Question 1

How do I calculate how much concrete I need for a slab?

Accepted Answer

Multiply the length × width × thickness (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. For example, a 12×10 foot slab at 4 inches thick (0.33 feet) = 12 × 10 × 0.33 = 39.6 cubic feet ÷ 27 = 1.47 cubic yards. Always add 10% for waste. This calculator does all the math automatically -- just enter your dimensions.

Question 2

How many 80 lb bags of concrete make a cubic yard?

Accepted Answer

An 80 lb bag of concrete mix yields approximately 0.60 cubic feet when mixed. Since there are 27 cubic feet in a cubic yard, you need about 45 bags (27 ÷ 0.60 = 45) to make one cubic yard. A 60 lb bag yields 0.45 cubic feet (60 bags per yard), and a 40 lb bag yields 0.30 cubic feet (90 bags per yard). For projects over 1 cubic yard, ready-mix delivery is usually more economical.

Question 3

How thick should a concrete slab be?

Accepted Answer

Standard residential slabs are 4 inches thick for patios, walkways, and garage floors. Driveways should be 4-6 inches thick (6 inches for heavy vehicles or trucks). Structural slabs and those subject to heavy loads should be 6 inches or more. Footings are typically 8-12 inches deep depending on frost depth in your area. Always check local building codes for minimum requirements.

Question 4

What is the difference between concrete and cement?

Accepted Answer

Cement is an ingredient in concrete, not the same thing. Concrete is a mixture of cement (typically Portland cement), sand (fine aggregate), gravel or crushed stone (coarse aggregate), and water. The cement acts as the binding agent. When people say "cement slab" or "cement sidewalk," they usually mean concrete. Pure cement paste is rarely used in construction -- it's always mixed with aggregates to make concrete.

Question 5

How long does concrete take to cure?

Accepted Answer

Concrete reaches approximately 70% of its design strength in 7 days and full strength (28-day strength) in about 4 weeks. However, you can typically walk on it after 24-48 hours and drive on it after 7 days. Curing is the process of keeping concrete moist and at the right temperature to allow proper hydration. Covering with plastic sheeting or applying a curing compound for the first 7 days significantly improves strength and durability.

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