Ready-mix trucks deliver 8–10 yards per load and save the labor of hand-mixing, which dominates the cost on anything but the smallest pours.

How do I calculate concrete for an irregular or L-shaped slab?

Split the shape into rectangles, calculate the volume of each (length × width × thickness ÷ 27), then add them together. For circular pads, use area = π × radius² × thickness. The Slab Concrete Calculator handles standard rectangles instantly.

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Related Guides

• How to Calculate Concrete for a 10×10 Slab
• How Many Bags of Concrete for a Slab
• How Much Does Concrete Weigh?
• How Much Does Concrete Cost?
• Concrete Volume Conversions Guide
• How to Reinforce a Concrete Slab: Rebar, Mesh & Fiber
• Gravel Coverage, Depth & Tonnage Guide
• Staircase Concrete Basics

Visit Concrete Calculator Max for the full suite of concrete estimation tools and reference guides.

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Summary

Calculating concrete for a slab is one formula plus one conversion:

• Volume (yd³) = Length ft × Width ft × Thickness ft ÷ 27
• Convert thickness from inches to feet first (÷ 12)
• Add 5–10% waste and round up to a practical order size
• 4 inches is the standard minimum thickness; 5–6 inches for vehicle loads
• Under ~1 yard, use bags; over 1 yard, order ready-mix

For a 12 × 12 slab at 4 inches, that's 1.78 yards calculated, 2 yards ordered. Run your own numbers in seconds with the Slab Concrete Calculator.