How to Calculate Concrete Quantity
Concrete is sold by the cubic yard for ready-mix delivery, and by the bag for retail. The math always starts with volume — length × width × depth — but the conversions and unit handling trip up first-time buyers. Our calculator handles all the math; here's the underlying formula so you can verify any result.
Cubic yards = Cubic feet ÷ 27
Bags of 80 lb = Cubic feet ÷ 0.60 (each bag = 0.60 ft³)
Bags of 60 lb = Cubic feet ÷ 0.45
The key gotcha: thickness is usually in inches but length and width are in feet. Convert thickness to feet first (divide by 12) before multiplying, or use a calculator that handles units automatically. Mixing units gives results that are wildly wrong — by a factor of 12 or 144.
Worked example: typical residential patio
A 20 ft × 20 ft patio slab at 4 inches thick:
- Volume in cubic feet: 20 × 20 × (4 ÷ 12) = 133.33 ft³
- Cubic yards: 133.33 ÷ 27 = 4.94 yd³
- With 10% waste factor: Order 5.5 yd³ from ready-mix
- Cost (ready-mix at $165/yd³): 5.5 × $165 = $908
- Bagged equivalent (245 bags of 80 lb at $6): $1,470 (61% more expensive)
Concrete Requirements by Project Type
Different concrete projects need different shapes, thicknesses, and strength specifications. Here are the most common applications.
Slabs (patios, garages, driveways, sheds)
Standard 4-inch thickness for patios and walkways. Garages and driveways: 4-6 inches. Heavy vehicle areas (RV pads, commercial): 6-8 inches. Always include 4 inches of crushed stone base underneath plus a 6 mil vapor barrier for interior slabs. Reinforce with #3 or #4 rebar grid at 16-24 inch spacing, or use wire mesh for residential thicknesses.
Footings (foundations, columns, fence posts)
Width and depth depend on what's being supported. Typical residential foundation footing: 16 inches wide × 8 inches deep. For 100 linear feet of footing: 1.65 cubic yards. Fence posts: 10-12 inch diameter × 24-36 inch deep holes. Footings always need to extend below the frost line in cold climates — 36-48 inches in northern US.
Columns and piers
Round or square columns for deck supports, pole barns, fence corners. A 12-inch diameter × 4 ft tall column uses 3.14 cubic feet (~0.12 cubic yards). For a deck with 4 such piers: 12.6 cubic feet (~0.47 cubic yards), or 21 bags of 80 lb. Use SonoTube cardboard forms for clean round columns.
Steps and stairs
Concrete steps are complex because each step adds a wedge of concrete on top of the previous. For 3 steps at 7-inch rise × 11-inch tread × 4 ft wide: roughly 7.7 cubic feet (~0.29 cubic yards). Formwork is the hard part — measure carefully or hire a contractor for stairs.
Driveway expansion (extension or repair)
Typical residential driveway extension: 10 ft × 20 ft × 4 inches = 200 sq ft × 0.333 ft = 66.67 cubic feet (2.47 cubic yards). For a full driveway pour: 20 × 40 ft at 5 inches = 12.35 cubic yards. Major project — schedule ready-mix delivery and have crew ready before the truck arrives.
Sidewalks and walkways
4-inch thick, 3-4 ft wide typically. For 50 linear feet at 3 ft wide × 4 inches: 50 ft³ (~1.85 cubic yards). Use expansion joints every 5 feet to prevent random cracking. Score control joints 1 inch deep within 24 hours of pour.
Concrete Coverage Reference
How many cubic yards or bags you need for common slab dimensions:
| Slab size | At 4 inches thick | At 5 inches | At 6 inches |
|---|---|---|---|
| 4 × 4 ft | 0.20 yd³ / 9 bags | 0.25 yd³ / 11 bags | 0.30 yd³ / 14 bags |
| 8 × 8 ft | 0.79 yd³ / 36 bags | 0.99 yd³ / 45 bags | 1.18 yd³ / 54 bags |
| 10 × 10 ft | 1.23 yd³ / 56 bags | 1.54 yd³ / 70 bags | 1.85 yd³ / 84 bags |
| 12 × 12 ft | 1.78 yd³ / 80 bags | 2.22 yd³ / 100 bags | 2.67 yd³ / 120 bags |
| 16 × 16 ft | 3.16 yd³ / 143 bags | 3.95 yd³ / 178 bags | 4.74 yd³ / 213 bags |
| 20 × 20 ft | 4.94 yd³ / 222 bags | 6.17 yd³ / 278 bags | 7.41 yd³ / 333 bags |
| 24 × 24 ft | 7.11 yd³ / 320 bags | 8.89 yd³ / 400 bags | 10.67 yd³ / 480 bags |
Bags assume standard 80 lb bags (0.60 ft³ each). Add 10% to these figures for waste — irregular slabs or sloped sites add 15%. Round up to whole bags or to the nearest half yard for ready-mix orders.
