Calculating Concrete Yardage for Slabs: Thickness, Waste Factor, and Ordering Right

You measured the slab, plugged numbers into a calculator, and ordered concrete. The truck showed up two yards short. Now your crew is standing around while you scramble to get a short load dispatched—at a premium. That scenario plays out on jobsites every week, and it almost always traces back to the same thing: a yardage calculation that skipped a step.

Here’s how to calculate concrete yardage for a slab correctly, with real numbers, proper waste factors, and the details that keep you from coming up short on pour day.

The Basic Formula for Concrete Yardage

Every concrete slab calculation starts with volume. You need three measurements in feet:

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

Then convert to cubic yards:

Cubic Yards = Volume (cubic feet) ÷ 27

Why 27? One cubic yard equals 3 ft × 3 ft × 3 ft = 27 cubic feet. That’s the unit ready-mix plants sell in, and it’s what your ticket will show when the truck arrives.

If your thickness is in inches (and it usually is—most slabs are spec’d at 4″ or 6″), divide by 12 first to convert to feet.

Worked Example: 20 × 24 Driveway Slab at 4 Inches

Let’s walk through a real scenario. You’re pouring a residential driveway: 20 feet wide, 24 feet long, 4 inches thick.

Step 1: Convert thickness to feet
4 inches ÷ 12 = 0.333 feet

Step 2: Calculate volume in cubic feet
20 ft × 24 ft × 0.333 ft = 159.84 cubic feet

Step 3: Convert to cubic yards
159.84 ÷ 27 = 5.92 cubic yards

So the raw volume is 5.92 yards. But you are not done yet.

Why the Raw Number Is Never Enough: Waste Factors

If you order exactly 5.92 yards for that driveway, you will almost certainly come up short. Here’s why:

• Subgrade variation—your gravel base is never perfectly flat. Low spots absorb extra concrete. Even a half-inch average deviation across a 480 SF slab adds roughly 0.75 yards.
• Form deflection—forms bow slightly under the weight of wet concrete, especially on longer runs.
• Spillage and overpour—some concrete inevitably ends up outside the forms or gets wasted during finishing.
• Thickness variation—screeding to a perfect 4″ across a 20-foot span is difficult. You’ll have spots at 4.25″ or 4.5″.

Industry-standard waste factors for concrete slabs, per ACI (American Concrete Institute) guidance and field experience:

For the 20 × 24 driveway on a standard residential subgrade, use 10%:

5.92 yards × 1.10 = 6.51 yards

Round up to the nearest quarter-yard when ordering: 6.75 cubic yards. Ready-mix dispatchers work in quarter-yard increments, and rounding up costs you roughly $40–50 extra but saves a $250+ short-load fee if you run tight.

The Short-Load Trap: Minimum Order Economics

Most ready-mix suppliers charge a short-load fee for orders under their minimum—typically 5 cubic yards per truck. A standard transit mixer holds 8–10 yards. If you order 3 yards, you’re paying to run a truck that’s less than half full.

Short-load fees typically range from $25–50 per yard under the minimum. On a 3-yard order with a 5-yard minimum, that’s an extra $50–100 on top of the concrete price. For small pours (under 2 CY), consider whether bagged concrete—60 lb or 80 lb bags—makes more economic sense:

• One 80 lb bag yields approximately 0.6 cubic feet
• One cubic yard = 27 cubic feet = approximately 45 bags (80 lb)
• At ~$6–7 per bag, one yard of bagged concrete costs $270–315 vs. $130–180 per yard for ready-mix

The crossover point is roughly 1–1.5 cubic yards: below that, bags may be cheaper after you factor in the short-load fee. Above that, ready-mix wins on both cost and labor.

Common Slab Thicknesses and Coverage Per Yard

This reference table saves time on quick estimates. One cubic yard of concrete covers:

To use the table: divide total square footage by the coverage number, then apply your waste factor. For the 20 × 24 driveway at 4″: 480 SF ÷ 81 = 5.93 yards × 1.10 waste = 6.52 yards. Same answer, faster math.

Ordering Concrete: What to Tell the Dispatcher

When you call the batch plant, they need four things:

1. Volume—in cubic yards, rounded to the nearest quarter yard
2. Mix design / PSI—most residential slabs are 3,000 or 4,000 PSI. Check your specs or local code. The International Building Code (IBC) and International Residential Code (IRC) set minimum compressive strength requirements by application.
3. Slump—typically 4–5 inches for slabs (how fluid the mix is). Higher slump is easier to work but weaker.
4. Delivery schedule—plan 1–2 days ahead. Once the truck arrives, you have roughly 10 minutes per yard to unload before truck-time charges start, usually $2–3 per minute.

Schedule your pour for morning when possible. Concrete sets faster in heat—an afternoon pour on a 90°F day gives you significantly less working time for finishing.

Multi-Shape Projects: Beyond the Simple Slab

Real projects rarely involve a single rectangle. A driveway with a turnaround, a patio with a step-down, a sidewalk that wraps around the house—each section needs its own calculation, then you total them up.

For each shape:

• Rectangles: L × W × D ÷ 27
• Circles (round pads): π × r² × D ÷ 27 (where r is radius in feet)
• Triangles: 0.5 × base × height × D ÷ 27

Add all sections together, then apply the waste factor once to the total. Do not apply waste to each section separately—that compounds the overage.

If you’re estimating a multi-shape project, a concrete volume calculator that handles slabs, footings, and columns in one estimate saves time and reduces errors compared to manual arithmetic on each shape.

Mistakes That Cost You Money

After years of watching concrete pours go sideways, these are the calculation errors that show up most often:

• Forgetting to convert inches to feet—entering 4 instead of 0.333 for thickness gives you a volume 12 times too large. It sounds obvious, but it happens under bid-day pressure.
• Using the wrong waste factor—5% is fine for a laser-graded warehouse floor with steel forms. It is not fine for a residential driveway on fill dirt with wood forms.
• Rounding down—ordering 6.5 yards when you calculated 6.51 feels like you’re being efficient. It’s actually gambling. Always round up to the next quarter yard.
• Ignoring the thickened edge—many slabs have thickened edges or turned-down footings at the perimeter. A 12″ × 12″ thickened edge around a 20 × 24 slab adds roughly 0.7 yards that the basic L × W × D formula misses.
• Not accounting for the subgrade—if your gravel base has settled unevenly, the actual average thickness may be 4.5″ or 5″ instead of 4″. Probe several spots with a ruler before calculating.

Quick-Check Your Numbers

Before you pick up the phone to order, run this sanity check. Per OSHA’s concrete and masonry resources, concrete weighs approximately 150 lbs per cubic foot—one cubic yard is about 4,050 lbs (just over 2 tons). If your calculated yardage seems off, multiply by 4,050 and ask yourself: does that weight make sense for this pour?

A 20 × 24 driveway at 6.75 yards = approximately 27,338 lbs of concrete. That’s about 3.4 truck loads if you were using a one-ton pickup (you’re not). It’s a reasonable single-truck load for a transit mixer. If your calculation suggests 20+ yards for a residential driveway, something is wrong—go back and check your unit conversions.

Getting the yardage right is the first step in every concrete estimate. Nail this calculation, apply the right waste factor for your conditions, and round up at ordering—and you will not be the contractor making that panicked call to the batch plant mid-pour.