How to use this calculator
- Measure the room. Length and width should be the inside dimensions of the foundation or shed walls. Most contractors round up by 1–2 inches to allow for the slip-form.
- Pick a thickness. 4 inches is the residential garage standard. Workshops with heavy equipment use 5–6 inches. Basements use 4 in over a vapor barrier.
- Add a waste %. 5–10% for an interior slab — the formwork is straight and the over-pour is small.
- Read your result. Cubic yards is the headline. For floors over 3 yd³, ready-mix is cheaper; smaller floors break even with bags.
Formula
Volume (yd³) = Length × Width × Thickness ÷ 27
Worked example
A 24 ft × 24 ft × 4 in garage floor equals 192 ft³ ÷ 27 = 7.1 cubic yards. Add 10% waste → order 7.8 yd³ from the ready-mix supplier. That is well above the short-load minimum, so no surcharge.
Tips for accurate results
- Garage floors should slope ¼ in per foot toward the door for drainage.
- Place a 6-mil polyethylene vapor barrier under any interior slab to keep moisture from wicking up.
- Use #4 rebar on 16-inch centers, or 6×6 W2.9×W2.9 wire mesh, in slabs that will see vehicle traffic.
- Cut control joints within 24 hours of pouring — every 8–12 ft for a 4-inch slab.