Concrete Slab Calculator
Calculate ready-mix concrete yardage, gravel base tonnage, and rebar reinforcement grid sticks for slabs, patios, and garage floors.
Area: 576 sq ft | ~0.8 Ready-Mix Truckloads (10-yd)
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What Is a Concrete Slab Calculator?
A concrete slab calculator models complete structural ground slabs for patios, driveways, garages, and shed foundations. Beyond basic concrete yardage, it calculates the underlying crushed gravel subbase, #4 rebar grid sticks (at 12" or 18" on-center), and ready-mix truckload cycles.
How to Use This Calculator
- Enter slab length and width in feet.
- Enter slab thickness in inches (4" for patios/sheds, 5–6" for garages/driveways).
- Enter gravel subbase thickness (typically 4 inches).
- Select rebar grid spacing (18" standard or 12" heavy duty).
- Review ready-mix yards, gravel tons, rebar sticks, and truckloads.
Complete Slab Engineering Takeoff
V_{\text{slab}} = \frac{L \times W \times (T / 12)}{27} \times 1.10, \quad \text{Rebar Sticks} = \left\lceil \frac{(N_L \times L + N_W \times W) \times 1.10}{20} \right\rceilIncludes 10% concrete contingency, 15% gravel compaction loss, and 10% rebar overlap splices.
Worked Example
Scenario: Pouring a 24 ft by 24 ft two-car garage slab, 4 inches thick, with 4 inches of gravel base and #4 rebar @ 18" OC.
Concrete volume: 576 sq ft × 0.333 ft ÷ 27 = 7.11 cu yds (+ 10% = 7.82 cu yds).
Gravel subbase: 7.11 cu yds × 1.15 compaction × 1.4 tons/yd = 11.45 tons.
Rebar grid (#4 20-ft sticks): 43 sticks.
Tips & Key Notes
- Place rebar on concrete "chairs" (plastic or wire stands) so rebar sits suspended in the middle-third of the slab thickness.
- Cut control joints at a depth of 1/4 the slab thickness (1 inch deep for 4" slab) every 8 to 10 feet to control shrinkage cracking.
- Pour early in the morning during summer to avoid rapid moisture evaporation caused by midday heat.
Frequently Asked Questions
How thick should a concrete patio slab be?
Standard residential patio slabs and walkways should be 4 inches thick. If the slab will support hot tubs, heavy storage sheds, or vehicle traffic, pour at 5 to 6 inches thick.
Do you need rebar in a 4-inch concrete slab?
While plain concrete can carry light foot traffic, adding #4 (1/2") rebar or 6x6 welded wire mesh prevents cracked segments from separating and shifting over time.
How many bags of concrete do I need for a 10x10 slab?
A 10 ft by 10 ft slab at 4 inches thick requires approximately 1.35 cubic yards of concrete, which equals about 62 eighty-pound bags or 82 sixty-pound bags.
How long must a concrete slab cure before building on it?
You can walk on a fresh slab after 24 to 48 hours and begin wooden wall framing after 7 days (when concrete reaches ~70% strength). Full cure is 28 days.
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