Concrete Calculator

How do I calculate how much concrete I need?

Multiply the three dimensions of your pour (converted to feet) to get the volume in cubic feet, then divide by 27 to get cubic yards — the unit ready-mix trucks and bag counts are priced by. Example: a 10 ft x 10 ft slab poured 4 inches thick is 10 x 10 x (4/12) = 33.3 cubic feet, or about 1.23 cubic yards, which takes about 56 bags of 80 lb pre-mixed concrete.

How do I calculate how much concrete I need?

Convert each dimension of your pour to feet, multiply length x width x thickness (slab), pi x radius^2 x height (cylinder), or width x length x depth (footing) to get cubic feet, then divide by 27 for cubic yards. Example: a 10 ft x 10 ft slab at 4 inches thick is 33.3 cubic feet, about 1.23 cubic yards, needing roughly 56 bags of 80 lb pre-mixed concrete.

Steps to calculate concrete volume and bags

1. Choose the shape that matches your project: slab (patio, walkway, floor), cylinder (fence post hole, deck footing), or footing (strip footing under a wall).
2. Choose feet/inches or meters/centimeters, then enter the dimensions for your shape.
3. The calculator converts every dimension to feet internally and computes volume in cubic feet.
4. Cubic feet is divided by 27 for cubic yards and multiplied by 0.0283168 for cubic meters, the two units concrete is typically ordered or estimated in.
5. Bag counts are calculated by dividing the volume by each bag's stated yield (80 lb = 0.60 ft3, 60 lb = 0.45 ft3, 40 lb = 0.30 ft3) and rounding up, since partial bags are not sold.
6. Toggle the 10% waste margin to add typical spillage, uneven subgrade and over-excavation allowance before the bag count is calculated.

Formula

• L, W = length and width in feet
• T = slab thickness in feet (inches entered are divided by 12)
• D, H = cylinder diameter and height in feet; qty = number of identical cylinders (e.g. post holes)
• Bag yield = cubic feet of concrete one bag produces when mixed: 0.60 ft3 for an 80 lb bag, 0.45 ft3 for 60 lb, 0.30 ft3 for 40 lb

Example concrete calculations

Frequently asked questions

Why do I need to round up the bag count?

Bags are sold as whole units, and running short mid-pour is far worse than having a partial bag left over, so the calculator always rounds the bag count up to the next whole bag after applying your chosen volume.

Should I always add the 10% waste margin?

Extra margin is commonly recommended for uneven ground, spillage, or subgrade that soaks up more mix than a perfectly flat, prepared surface would. For a well-prepared, formed pour some builders skip it, but it is a safer default for DIY projects.

Can I mix different bag sizes to hit the volume exactly?

Yes. The three bag counts (80 lb, 60 lb, 40 lb) are independent alternatives calculated from the same total volume, not meant to be combined — pick whichever bag size is available or easiest for you to carry, and buy that count.

Does this account for rebar, wire mesh or gravel base?

No. This calculator only estimates the concrete volume itself. Reinforcement (rebar, wire mesh) and a compacted gravel base are common additions to a real pour but are not included in the volume or bag count.

This calculator estimates volume from simple geometric shapes and standard published bag yields. Actual concrete needs can vary with subgrade unevenness, formwork leakage, and mix consistency, so treat the result as a planning estimate and consider rounding up further for irregular or uneven surfaces.