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Concrete Slab Volume & Cost Estimator

Calculate the exact cubic yardage of concrete needed for a slab, patio, or driveway. Includes waste factors and estimates total material costs.

Concrete Slab Volume & Cost Estimator

Ordering exactly the mathematical volume of concrete is a rookie mistake. Subgrades are never level, forms bow under pour pressure, and spills happen. A 10% waste factor is the professional standard for residential slabs. Running short mid-pour means a cold joint — a structural weakness that can cause cracking along the seam.

Thickness Presets

5–10% standard

Material only

Thickness = 4" / 12 = 0.3333 ft
Volume = 20 × 20 × 0.3333 = 133.33 ft³
Cubic yards = 133.33 / 27 = 4.94 yd³
With waste = 4.94 × (1 + 10%) = 5.43 yd³
Cost = 5.43 × $150/yd³ = $815
Pure Volume
4.94
yd³ (no waste)
Order This Much (+10%)
5.43
yd³ with waste factor
≈ 1 truck load (8 yd³/truck)
Estimated Material Cost
$815
Concrete only (ex. labor, forms, finish)
Volume & Cost by Thickness (20 × 20 ft, 10% waste factor)
3.5" Sidewalk
4.75 yd³ / $713
4" Slab/Patio
5.43 yd³ / $815
5" Driveway
6.79 yd³ / $1,019
6" Light Truck
8.15 yd³ / $1,222

Practical Example

A 20 × 20 ft driveway at 4" thick:

Pure volume = 20 × 20 × (4/12) / 27 = 400 × 0.333 / 27 = 4.94 cubic yards
With 10% waste = 4.94 × 1.10 = 5.43 cubic yards to order
At $150/yd³ = $814 in concrete material costs

Standard concrete trucks deliver 8–10 yards. You'd order 1 partial truck. Anything under ~5 yards usually incurs a short-load fee of $50–$150. Factor this into your budget for small pours.

💡 Field Notes

  • Why waste factor matters: Subgrades are never perfectly flat — even a surveyed grade has low spots. Forms bow outward under the weight of wet concrete. Running short mid-pour forces a "cold joint" where fresh concrete is poured on partially-set concrete, creating a visible line and structural weakness. Always order more than the math says.
  • Thickness is cubed in importance: Doubling thickness from 4" to 8" doubles the concrete volume (and cost), but more than doubles load capacity. A 6" slab carries heavy equipment (forklifts, RVs); a 4" slab is for foot traffic and passenger vehicles only. Using the wrong thickness is a structural failure waiting to happen.
  • Mix design and PSI: Standard residential slabs use 3,000–4,000 PSI mix. Driveways should be 4,000 PSI minimum. Industrial floors require 4,500–5,000 PSI with fiber reinforcement. This calculator estimates volume only — always confirm mix design with your ready-mix supplier based on the load application.
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What is Concrete Volume Estimating and Subgrade Preparation?

Concrete estimating errors are among the most costly mistakes in construction. Ordering too little creates cold joints — structural weaknesses where fresh concrete is poured on partially-cured concrete. Ordering too much wastes expensive material. The solution is accurate volume math combined with a mandatory waste factor.

Mathematical Foundation

Slab Volume (Imperial)
Vyd3=L×W×(T/12)27V_{yd^3} = \frac{L \times W \times (T / 12)}{27}
LL= Slab length in feet.
WW= Slab width in feet.
T/12T / 12= Slab thickness in feet (divide inches by 12).
/27/ 27= 27 cubic feet per cubic yard. Concrete is sold by the cubic yard in the US.
Slab Volume (Metric)
Vm3=L×W×(T/1000)V_{m^3} = L \times W \times (T / 1000)
T/1000T / 1000= Thickness in meters (divide mm by 1000). Concrete is sold by the cubic meter in metric countries.
Order Quantity with Waste
Vorder=Vpure×(1+WF100)V_{order} = V_{pure} \times \left(1 + \frac{WF}{100}\right)
WFWF= Waste factor: 5% for flat industrial slabs, 10% for typical residential, 15%+ for irregular shapes.
VorderV_{order}= Quantity to order — round up to the nearest quarter-yard.
Material Cost
Cost=Vorder×CCost = V_{order} \times C
CC= Cost per cubic yard. US average: $125-$200/yard delivered. Material only — excludes labor, forms, rebar, finishing.

