How Much Concrete Do I Need for a Slab? (Contractor's Calculation Guide)

Getting your concrete quantity right for a slab is straightforward math, but the details matter. Under-order and you get a cold joint mid-pour. Over-order and you’re paying $130-155 per yard for concrete that ends up in a pile on the lawn.

I’ve poured concrete slabs for over 20 years, from small utility pads to full garage floors. The formula never changes, but the decisions around slab thickness, waste factor, and whether to call a ready-mix plant or haul bags will make or break your budget. This guide covers all of it with real numbers.

Quick Answer

For a standard 4-inch residential slab, use this formula:

Length (ft) x Width (ft) x 0.0123 = Cubic Yards (before waste)

Then multiply by 1.10 to add your 10% waste factor.

Example: a 12 x 20 slab at 4 inches thick = 240 sq ft x 0.0123 = 2.95 yards x 1.10 = 3.24 yards (order 3.5).

That shortcut only works at 4-inch thickness. For 5-inch or 6-inch slabs, use the full formula in the next section or run it through the concrete calculator directly.

The Full Formula (Works at Any Thickness)

Length (ft) x Width (ft) x Thickness (ft) / 27 = Cubic Yards

The key thing most people mess up: thickness has to be in feet, not inches. Four inches of slab is not 4, it’s 0.333. If you plug in 4 instead of 0.333 you will get a number twelve times too high. I’ve watched contractors do this and nearly order a full truck for a job that needed less than a yard.

Convert inches to feet by dividing by 12:

Thickness (in)	Thickness (ft)
3.5 in	0.292 ft
4 in	0.333 ft
5 in	0.417 ft
6 in	0.500 ft
8 in	0.667 ft

Divide your final cubic feet answer by 27 to convert to cubic yards (there are 27 cubic feet in one cubic yard). That gives you your base volume. Then add the waste factor.

Choosing the Right Slab Thickness

Thickness is the variable with the biggest impact on how much concrete you order, and it’s the one I see contractors underspecify most often.

Application	Recommended Thickness	Notes
Walkways, sidewalks	4 in	Standard residential
Patio slabs	4 in	Proper compacted sub-base required
Shed and utility pads	4 in	Light storage, no heavy equipment
Garage floors	5-6 in	Regular passenger vehicles, 5 in minimum
Garage with heavy trucks	6 in	Trucks, SUVs, trailers
Driveways	5-6 in	Freeze-thaw climates push toward 6 in
Workshop floors	6 in	Tools, lifts, heavy machinery

Four inches is not always enough. Too many garage floors crack because someone poured 4 inches and thought they’d saved money. You save $50-80 per yard going thinner. You spend $3,000-5,000 on repairs five years later. Pour thick enough.

Going from 4 to 5 inches adds 25% to your concrete volume. Going from 4 to 6 inches adds 50%. Calculate both options and see what the material cost difference actually is before you decide.

The Waste Factor: Why You Always Add 10%

Every professional orders 10% over their calculated volume, minimum. Here’s what that buffer actually covers:

Sub-base variation. Your gravel base is never perfectly level. Low spots eat more concrete than your math predicted.
Spillage during the pour, especially around the chute and when screeding.
Concrete that stays in the drum. Ready-mix trucks don’t empty completely.
Form bow. Even a half-inch of bow across a 20-foot run adds volume.
Drainage slope. Slabs sloped 1/4 inch per foot for drainage are thicker on one edge.

On jobs under 5 yards, I use 10% every time. On large pours over 10 yards, I’ll drop to 8%, but never less. Running short mid-pour is always more expensive than a little leftover material.

Use the concrete calculator to run your base number, then apply your waste factor based on site conditions.

Worked Example 1: 20 x 24 Garage Slab at 5 Inches

This is a typical two-car garage slab. Going 5 inches because the homeowner has an F-250 and a utility trailer.

