Calculate exactly how much concrete you need — in cubic yards, cubic feet, and number of bags. Get cost estimates for slabs, footings, columns, stairs, and fence posts.
Concrete is the most widely used building material on Earth — and one of the most commonly miscalculated. Order too little and you'll face cold joints, delays, and a second delivery charge. Order too much and you're paying for wasted material plus disposal. This comprehensive guide teaches you how to calculate concrete quantities accurately for any residential or commercial project.
The fundamental formula for concrete volume is simple: Length × Width × Depth. The challenge is getting the units right and accounting for real-world conditions. Always convert all measurements to the same unit before multiplying. Since concrete is sold by the cubic yard, divide cubic feet by 27 to get cubic yards.
For small projects, bagged concrete is practical. The three standard bag sizes have different yields: an 80 lb bag produces approximately 0.6 cubic feet of mixed concrete, a 60 lb bag produces about 0.45 cubic feet, and a 40 lb bag produces roughly 0.3 cubic feet. The most common brands (Quikrete, Sakrete, Rapid Set) all have similar yields within each weight class.
| Bag Size | Yield (cu ft) | Bags per Cu Yd | Avg Price | Cost per Cu Yd |
|---|---|---|---|---|
| 80 lb | 0.60 | 45 | $5.00–$6.50 | ~$250 |
| 60 lb | 0.45 | 60 | $3.50–$5.00 | ~$270 |
| 40 lb | 0.30 | 90 | $2.50–$3.50 | ~$270 |
Choosing the correct thickness is critical for both structural integrity and cost. A slab that's too thin will crack under load; a slab that's too thick wastes concrete and money. Here are the standard thicknesses recommended by the ACI (American Concrete Institute) for residential applications:
4 inches is the minimum for any load-bearing slab and is standard for sidewalks, patios, shed floors, and light-duty areas. 5 inches is ideal for residential driveways with normal car traffic. 6 inches is recommended for driveways with heavier vehicles (trucks, RVs), garage floors, and workshop floors. Going from 4 inches to 6 inches increases concrete volume by 50%, so thickness matters enormously for cost.
Footings support walls and posts, distributing their load into the ground. A typical residential footing is 12–16 inches wide and 6–8 inches deep, running the full length of the wall it supports. Column footings (also called pier or pad footings) are square or circular pads, typically 24×24 inches and 12 inches deep for deck posts.
For round tube forms (Sonotubes), the formula is π × r² × height. A common 12-inch diameter Sonotube that is 4 feet deep requires: π × 0.5² × 4 = 3.14 cubic feet, or about 5.2 bags of 80 lb concrete. Multiple the per-tube amount by the number of tubes for the total project requirement.
Not all concrete is equal. Mix strength is measured in PSI (pounds per square inch) and affects price. 2,500 PSI is the minimum for non-structural applications like backfill and fence posts. 3,000 PSI is standard for residential slabs, sidewalks, and patios. 4,000 PSI is used for driveways, garage floors, and basement floors. 5,000+ PSI is for commercial and heavy-load applications. Each 500 PSI increase adds roughly $5–$10 per cubic yard to the ready-mix price.
The breakeven point between bagged and ready-mix concrete is typically around 0.5–1.0 cubic yards. Below that, bagged concrete is more practical despite the higher per-yard cost — you avoid minimum order charges (most ready-mix companies require 1 cubic yard minimum) and delivery scheduling. Above 1 cubic yard, ready-mix is almost always more economical, plus the quality and consistency are superior.
Most concrete slabs benefit from reinforcement. Welded wire mesh (6×6 W1.4/W1.4) is standard for sidewalks and patios — it helps control cracking but doesn't add significant structural strength. Rebar (#3 or #4 bars on 12-inch or 18-inch centers) provides true structural reinforcement for driveways, garage floors, and any slab thicker than 5 inches. Fiber reinforcement (mixed into the concrete) is increasingly popular for flatwork as a partial replacement for wire mesh.