Laws & Principles

  • 10% waste is mandatory: Subgrades are never flat, forms bow outward under wet concrete pressure, and truck transitions cause spillage. Ordering the exact mathematical volume virtually guarantees coming up short. Running short mid-pour is a construction emergency.
  • Cold joints — fresh concrete placed against partially-cured concrete — create a plane of weakness susceptible to cracking and water infiltration. The only fix is to chip out and replace the section. Extra waste material ($75-150) is cheap insurance against a $2,000+ repair.
  • Slab load capacity scales with the square of thickness — a 6-inch slab carries about 2x the load of a 4-inch slab. Sidewalks: 3.5-4 inches. Driveways: 4 inches. Heavy vehicles: 5-6 inches. Forklifts: 6-8 inches with rebar.
  • Standard mixer trucks carry 8-10 cubic yards. Orders under 5 yards incur short-load fees ($50-$200). For pours under 3 yards, consider bagged concrete mixed on-site.

Step-by-Step Example Walkthrough

" A contractor pours a 20 x 20 ft residential driveway at 4-inch thickness. Ready-mix costs $150/cubic yard. "

  1. 1. Convert thickness: 4 in / 12 = 0.333 ft.
  2. 2. Volume: 20 x 20 x 0.333 = 133.3 cu ft.
  3. 3. Convert: 133.3 / 27 = 4.94 cubic yards.
  4. 4. Apply 10% waste: 4.94 x 1.10 = 5.43 yards.
  5. 5. Round up: order 5.5 cubic yards.
  6. 6. Material cost: 5.5 x $150 = $825.
  7. 7. Add short-load fee (~$100): total ~$925.
Final Result: Order 5.5 cubic yards at ~$825-925 in material costs. The 0.5-yard waste buffer ($75) is cheap insurance against a cold joint repair costing $2,000+.

Quick Answer: How much concrete do I need for a slab?

Multiply length × width × thickness (all in feet), divide by 27 to get cubic yards, then add 10% waste. A typical 20×20 ft driveway at 4 inches thick requires 5.5 cubic yards (including waste). At $150/yard delivered, that is about $825-$925 in material cost.

The Volume Formula

Yards = (L × W × T/12) ÷ 27 × (1 + Waste%)

Divide thickness in inches by 12 to convert to feet. Divide total cubic feet by 27 to convert to cubic yards. Always add a waste factor: 5% minimum for flat industrial slabs, 10% for typical residential, 15%+ for irregular shapes or uneven subgrades. Round up to the nearest quarter-yard — you cannot order fractional amounts from a ready-mix plant.

Concrete Yardage Quick Reference

Slab Size 4" Thick 5" Thick 6" Thick Est. Cost (4")
10 × 10 ft1.4 yd³1.7 yd³2.1 yd³$210-$280
12 × 12 ft2.0 yd³2.5 yd³3.0 yd³$300-$400
20 × 20 ft5.5 yd³6.9 yd³8.3 yd³$825-$1,100
24 × 24 ft7.9 yd³9.9 yd³11.9 yd³$1,185-$1,580
30 × 30 ft12.3 yd³15.4 yd³18.5 yd³$1,845-$2,460

All volumes include 10% waste factor. Cost estimates at $150-$200/yard delivered (2024 US average). Actual prices vary by region, mix design, and delivery distance. Short-load fees ($50-$200) apply to orders under 5 yards. Does not include labor, formwork, rebar, or finishing.

Real-World Scenarios

Residential Patio Pour

A 12×16 ft backyard patio at 4-inch thickness. Volume: (12 × 16 × 0.333) / 27 = 2.37 yards. At 10% waste: 2.6 yards (round to 2.75). Cost at $160/yard: $440. Since this is under 5 yards, expect a $75-$150 short-load fee from the ready-mix plant. Total material: ~$515-$590. Consider adding 6×6 W2.1/W2.1 welded wire mesh ($45 per roll) and control joints every 8 feet.

