How much concrete do I need? Slab, footing, and pier yardage

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.

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.