Concrete Cost Breakdown 2026
| Type | Per cubic yard | Per bag (retail) | Notes |
|---|---|---|---|
| Standard ready-mix (3,000 psi) | $130–160 | — | Most residential use |
| 4,000 psi ready-mix | $150–180 | — | Driveways, structural |
| 5,000 psi ready-mix | $170–210 | — | Heavy commercial |
| Bagged 80 lb (Quikrete/Sakrete) | $270 equivalent | $5–7 | 0.60 ft³ per bag |
| Bagged 60 lb | $300 equivalent | $4–6 | 0.45 ft³ per bag |
| Bagged 40 lb (small repair) | $340 equivalent | $3–4 | 0.30 ft³ per bag |
| Rapid-set (60 min cure) | — | $8–12 / 50 lb | Posts, repairs only |
| Concrete pumping (if needed) | $200–500 setup + per yard fee | — | Pools, far pours |
Additional costs to budget: short-load fee ($50-150) for orders under 8 yards, wait time beyond 5 minutes ($1-3/minute), Saturday/holiday delivery premium ($100-200), fuel surcharge ($25-100). For DIY projects with bagged concrete: rent a concrete mixer for $40-80/day or mix in a wheelbarrow for very small batches.
Reinforcement: Rebar and Wire Mesh
Concrete handles compression well but is weak in tension. Reinforcement adds tensile strength and prevents cracking. Choose between rebar and wire mesh based on slab type:
Wire mesh (for thin slabs)
6x6 inch W1.4×W1.4 welded wire mesh is standard for residential 4-inch slabs. Comes in 5x10 ft sheets ($25-40 each) or 5-ft wide rolls. Position in the middle of the slab thickness using mesh chairs or rocks. Don't lay flat on the ground — it must be elevated to work properly.
Rebar (for thicker slabs and driveways)
#3 rebar (3/8 inch) for residential slabs at 16-24 inch grid spacing. #4 rebar (1/2 inch) for driveways at 12-16 inch spacing. #5 (5/8 inch) for heavy commercial use. Cost: $4-8 per 10-ft section depending on size. Lap rebar 12-16 inches at all joints; wire-tie at every intersection.
Fiber reinforcement
Synthetic or steel fibers added to the concrete mix at the plant. Adds tensile strength without separate rebar work. Cost: $5-15 per cubic yard upcharge. Works great for thin residential slabs but not a substitute for primary reinforcement in structural applications.
How to Order Ready-Mix Concrete
- Calculate volume. Use this calculator. Add 10% waste factor for safety.
- Choose strength (psi). 3,000 psi for residential slabs; 4,000 psi for driveways; 5,000+ for heavy commercial.
- Specify slump. 4-inch slump is standard for most slabs (slightly stiff). 5-6 inch slump for pours that need to flow into forms.
- Add air entrainment if needed. Required for exterior slabs in freeze-thaw climates (5-7% air content).
- Schedule delivery. Confirm 24 hours ahead. Have 30-minute unload window — truck waits cost $50-100/hour beyond that.
- Prep forms and reinforcement before truck arrives. Vapor barrier, rebar/mesh, edge forms, control joint plans.
- Have enough labor for the pour. Concrete sets fast. Plan 1-2 people per cubic yard delivered, depending on access.
- Finishing tools ready. Bull float, edger, fresno trowel, broom for non-slip texture. Hire a finisher for jobs over 10 yards.
Concrete Curing: The Critical 28 Days
Concrete strength develops over time, not instantly. The curing process determines whether your concrete reaches its rated strength.
- 0-4 hours: Initial set. Concrete becomes too stiff to remix.
- 4-24 hours: Final set. Concrete is firm but still gaining strength rapidly.
- 1-7 days: Strength develops to about 50-70% of final. Keep concrete moist with plastic sheeting, wet burlap, or curing compound.
- 7 days: Generally drivable with passenger vehicles, but not heavy trucks.
- 14 days: About 90% of design strength.
- 28 days: Full design strength reached. The standard reference for concrete strength testing.
Critical mistake: letting concrete dry too quickly. In hot, dry, or windy conditions, surface evaporation pulls water out faster than the concrete can use it for hydration. Cover with plastic sheeting within 2-3 hours of finishing, mist throughout the first 24 hours, and keep moist for at least 7 days.
Common Concrete Mistakes
- Skipping the base layer: No crushed stone under slab = drainage issues, frost heave, premature cracking.
- Mixing units: Thickness in inches × length in feet = wrong answer. Convert first.
- Adding too much water: "Wet" mix is easier to work but dramatically weaker. Stick to the recommended water-cement ratio.
- Skipping reinforcement: 4-inch slabs need wire mesh minimum. Driveways need rebar. No reinforcement = cracks within 1-2 years.
- No control joints: Score joints every 5-10 feet within 24 hours. Without them, concrete cracks randomly where you don't want it.
- Pouring in extreme weather: Below 40°F or above 90°F creates major curing problems. Schedule accordingly.
- Not curing properly: Letting fresh concrete dry out is the most common DIY failure. Keep moist 7 days minimum.