Concrete Calculator

How Concrete Calculator Works

Volume = Area × Depth | Cubic Yards = Cubic Feet / 27

V

Volume & Depth — The total concrete volume needed. For slabs: V = L × W × (D/12). Depth in inches is converted to feet. For cylinders: V = πr²h.

yd³

Cubic Yards — Standard concrete ordering unit. 1 cubic yard = 27 cubic feet. Most ready-mix trucks carry 8-10 cubic yards.

W

Waste Allowance — A percentage added to account for spillage, uneven subgrades, and slight depth variations. Standard practice is 10%.

How to Use

1. Choose your shape (slab, hole, column, or stairs).
2. Enter dimensions in feet and inches as requested.
3. Optionally add waste allowance (recommended: 10%).
4. Get your concrete volume and exact bag count for Home Depot.
5. Review the weight estimate to plan for transport and handling.

Quick Reference

Understanding the Result

Worked Examples

Frequently Asked Questions

How many 80lb bags do I need per cubic yard?

One cubic yard = 27 cubic feet. An 80lb bag covers approximately 0.6 cubic feet, so you need 27 / 0.6 = 45 bags per cubic yard. A 60lb bag covers about 0.45 cubic feet, so 27 / 0.45 = 60 bags per cubic yard. For a typical two-car driveway needing 5 cubic yards, that is 225 80-lb bags or 300 60-lb bags — this is why ready-mix delivery is almost always the right choice for projects over 1 cubic yard.

What is the standard waste allowance and when should I increase it?

10% is the standard waste allowance for most projects. Increase it to 15% for complex pours with multiple obstacles, rebar-heavy footings, or when pouring in difficult terrain. For simple flatwork like a patio slab, 5% may be sufficient. The waste factor accounts for spillage during transport and placement, slight depth variations from uneven subgrade, and concrete that remains in the chute of the mixer or pump hose. If your subgrade is particularly uneven (rocky or sloped), bump the waste factor by an additional 5%.

Should I use fiber mesh or rebar for concrete reinforcement?

It depends on the application. Fiber mesh (microsynthetic fibers) is ideal for flatwork like sidewalks, patios, and driveways — it controls plastic shrinkage cracking and provides three-dimensional reinforcement throughout the slab. Rebar is better for structural applications like footings, foundations, and columns that bear heavy loads. Many contractors use both: rebar for structural integrity and fiber mesh for crack control. For a standard 4-inch thick driveway slab on good soil, a 6x6 inch welded wire mesh or a 1/2 inch rebar grid on 24-inch centers is common. Control joints should be cut at intervals equal to 2-3 times the slab thickness in inches — for a 4-inch slab, cut joints every 8-12 feet.

How do I calculate concrete for staircases?

Stairs are approximated as triangular prisms. The formula multiplies stair length by width, then by riser height (in feet), then by the number of steps, and divides by 2 because each step is approximately a triangle in cross-section. For example, a set of 10 stairs that are 3 feet wide, with 7-inch risers and a total run of 10 feet: Volume = 10 ft × 3 ft × (7/12 ft) × 10 steps × 0.5 = 8.75 cubic feet = 0.32 cubic yards. The 0.5 factor accounts for the triangular shape. For more precise estimates on complex staircases with landings, treat the landing as a separate slab calculation and add it to the stair volume.

How do I calculate concrete for round footings or columns?

Round footings and columns use the cylinder volume formula: V = π × r² × h, where r is the radius (half the diameter) in feet and h is the depth in feet. For example, a 24-inch diameter footing (r = 1 ft) that is 4 feet deep: V = 3.14159 × 1² × 4 = 12.57 cubic feet = 0.47 cubic yards. With 10% waste: 0.51 cubic yards. That is about 23 80-lb bags or 31 60-lb bags. For sonotube (cardboard form) columns, the diameter is the inside diameter of the tube. Common sonotube sizes are 8, 10, 12, 18, and 24 inches. Always flare the bottom of the footing wider than the column for stability — building codes typically require the footing to be twice the column diameter and at least 6 inches thick at the base.

When is ready-mix concrete delivery worth it versus mixing from bags?

Ready-mix is cost-effective for pours over 1 cubic yard (roughly 45 80-lb bags). Below 1 cubic yard, bagged concrete is practical for a DIY project. At 1-2 cubic yards, both options are viable — ready-mix costs about $130-$180 per yard plus a $150-$200 delivery fee, while bags cost $225-$315 per cubic yard equivalent but have no delivery minimum. For 2+ cubic yards, ready-mix is the clear winner on both cost and labor. A full truck carries 8-10 cubic yards. If your pour is less than 4-5 yards, you will likely pay a short-load fee of $50-$100 on top of the per-yard price. Always call at least three local ready-mix suppliers for quotes.

Pro Tips

• →Order concrete for a morning delivery — cooler temperatures give you more working time. Concrete delivered on a hot afternoon can lose 30-60 minutes of workability. In temperatures above 85°F, use a set-retarding admixture or plan to finish the concrete within 30 minutes of placement.
• →Build sturdy formwork with 2x4 lumber secured with stakes every 2-3 feet. Check that forms are level with a 4-foot level before pouring. Apply a release agent (diesel fuel or commercial form oil) to the inside of forms so they strip cleanly after the concrete sets. Use duplex nails so forms can be disassembled easily.
• →Have all tools ready before the truck arrives: wheelbarrow (or concrete buggy for large pours), shovel, rake, screed board (a straight 2x4), bull float, magnesium hand float, edging tool, and groover/jointer tool. The concrete will not wait while you search for a missing trowel — you typically have 60-90 minutes from truck arrival to finish.
• →For bagged concrete, rent a portable drum mixer ($35-$50/day) for projects requiring more than 20 bags. Hand-mixing in a wheelbarrow with a hoe is exhausting beyond about 10-12 bags and produces inconsistent results. A mixer ensures uniform water distribution and strength throughout the pour.
• →Cure concrete properly: keep the surface moist for at least 7 days. Cover with wet burlap, plastic sheeting, or apply a curing compound. Do not let freshly poured concrete dry out in the first 24-48 hours — rapid drying causes surface cracking and reduces strength. In hot weather, mist the surface with water 2-3 times per day.
• →Cut control joints within 24 hours of pouring, while the concrete is still green. For a 4-inch slab, cut joints every 8-12 feet and to a depth of at least 1 inch (25% of slab thickness). Joints direct cracking to predetermined locations. For sidewalks, place joints at intervals equal to the width of the walkway.

Limitations to Know

• •This calculator provides estimates for common geometric shapes. For irregular or curved shapes, break the area into rectangles and triangles, calculate each section separately, and sum the results. Complex shapes with multiple radii or organic curves will have higher waste — add an additional 5% for each complex feature.
• •Bag estimates assume standard yields: 0.6 cubic feet per 80-lb bag and 0.45 cubic feet per 60-lb bag. Actual yield varies based on water content, mixing method, and aggregate moisture. Overwatering to improve workability reduces yield per bag and weakens the concrete. Always follow the water-to-mix ratio printed on the bag.
• •The calculator does not account for reinforcement (rebar or mesh), formwork lumber costs, subbase gravel, vapor barriers, or labor. These additional costs typically represent 40-60% of the total project cost beyond the concrete material itself. For a budget estimate, multiply the concrete cost by 2.5-3x for the all-in project cost.
• •Stair calculations use an approximation (triangular prism model with 0.5 factor). For staircases with landings, curved steps, or irregular riser heights, this estimate should be treated as an approximation. Calculate landings separately as slabs and add them to the stair volume.

Sources

• Concrete
• Portland Cement Association
• American Concrete Institute
• Volume