Concrete Calculator: How to Never Order Too Much — or Too Little

Running short on a concrete pour is one of the most expensive mistakes in construction. The ready-mix truck is on the clock. Your crew is standing around watching the first pour begin to set. A short-load delivery from the plant costs $150-300 in additional fees, and if you cannot get one in time, you may be looking at a cold joint — a structural weakness where new concrete meets partially set concrete.

Our concrete calculator handles slabs, footings, columns, walls, steps, and mixed multi-section projects. This guide covers the method and the mistakes to avoid.

The universal formula: volume in cubic yards

Concrete is sold by the cubic yard. One cubic yard = 27 cubic feet. Every calculation reduces to:

Volume (cubic yards) = Length (ft) x Width (ft) x Depth (ft) / 27

Always convert depth from inches to feet before calculating: divide by 12.

Worked example: standard driveway slab

20 ft wide, 40 ft long, 4 inches thick

Depth in feet: 4 / 12 = 0.333 ft

Volume = 20 x 40 x 0.333 / 27 = 266.4 / 27 = 9.87 cubic yards

Add 10% waste factor: 9.87 x 1.10 = 10.86 → order 11 yards

Waste factors by application

Application	Waste factor	Reason
Simple rectangular slab	8-10%	Minor form imprecision, spillage
Footings and walls	10-15%	Irregular soil, form bulge
Steps and irregular shapes	15-20%	Complex geometry
Pump jobs (concrete pumped)	Add 0.5 yards minimum	Concrete left in pump lines
Old soil subgrade	Add 5%	Soft spots may require extra depth

Common slab applications

Garage floor

A standard two-car garage: 20 ft x 20 ft x 4 inches (0.333 ft)

Volume = 20 x 20 x 0.333 / 27 = 133.2 / 27 = 4.93 yards

With 10% waste = 5.5 yards

Sidewalk

4 ft wide, 60 ft long, 4 inches thick

Volume = 4 x 60 x 0.333 / 27 = 79.9 / 27 = 2.96 yards

With 10% = 3.3 yards → order 3.5 yards

Patio

12 ft x 16 ft, 4 inches thick

Volume = 12 x 16 x 0.333 / 27 = 63.9 / 27 = 2.37 yards

With 10% = 2.6 yards → order 2.5 or 3 yards

Continuous footings

Foundation footings run around the perimeter and under load-bearing walls. Calculate the linear footage and then apply the cross-section dimensions.

Volume = Linear ft x Width (ft) x Depth (ft) / 27

Example: perimeter footing

House footprint: 40 ft x 30 ft = 140 linear feet of perimeter. Footing: 18 inches wide, 12 inches deep.

Volume = 140 x 1.5 x 1.0 / 27 = 210 / 27 = 7.78 yards

With 12% waste = 8.7 yards → order 9 yards

Round and cylindrical concrete

For column footings, piers, and sonotubes:

Volume = π x (diameter/2)² x height / 27

Example: sonotube for deck post

12-inch diameter tube (0.5 ft radius), 3.5 ft deep (48 inches below frost line)

Volume = 3.14159 x (0.5)² x 3.5 / 27 = 3.14159 x 0.25 x 3.5 / 27 = 2.75 / 27 = 0.102 yards

For 8 posts: 0.102 x 8 = 0.82 yards. With 15% waste = 0.94 yards → 1 yard, or use bags

When to use bags vs ready-mix

Project size	Best option	Notes
Under 0.5 yards	Bagged concrete mix	Ready-mix minimum load fees make it expensive
0.5 - 1.0 yards	Either (compare costs)	Ready-mix short-load fees may apply
1.0+ yards	Ready-mix	Labor and time savings justify cost
Large pours (8+ yards)	Ready-mix, possibly pump	Pump adds cost but reduces labor for large slabs

Bags needed for small jobs

Standard bags yield approximately:

• 40 lb bag: 0.011 cubic yards (0.30 cubic feet)
• 60 lb bag: 0.017 cubic yards (0.45 cubic feet)
• 80 lb bag: 0.022 cubic yards (0.60 cubic feet)

For 0.5 yards using 80 lb bags: 0.5 / 0.022 = 22.7 → 23 bags

Concrete mix design by application

Application	Min compressive strength	Common mix
Residential driveway	4,000 PSI	Air-entrained in freeze-thaw climates
Sidewalk, patio	3,500 PSI	Air-entrained if exposed to deicers
Foundation, footings	3,000-4,000 PSI	Per structural engineer specs
Garage floor	4,000 PSI	Air-entrained; fiber reinforcement optional
Countertops	5,000+ PSI	GFRC or specialized mix

The three most common ordering mistakes

1. Forgetting to convert inches to feet. Entering 4 (inches) instead of 0.333 (feet) gives a result 12x too large. Always divide inch measurements by 12 first.
2. Skipping the waste factor. Even a perfectly formed rectangular slab needs 8-10% extra. Forms bow under pressure, subgrade has soft spots, and spillage is inevitable.
3. Rounding down instead of up. Round up to the nearest half-yard when ordering. Running short is far more disruptive than having a small overage.

Use the concrete calculator for any shape or combination of shapes in a single project — it adds them all up and applies the right waste factor for each section.