Concrete Yardage — How to Calculate a Slab Pour Without Buying Too Much

A homeowner is pouring a 20-foot by 25-foot patio at 4 inches thick. The ready-mix supplier asks how many yards to deliver. The homeowner converts 4 inches to 0.333 feet, multiplies, and rounds — coming up with 6.2 yards. The truck arrives with 7 yards (suppliers round up). After the pour, there are 2.5 yards left in the chute. The homeowner overpaid by hundreds of dollars and is now scrambling to find somewhere for excess concrete on a job that already failed before it set.

The miscalculation has two parts: the conversion math, and the waste factor. Both are simple. Both go wrong with surprising frequency.

The Volume Formula

Concrete volume in cubic yards equals length (feet) times width (feet) times depth (feet), divided by 27.

Why 27? A cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet. Divide cubic feet by 27 to get cubic yards.

The depth conversion is where mistakes happen. A 4-inch slab is not 4 feet thick. Convert inches to feet by dividing by 12. So 4 inches is 4/12 = 0.333 feet. A 6-inch slab is 0.5 feet. An 8-inch slab is 0.667 feet.

Worked example: 20 × 25 × 4 inches.

• Volume in cubic feet: 20 × 25 × 0.333 = 166.7
• Volume in cubic yards: 166.7 / 27 = 6.17 cubic yards

Order 6.5 yards, not 6.2 or 7. The supplier's minimum order increments are usually 0.25 or 0.5 yards.

The Waste Factor

The volume calculation gives you the geometric volume of the form. The actual amount you need is higher for several reasons.

Sub-base variation. A slab that's nominally 4 inches thick is rarely exactly 4 inches everywhere. Sub-base settling, screeding tolerance, and uneven excavation all add 5–10% to the actual concrete volume needed.

Spillage and waste. A pour produces some spillage at the edges, some in the chute, some in the wheelbarrow. Typical waste is 3–5% for a contained slab, more for an oddly-shaped pour.

Strike-off allowance. Floating the surface requires slightly more material than the geometric volume — the bull float pulls material around and the surface needs to be slightly over-filled before screeding.

Total practical waste: 8–12% over the geometric volume for typical residential slabs.

For the 6.17-yard calculation above, add 10% waste = 6.79 yards. Order 7 yards. The "rounded up to 7" the supplier suggested is actually the correct order — not because they were rounding generously, but because they know waste is real.

Common Slab Calculations

4-inch patio or sidewalk: Length × width × 0.0124 = yards. (The 0.0124 is 0.333 / 27, the per-square-foot yardage for 4-inch thickness.)

6-inch driveway: Length × width × 0.0185 = yards.

8-inch driveway (heavy duty): Length × width × 0.0247 = yards.

4-inch slab footing 16 inches wide: Length × 1.33 × 0.0124 = yards.

For a 20 × 25 patio at 4 inches: 20 × 25 × 0.0124 = 6.2 yards. Add 10% waste: 6.82 yards. Order 7.

What Concrete Actually Costs

In 2026 markets, ready-mix concrete pricing varies regionally but follows a consistent structure.

Base concrete cost: $145–$185 per cubic yard for standard 3,000–4,000 psi mix. Mountain markets and the Northeast trend higher.

Short-load fee. Any order under 5 yards is typically charged a short-load fee of $80–$150. A 3-yard order doesn't cost 3/5 of a 5-yard order — it often costs nearly as much because the truck is the same.

Saturday or after-hours fee. Weekend and evening deliveries typically add $75–$150.

Stand-by fee. If the truck waits longer than the included pour time (usually 5–7 minutes per yard, so 35–50 minutes for a 7-yard load), stand-by is $2–$4 per minute.

For the example 7-yard pour: $1,050–$1,300 for the concrete itself, plus delivery (usually included up to 20 miles), tax, and any time overages. A pour that runs over the included time by 20 minutes adds $40–$80.

The Bags vs. Truck Threshold

For small slabs, bag concrete from the home center is sometimes cheaper.

An 80-lb bag of concrete mix produces 0.6 cubic feet, or about 0.022 cubic yards. That means 45 bags per cubic yard.

At $5–$6 per 80-lb bag, bag concrete costs $225–$270 per cubic yard — more than ready-mix, but no delivery fee.

The break-even. Below about 1.5–2 cubic yards, bags can be cheaper than ready-mix after delivery fees, especially with short-load surcharges. Above that, ready-mix is cheaper and dramatically less labor.

A 6-yard patio in bags would require 270 bags, weighing 21,600 pounds, mixed in batches by hand or in a rented mixer. The labor alone exceeds any cost saving. Bag concrete is for footings, post holes, and very small slabs — not patios.

The Mistake That Costs the Most

The single most common error in DIY concrete pricing is forgetting to add waste.

A homeowner calculates 6.2 yards, orders exactly 6.25 — and runs out on a 70-foot pour with the truck still on site. The supplier dispatches a small "hot load" truck with an additional yard. The hot load is $300–$500 because of the urgency and the second truck. Total: $1,500+ for what would have been a $1,100 pour with proper waste calculation.

The honest math: pour volume + 10% waste, rounded up to the nearest quarter or half yard. That number is what to order. It's almost always slightly more than seems necessary, and almost always exactly right.

Concrete is the rare material where buying a little extra is genuinely better than buying a little too little. Excess is wheelbarrowed off, used to set a fence post, or poured into a wood form for a stepping stone. Underordering creates an emergency. The asymmetry of those outcomes is what the waste factor is paying for.