Step 1 - Area: 20 ft x 24 ft = 480 sq ft

Step 2 - Convert thickness: 5 in / 12 = 0.417 ft

Step 3 - Cubic feet: 480 x 0.417 = 200.16 cubic feet

Step 4 - Cubic yards: 200.16 / 27 = 7.41 cubic yards

Step 5 - Add 10% waste: 7.41 x 1.10 = 8.15 cubic yards

What I’d order: 8.5 yards

At $140/yard delivered, that’s $1,190 in concrete material. If I’d poured this at 4 inches, I’d have needed 6.5 yards, saving about $280 in material. But on a two-car garage for a truck owner, 4 inches was never the right spec. The extra $280 buys 20+ years of slab life. Spend the money.

Worked Example 2: 12 x 16 Shed Pad at 4 Inches

A common backyard utility pad. Simple rectangular shape. Properly compacted gravel sub-base already in place.

Step 1 - Area: 12 ft x 16 ft = 192 sq ft

Step 2 - Convert thickness: 4 in / 12 = 0.333 ft

Step 3 - Cubic feet: 192 x 0.333 = 63.94 cubic feet

Step 4 - Cubic yards: 63.94 / 27 = 2.37 cubic yards

Step 5 - Add 10% waste: 2.37 x 1.10 = 2.60 cubic yards

What I’d order: 3 yards

Here’s where the short-load fee comes into play. Most ready-mix plants have a minimum delivery of 1-2 yards and charge a short-load fee for anything under their minimum, often $75-150 extra. At 2.6 yards, you’re already close to the 3-yard round-up, and ordering 3 yards might cost the same delivered as ordering 2.6 if the plant charges you to their minimum. Always call the plant and ask about minimums and short-load fees before you finalize your order. It changes the math on small jobs.

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Slab Size Reference Table (4-Inch and 6-Inch Thickness)

Slab Size	Sq Ft	Yards at 4 in	With 10% Waste	Yards at 6 in	With 10% Waste
10 x 10	100	1.23	1.35	1.85	2.04
10 x 20	200	2.47	2.72	3.70	4.07
12 x 16	192	2.37	2.60	3.56	3.91
12 x 20	240	2.96	3.26	4.44	4.88
16 x 20	320	3.95	4.35	5.93	6.52
20 x 24	480	5.93	6.52	8.89	9.78
20 x 30	600	7.41	8.15	11.11	12.22
24 x 30	720	8.89	9.78	13.33	14.67
30 x 40	1,200	14.81	16.29	22.22	24.44

These numbers are base calculations before any site-specific adjustments. If your sub-base has significant variation, bump the waste factor to 12-15%.

Ready-Mix vs. Bags: The Honest Numbers

Every contractor gets asked this. Here’s my straightforward answer after two decades of pours.

Bags: When They Make Sense

Bags are practical for small pours under 0.5 cubic yards, and for sites where a ready-mix truck can’t get in. Narrow side yards, backyard-only access through a gate, urban lots with no truck turnaround. In those situations, bags and a portable mixer or wheelbarrow relay are your options.

One 60-lb bag covers about 0.45 cubic feet. One 80-lb bag covers about 0.60 cubic feet.

To fill 1 cubic yard (27 cubic feet): 60 bags of 60-lb material.

Ready-Mix: The Cost Math

At $4.75 per 60-lb bag, a 2.5-yard job by the bag route costs:

2.5 yards = 67.5 cubic feet
67.5 / 0.45 = 150 bags
150 x $4.75 = $712 in bags alone
Plus a full weekend of mixing labor

By ready-mix truck: 2.5 yards x $140 = $350 delivered, poured in under two hours.

The bag option is almost always more expensive in material cost and always more expensive in labor. The ready-mix truck wins on every job with site access.

The exception: if you own a mixer and you’re doing a 0.5-yard job yourself, grabbing bags from the hardware store beats scheduling a truck. Beyond that, call the plant.

For current ready-mix pricing in your region, check the concrete cost per yard calculator before you call the plant. Regional pricing varies by $30-60 per yard depending on location and fuel costs.

