Concrete
How to Calculate Concrete for a Slab
Slab-specific concrete estimating: choosing thickness, handling thickened edges and footings, base and vapor barrier allowances, plus a full worked takeoff.
Slabs look like the simplest possible concrete estimate, and the field portion is. What catches people is everything a "length × width × thickness" mental model leaves out: thickened perimeter edges, interior footings under posts, the extra volume lost to a rough subgrade, and the base layers that have their own takeoff. This guide walks the full slab estimate the way an estimator would.
Step 1: Choose the right thickness
| Use | Typical thickness | Notes |
|---|---|---|
| Patio, walkway | 4 in | Foot traffic only |
| Shed slab | 4 in | Thicken to 6–8 in under heavy equipment |
| Driveway (cars) | 4–5 in | 5 in strongly preferred; add 1 in in freeze climates |
| Garage floor | 5–6 in | Often with thickened perimeter |
| RV pad / light truck | 6 in | Reinforcement per design |
Thickness drives volume linearly - going from 4 to 5 inches adds 25% to the order. Decide thickness first, from use and local code, then estimate.
Step 2: Field slab volume
For a 24 × 24 ft garage at 5 in: 24 × 24 × 0.417 ÷ 27 = 8.89 yd³.
Step 3: Thickened edges - the volume everyone forgets
Monolithic slabs thicken at the perimeter, typically to 12 in wide × 12 in deep. That band is additional depth below the field slab:
Garage example: perimeter = 96 ft. Extra depth = 12 − 5 = 7 in = 0.583 ft. Edge volume = 96 × 1.0 × 0.583 = 56 ft³ = 2.07 yd³ - a 23% addition that flat math misses entirely.
Step 4: Interior thickenings and pads
Post footings, equipment pads, and bearing-wall thickenings inside the slab each add volume. Treat each as a small rectangle or cylinder at its extra depth and add them up. Six 2×2 ft interior pads 7 in deeper than the field slab add 6 × 2 × 2 × 0.583 = 14 ft³ (0.5 yd³).
Step 5: Total with waste
Garage running total: 8.89 + 2.07 + 0.5 = 11.46 yd³. With 10% waste = 12.6 yd³. Order 13 yards - likely two truckloads, so coordinate spacing with the plant.
Estimate the field slab in seconds, then add edge bands using the method above.
Open the Concrete CalculatorDon't forget the base takeoff
The slab estimate isn't finished until the layers under it are counted: typically 4 in of compacted gravel base (sized 12 in beyond the slab footprint on all sides), a vapor barrier for interior slabs, and reinforcement. The base is a separate material order in tons - run it through the gravel calculator, and see What Goes Under a Concrete Slab? for the full stack.
Slab estimating mistakes ranked by cost
1. Missing thickened edges (15–25% short). The truck leaves, the perimeter is half-filled, and the fix is a cold joint in the most highly stressed part of the slab.
2. Measuring formed dimensions wrong (5–15% either way). Measure the actual inside-of-forms dimensions after formwork is set, not the plan dimensions.
3. Trusting a "flat" subgrade (5–10% short). Every low spot fills with concrete first. Compact and screed the base to grade before pour day.
4. Ordering exact volume (risk of catastrophe). Round up to the half yard. Have a plan for surplus - a couple of form tubes for future post footings make good use of leftovers.
When to call a professional
Slabs on expansive clay, uncontrolled fill, or slopes; slabs supporting structures; and any slab where a crack would be expensive deserve engineered design. A geotechnical opinion on the subgrade costs far less than replacing a failed slab.
Frequently Asked Questions
How thick should a concrete slab be?
4 inches for foot traffic (patios, walkways, shed floors); 5-6 inches for passenger vehicles (driveways, garages); 6+ inches with engineered reinforcement for trucks and equipment. Codes and soil conditions can require more.
What is a thickened edge slab?
A slab whose perimeter deepens to 8-12 inches to stiffen the edge and spread loads - common for garages, sheds, and monolithic (slab-plus-footing-in-one-pour) construction. The thickened band adds significant volume that flat-slab math misses.
Do I subtract the rebar volume from concrete?
No. Reinforcing steel displaces well under 1% of slab volume, far less than the waste factor. Estimators ignore it.
How many bags of concrete for a 12x12 slab?
At 4 inches: 48 cubic feet, about 53 cubic feet with 10% waste, or 88 bags of 80 lb mix. That's a hard day of mixing - at this size, compare ready-mix seriously.
Should the slab overhang its gravel base?
No - the base should extend 12 inches beyond the slab edge on all sides for edge support and drainage. Size the base area larger than the slab, not equal to it.