Order short, you're stopping the truck and paying short-load fees. Order long, you're paying for concrete that's hardening in the chute. Either way you've made the day worse and added cost. The math is simple — what gets people in trouble is forgetting overage and assuming the form is the size on the drawing.

The three formulas

Slab (rectangular):
  yards = (L × W × T) / 27
  L, W, T in feet (T = thickness, e.g. 4" = 0.333')

Footing (continuous):
  yards = (L × W × D) / 27
  L = total run length, W = width, D = depth (all feet)

Pier or column (cylindrical):
  yards = (π × r² × H) / 27
  r = radius (feet), H = height (feet)

27 ft³ = 1 cubic yard

Slab thickness reference

• 4" — residential interior slabs, sidewalks, patios — 0.333 ft
• 5" — driveways, light loads — 0.417 ft
• 6" — heavy driveways, garage floors, light commercial — 0.500 ft
• 8" — shop floors, equipment pads, occasional truck loads — 0.667 ft
• 10–12" — heavy industrial, frost-heave-protected slabs — 0.833–1.000 ft

Worked example: 24 × 36 garage slab at 6"

Dimensions:    24 ft × 36 ft × 0.500 ft (6")
Volume:        24 × 36 × 0.500 = 432 ft³
Yards:         432 / 27 = 16.0 cu yd

Add 10% overage:
  16.0 × 1.10 = 17.6 cu yd

Order 17.75 cu yd (round up to 1/4 yard).
Most plants charge by 1/4 or 1/2 yard increments.

Worked example: 80 ft of 16" × 8" continuous footing

Dimensions:    80 ft × 1.333 ft × 0.667 ft
Volume:        80 × 1.333 × 0.667 = 71.1 ft³
Yards:         71.1 / 27 = 2.63 cu yd

Add 15% overage:
  2.63 × 1.15 = 3.03 cu yd

Order 3.25 cu yd. Footings always run heavy because the trench
isn't perfectly cut — call it 15% minimum.

Worked example: six 12" piers × 4 ft deep

Per pier:      π × (0.5)² × 4
               = 3.1416 × 0.25 × 4
               = 3.14 ft³
Six piers:     6 × 3.14 = 18.8 ft³
Yards:         18.8 / 27 = 0.70 cu yd

Add 20% overage (drilled holes are never the spec diameter):
  0.70 × 1.20 = 0.84 cu yd

Order 1.0 cu yd. Most plants have a 1 yd minimum anyway.

The 90-minute rule

Most ready-mix has a 90-minute window from batching to placement. Past that, the load is supposed to be rejected — though in summer heat the real window can be closer to 60 minutes. If you've got a long job, schedule trucks in waves rather than parking three at once.

Mix design quick reference

• 2,500 psi — light residential, footings under 4 ft
• 3,000 psi — most residential slabs, footings, walls
• 4,000 psi — driveways, garages, exposed concrete (default for "spec it strong")
• 4,500–5,000 psi — commercial, heavy loads, frost-exposed
• 3/8" pea gravel — pumped jobs, thin slabs, decorative
• 3/4" stone — most slab work — the default
• 1" stone — heavy footings, mass pours
• 4–6% air entrainment — anywhere with freeze/thaw

Rebar quick reference

• 4" slab, residential — #3 @ 18" o.c. each way, or fiber mesh
• 5–6" driveway — #4 @ 16" o.c. each way
• Garage / shop floor — #4 @ 12" o.c. each way
• Footings — 2× #4 continuous top + bottom (residential), more for engineered
• Cover: 3" when poured against earth, 2" when formed and exposed to weather, 1.5" interior slabs

Common mistakes

• Calculating to the spec dimension instead of the actual form. Forms move. Trenches widen.
• Forgetting overage. The truck won't wait while you measure.
• Pouring on frozen or saturated subgrade. Both kill long-term performance.
• Not having enough hands. A 6+ yd slab pour is a 4-person minimum.
• Skipping the control joints. Concrete cracks. Tell it where.

Run it on your phone

The BuilderCalc app does all of this — slab, footing, pier, and column volumes with overage built in, plus beam span, joist, rafter, IRC stairs, drywall, paint, and material takeoff. 100% offline, free on the App Store and Google Play.

Related

• BuilderCalc — construction calculator on iOS + Android
• NEC wire sizing for branch circuits

Note: Starting points only. Always check local code, your engineer's drawings, and the ready-mix supplier's recommendations for the actual mix design and conditions on site.