Concrete Calculator
Calculate concrete in cubic yards and 80-lb, 60-lb, or 50-lb pre-mixed bags for slabs, footings, cylindrical sonotubes, and fence posts. Compare ready-mix batch delivery against bagged mix economics.
Concrete Slab, Footing & Column Calculator
Calculate yards for ready-mix dispatch or bag counts for 80-lb, 60-lb, and 50-lb mixes
ACI recommendation: +10% over-pour margin for slabs
Net pour: 2.47 yd³ (73.3 ft³)
How to Calculate Concrete Yards: Formulas for Slabs & Cylinders
Commercial ready-mix plants batch concrete in cubic yards (27 cubic feet). Formwork lumber, excavation depth, and post holes are measured in inches. Dimensional consistency is essential:
Add 8% to 10% allowance for form board deflection and uneven subgrade gravel.
Where Radius in feet = (Diameter in inches ÷ 2) ÷ 12.
Worked Contractor Example: 24' × 24' Garage Slab at 4" Thickness
Suppose you are pouring a standard two-car residential garage slab measuring 24 feet by 24 feet with a 4-inch nominal thickness:
- Calculate surface area: 24 ft × 24 ft = 576 sq ft
- Calculate net geometric volume: (576 sq ft × 4 in) ÷ 324 = 7.11 cu yds
- Add 10% allowance for 2x4 form flex and subgrade variation: 7.11 × 1.10 = 7.82 cu yds
- Ready-Mix Plant Order: Round up to 8.00 cubic yards (plants batch in 0.25-yd increments).
- Bagged mix alternative: 7.82 cu yds × 45 bags/yd = 352 eighty-pound bags (approx. 28,160 lbs)
Ready-Mix Truck Ordering: Short-Load Fees & Dispatch Rules
Coordinating a concrete pour requires navigating commercial dispatch regulations:
Standard 3-axle or 4-axle front/rear discharge mixer trucks carry up to 10 yards. Gross vehicle weight exceeds 65,000 to 75,000 lbs. Ensure jobsite access routes can support the weight.
Ordering less than a minimum load incurs an under-load fee. For pours between 1.0 and 3.5 yards, calculate whether the fee still makes ready-mix cheaper than 150 bags of dry concrete.
Concrete must be discharged within 90 minutes of plant water contact. Plants provide 5 to 7 minutes per cubic yard of free unloading time, billing $2 to $4/minute for delays.
Bagged Concrete vs. Ready-Mix Truck: Financial & Labor Thresholds
The decision between bags and a truck is both economic and structural:
| Volume Required | 80-lb Bags (0.60 cu ft) | Total Bag Weight | Estimated Bagged Cost | Ready-Mix Truck Cost | Recommended Method |
|---|---|---|---|---|---|
| 0.5 cu yd (13.5 cu ft) | 23 bags | 1,840 lbs | $138.00 | $275.00 (min fee) | Bagged (Wheelbarrow Mix) |
| 1.0 cu yd (27.0 cu ft) | 45 bags | 3,600 lbs | $270.00 | $310.00 (short-load) | Towable Drum Mixer / Bags |
| 2.0 cu yds (54.0 cu ft) | 90 bags | 7,200 lbs | $540.00 | $380.00 | Ready-Mix Truck (Save $160) |
| 6.0 cu yds (162.0 cu ft) | 270 bags | 21,600 lbs | $1,620.00 | $840.00 | Ready-Mix Truck (Save $780) |
The primary danger of hand-mixing large concrete slabs in a wheelbarrow is the formation of cold joints. Concrete begins initial setting within 60 to 90 minutes. If you mix 50 bags by hand, the first section will begin stiffening before you finish pouring the middle. When the two batches meet, they will not knit together, leaving a permanent structural fissure that cracks under freezing weather.
Compressive Strength (PSI), Slump & Cylindrical Sonotube Depths
Specifying concrete requires balancing compressive strength (pounds per square inch after 28 days of moist curing) and workability (slump measured via the ASTM C143 slump cone test):
Standard mix for residential sidewalks, air conditioner pads, post footings, and interior basement slabs protected from freezing temperatures.
Required for passenger vehicle driveways and garage floors. In northern regions, specify 5% to 7% air entrainment to resist freeze-thaw spalling and winter deicing salts.
Engineered for heavy delivery truck loading, commercial approach ramps, and rapid form stripping. Contains high Portland cement ratios and water-reducing admixtures.