Commercial Warehouse Floor

A 60×100 ft warehouse floor at 6-inch thickness for forklift traffic. Volume: (60 × 100 × 0.5) / 27 = 111.1 yards. At 5% waste (flat, controlled pour): 116.7 yards — about 12 truckloads. Cost at $140/yard: $16,338. At this volume, negotiate a project price with the batch plant. Schedule trucks at 15-20 minute intervals to avoid cold joints. Budget 4-6 finishers and a laser screed machine for flatness tolerance.

Pro Tips

Do This

  • ✓Always order 10% more than the calculated volume. Forms bow outward, subgrades have dips, chute transitions spill. The $75-150 cost of extra concrete is cheap insurance against a cold joint repair ($2,000+). The most expensive concrete on any project is the second truck you did not plan for.
  • ✓Convert all measurements to feet before calculating. Thickness is usually specified in inches — divide by 12 to get feet before multiplying. A common error is calculating 20 × 20 × 4 = 1,600 and dividing by 27 to get 59 yards. The correct volume is 20 × 20 × (4/12) = 133.3 cu ft ÷ 27 = 4.94 yards.
  • ✓Use bagged concrete for pours under 2 cubic yards. At ~0.6 cu ft per 80 lb bag, 2 cubic yards requires ~90 bags ($450-$540 at $5-6/bag). No delivery fee, no minimum order, and you can pace your pour across a weekend. Above 2 yards, ready-mix is almost always cheaper and faster.

Avoid This

  • ✗Don't forget to deduct for monolithic footings or thickened edges. Driveways and garage slabs often have thickened edges (12-18 inches) where the slab meets the foundation. These consume significantly more concrete than the 4-inch flat area. Calculate the thickened edge volume separately and add it to the slab volume.
  • ✗Don't use a sidewalk thickness for a driveway. A 3.5-inch sidewalk is designed for foot traffic only. A passenger car weighs 4,000 lbs — a pickup truck 6,000+. Driveways need 4 inches minimum, and 5-6 inches if heavy trucks, RVs, or equipment will park on them. Under-thickness slabs crack within 2-3 years.
  • ✗Don't order without checking delivery access. A loaded concrete truck weighs 60,000+ lbs and is 35 feet long. Verify the truck can reach the pour site without crossing septic systems, underground utilities, or hillside terrain that cannot support the weight. If the truck cannot get close, you need a concrete pump — add $800-$1,500 to the budget.

Frequently Asked Questions

How many cubic yards of concrete do I need for a 20×20 slab?

At 4 inches thick with 10% waste: 5.5 cubic yards. At 6 inches thick: 8.3 yards. Formula: 20 × 20 × (4/12) = 133.3 cu ft ÷ 27 = 4.94 yards, plus 10% = 5.43, rounded up to 5.5. At $150/yard delivered, budget $825 in material cost plus a possible short-load fee since 5.5 yards is under a full truck load.

What is a cold joint and why is it dangerous?

A cold joint occurs when fresh concrete is poured against concrete that has already begun its initial set (2-4 hours after placement). The new concrete cannot chemically bond to the hardened surface, creating a visible seam that is a plane of weakness. Cold joints crack under load, allow water infiltration, and in structural applications can be a code violation. The fix requires chipping out and replacing the affected section — always more expensive than the extra concrete that would have prevented running short.

Should I use bagged concrete or ready-mix?

For pours under 2 cubic yards (about 90 bags at 80 lbs each), bagged concrete is often cheaper because there is no delivery fee or short-load charge. For 2-5 yards, ready-mix is faster but expect a short-load fee of $50-$200. For over 5 yards, ready-mix is always cheaper and far more practical. A single ready-mix truck delivers in 10 minutes what would take a crew hours to mix by hand from bags.

How much does a concrete slab cost per square foot?

Material only: $1.85-$2.50 per sq ft for a 4-inch slab ($2.75-$3.75 for 6-inch). Installed (material + labor + forms + grading): $6-$12 per sq ft for a basic broom-finished slab, $10-$18 for stamped or colored concrete. Costs vary significantly by region — metro areas are 20-40% higher than rural. For large projects over 500 sq ft, get at least three contractor bids. The cheapest bid often means thinner concrete, less rebar, and poor drainage prep.

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