Do You Need Rebar in a Slab?

Rebar doesn’t change your concrete volume calculation - it sits inside the slab, not in addition to it. But it does determine how long that slab lasts. For most residential slabs:

Patios and utility pads: wire mesh (6x6 welded) or fiber reinforcement added at the plant is usually enough.
Garage floors and driveways: use #3 or #4 rebar on an 18-24 inch grid at mid-slab depth. Any slab under vehicle load needs real reinforcement.
Fiber reinforcement: polypropylene fibers mixed at the plant add crack resistance and work well alongside rebar or mesh. Costs about $5-7 per yard extra.

Use the rebar spacing calculator to calculate bar counts and linear footage for your grid before ordering material.

Common Mistakes That Cost Real Money

Leaving thickness in inches. Convert inches to feet before you calculate. Four inches is 0.333, not 4. Get this wrong and your estimate is off by a factor of twelve.

Skipping the waste factor. Running short mid-pour means a cold joint, which is a structural weak point. The cost of ordering extra is always less than the cost of fixing a bad joint. Ten percent is not optional.

Not asking about short-load fees. On any job under 3 yards, call the plant before you finalize the order. Ask about minimums and surcharges. It changes the math.

Pouring over uncompacted sub-base. Low spots eat more concrete than your calculation predicted, and a slab on a poor base will settle unevenly. Compact before you set your forms.

Not calling the plant early enough. Spring and summer, plants book up fast. Three to five days minimum notice. Confirm the morning of the pour.

Frequently Asked Questions

How many cubic yards for a 10 x 10 slab?

At 4 inches thick: 10 x 10 x 0.333 / 27 = 1.23 yards base. With 10% waste = 1.35 yards. I’d order 1.5 yards and ask about the short-load fee structure at the plant.

How much concrete for a 20 x 20 slab at 4 inches?

20 x 20 x 0.333 / 27 = 4.93 yards. Add 10% = 5.42 yards. Order 5.5 yards.

What thickness is standard for a garage slab?

Minimum 5 inches for standard passenger vehicles. Use 6 inches for anything heavier, or if you’re in a freeze-thaw climate. The extra inch costs roughly $5-7 per square foot in concrete material. It’s worth it on a garage floor you want to last 30 years.

How many bags of concrete equal one cubic yard?

60-lb bags (0.45 cubic feet each): 60 bags per yard. 80-lb bags (0.60 cubic feet each): 45 bags per yard.

Do I need rebar in a residential slab?

For patios and shed pads: wire mesh or fiber reinforcement is typically enough. For garage floors and driveways, use #3 or #4 rebar on an 18-inch grid. The cost of rebar is low compared to the cost of a cracked slab. Add it.

What causes slabs to crack?

Insufficient thickness, no reinforcement, poor sub-base prep, too much water added at the jobsite, no control joints cut, and curing too fast. Most residential slab cracking is preventable.

Can I use bags for a slab over 200 sq ft?

You can, but at 200 sq ft and 4-inch thickness you need about 2.5 yards, or roughly 150 bags. That’s $700+ in bags plus a full weekend of mixing versus $350 for a ready-mix delivery. Call the truck when site access allows.

What is a short-load fee?

A surcharge from ready-mix plants for orders below their delivery minimum, typically 1-3 cubic yards. Fees run $75-150. Ask about minimums before you order.

Putting It All Together

Slab estimating comes down to four things: accurate area measurement, correct thickness spec, proper waste factor, and understanding the ordering economics at your local plant.

Run your numbers with the formula, verify them with the concrete calculator, and call two or three local plants before you commit. Prices swing $20-40 per yard between suppliers in the same metro area. On a 10-yard job that’s real money.

For irregular slab shapes, the square footage calculator handles L-shapes and custom dimensions before you feed the number into your concrete estimate.

The formula is simple. The discipline to use it every time, including the waste factor and the thickness check, is what separates contractors who run tight estimates from the ones always scrambling at the pour.