Every slab estimate comes down to the same list. Concrete, rebar, base, forms, and labor. Here is how to figure each one, using a 24 × 24 garage slab poured 4 inches thick.
1. Concrete
Length × width × thickness (in feet) ÷ 27 = cubic yards. Four inches is 0.333 feet.
24 × 24 × 0.333 ÷ 27 = 7.11 yards
Add 10% so you don't run short. That is 7.82 yards. Order 8.
2. Rebar
Say the rebar goes 18 inches apart, both ways. That is 1.5 feet.
24 ÷ 1.5 = 16 spaces, so 17 bars each way
17 bars × 24 feet × 2 directions = 816 feet
Add 10% for laps. That is about 898 feet, or 45 sticks of 20-foot bar.
3. Gravel base
A 4-inch base under this slab is the same math as the concrete. About 7 yards of gravel. Ask your pit how many tons that is for their rock.
4. Forms
If the slab needs forms, measure the outside edge. Here that is 24 × 4 = 96 feet of form boards, plus stakes.
5. Labor
Count the hours for each task: prep, forming, rebar, the pour, finishing, and saw cuts. Then multiply by what an hour really costs you. That is more than the wage. It includes taxes and insurance too.
6. Add it up, then add margin
Total every cost. Then divide by 0.75 for a 25% margin. Don't multiply by 1.25. That only gets you 20%. See how to bid a concrete driveway for the full example.
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More guides: How to bid a concrete driveway · Concrete price per square foot