TOOL_03 · Concrete
Concrete calculator
Slabs, footings, post holes. Ready-mix yards with a 10% waste factor, or bag counts — the order-the-truck number, not the wishful one.
① Your pour
Ready-mix order
0.0cu yd
Includes 10% waste — always order a little extra
Bagged (80 lb)
0bags
89 × 60 lb or 134 × 40 lb
Material Cost Estimator
Estimated Total for 80 lb Concrete Bags
$435.50
67 units × $6.50 per 80 lb bag
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How the math works
Slabs & footings: length × width × thickness (inches ÷ 12). Post holes: π × radius² × depth. Cubic feet ÷ 27 = cubic yards. The result card shows the raw yardage and the order number with 10% waste added — subgrade is never as flat as it looks.
Bag yields: an 80-lb bag yields about 0.6 cu ft, 60-lb about 0.45, 40-lb about 0.3. Bag counts are rounded up to the whole bag.
Rule of thumb: over ~1 cubic yard (45× 80-lb bags), schedule a ready-mix delivery. Your back will agree.
Common questions
How much concrete do I need for a 12x12 slab?
A 12×12 slab at 4 inches thick is 48 cubic feet — 1.78 cubic yards. Order 2 yards to cover waste and uneven subgrade. At 6 inches thick (driveway spec), it's 2.67 yards, so order 3.
How many 80-lb bags of concrete are in a yard?
One cubic yard is 27 cubic feet. An 80-lb bag yields about 0.6 cu ft, so a full yard takes 45 bags. That's why anything over roughly 1 yard is usually cheaper and far easier as a ready-mix truck delivery.
How deep should a post hole be for a fence?
Bury ⅓ of the post's total length: a 6-ft fence with 8-ft posts means a 2.5–3 ft hole. Diameter should be 3× the post width — 12 inches for a standard 4×4. This calculator's post-hole mode handles the cylinder math.
Should I order ready-mix or buy bags?
Bags make sense under about 1 cubic yard (45× 80-lb bags). Beyond that, a ready-mix truck is cheaper per yard, takes minutes instead of hours of mixing, and gives consistent strength. Most batch plants have a 1-yard minimum delivery — this calculator's waste-included yardage tells you exactly what to order.
Estimates are for planning purposes only. Structural pours require engineered specifications — consult a professional for load-bearing work.