Sonotube Post Hole & Pier Concrete Requirements
Deck footings, pergolas, and pole barns must extend below local geographic frost lines (typically 36" to 48" in northern US states) to prevent winter frost heaving. The reference table below details volume and bag requirements per cylindrical pier:
| Sonotube Diameter | Hole Depth | Volume (cu ft) | Volume (cu yds) | 80-lb Bags per Pier | Typical Pier Application |
|---|---|---|---|---|---|
| 8-Inch Round Tube | 36 inches (3 ft) | 1.05 cu ft | 0.039 yd³ | 1.8 (use 2) | Standard fence post, mailbox |
| 10-Inch Round Tube | 36 inches (3 ft) | 1.64 cu ft | 0.061 yd³ | 2.7 (use 3) | Single-level residential deck pier |
| 10-Inch Round Tube | 48 inches (4 ft) | 2.18 cu ft | 0.081 yd³ | 3.6 (use 4) | Deep frost deck footing (Northern US) |
| 12-Inch Round Tube | 48 inches (4 ft) | 3.14 cu ft | 0.116 yd³ | 5.2 (use 6) | Multi-story deck column, porch roof support |
Concrete Calculator FAQs
How do you calculate cubic yards of concrete for a slab?
Measure the length and width of the slab in feet, and thickness in inches. Multiply length by width to find square footage, multiply by thickness in inches, and divide by 324: Formula: Cubic Yards = (Length ft × Width ft × Thickness in) ÷ 324. Always add an 8% to 10% waste allowance to compensate for subgrade unevenness, formwork deflection, and spillage. For example, a 20 ft by 20 ft slab at 4 inches thick requires (20 × 20 × 4) ÷ 324 = 4.94 cu yds + 10% = 5.43 cubic yards.
How many 80-lb or 60-lb bags of concrete make one cubic yard?
Based on ASTM C387 volumetric specifications: an 80-lb bag yields approximately 0.60 cubic feet, requiring exactly 45 bags per cubic yard (27 ÷ 0.60 = 45). A 60-lb bag yields approximately 0.45 cubic feet, requiring exactly 60 bags per cubic yard (27 ÷ 0.45 = 60). A 50-lb fast-setting bag yields 0.375 cubic feet, requiring 72 bags per cubic yard.
When should I order a ready-mix truck instead of mixing bags by hand?
The contractor rule of thumb is 1.5 cubic yards. Any project requiring more than 1.5 cubic yards (68 eighty-pound bags, or over 5,400 lbs of dry material) should be ordered from a ready-mix batch plant. Hand-mixing that volume in a wheelbarrow takes hours, creating dangerous "cold joints" where previously placed batches begin setting before new batches are poured. For pours under 1.0 to 1.5 yards, bagged mix avoids ready-mix short-load fees.
What is a ready-mix short-load fee?
Commercial ready-mix concrete suppliers operate large transit mixer trucks with 9 to 11 cubic yard drum capacities. Because running an 80,000-lb truck with a small load costs nearly as much in fuel and driver labor as a full load, suppliers charge a "short-load fee" (typically $120 to $250) on orders smaller than 4 to 5 cubic yards.
What is the ASTM C94 90-minute concrete discharge rule?
ASTM C94 (Standard Specification for Ready-Mixed Concrete) mandates that concrete must be discharged from the truck drum within 90 minutes after batch water is added to the cement at the plant, or before the drum has turned 300 revolutions. In ambient summer temperatures exceeding 85°F (29°C), hydration accelerates and this window narrows to 60 minutes.
How much concrete is needed for a cylindrical post hole or sonotube?
Calculate cylindrical volume using the equation: Volume (yd³) = [ π × (Diameter in ft ÷ 2)² × Depth ft ] ÷ 27. For example, a standard 10-inch diameter post hole (0.417 ft radius) dug 3 feet deep holds [3.14159 × (0.417)² × 3] ÷ 27 = 0.0608 cu yds (1.64 cu ft). That equals approximately 2.7 eighty-pound bags (round to 3 bags) per post hole.
What concrete compressive strength (PSI) should I specify?
For residential applications: use 2,500 PSI for non-structural fence posts and stepping pads; 3,000 PSI for standard walkways, shed floors, and basement slabs; 4,000 PSI for garage slabs, driveways, and all exterior flatwork in northern freeze-thaw climates (accompanied by 5% to 7% air entrainment to resist road salts).
Why does adding water on the jobsite weaken concrete?
Concrete cures through a chemical reaction called hydration, not drying. Adding excessive water on the jobsite increases workability (slump) but dilutes the water-cement ratio. Every additional gallon of unapproved water added to a 10-yard mixer truck increases slump by about 1 inch but permanently reduces compressive strength by 150 to 200 PSI and increases drying shrinkage cracking.
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