Loan Concrete Calculator
Pick your shape — slab, footing, sonotube, steps or curb — and get cubic yards, the exact bag count in both sizes, and a shopping list with prices. No guessing in the hardware-store aisle.
Your shopping list
Bag counts include your waste allowance and are rounded up — you cannot buy a partial bag. As a rule of thumb, once a job passes about 1.5–2 yd³ (roughly 100+ bags), a ready-mix truck is usually cheaper and far less work than mixing by hand.
How it works
Everything starts from volume in cubic feet. A slab is length × width × thickness; a sonotube is π × radius² × depth; steps are counted as stacked boxes — step 1 is one riser high, step 2 is two, and so on, giving width × tread × riser × n(n+1)/2. Cubic yards are ft³ ÷ 27, and bag counts come from real yields: an 80-lb bag makes 0.60 ft³ and a 60-lb bag makes 0.45 ft³ of finished concrete. Waste is added before rounding bags up, never after.
Frequently asked questions
How many 80-lb bags make a cubic yard?
A cubic yard is 27 ft³ and an 80-lb bag yields 0.60 ft³, so 27 ÷ 0.60 = 45 bags per yard. In 60-lb bags (0.45 ft³ each) it is 60 bags per yard — a useful sanity check before you order.
How much extra concrete should I order?
Five to ten percent beyond the exact volume is the standard allowance for uneven ground, spillage and form movement. This calculator defaults to 10% and applies it before rounding bag counts up.
When is ready-mix cheaper than bags?
Usually around 1.5–2 cubic yards. Below that, bags win on total cost despite the labor; above it, you are mixing 100+ bags by hand. The results compare both prices for your exact job so you can see the crossover.
How thick should my slab be?
Four inches covers patios, walkways and shed pads. Driveways and anything vehicles cross should be 6 inches, ideally with reinforcement. Footings must reach below your local frost line — check local code for depth.
📐 Need the area first?
The Square Footage Calculator handles four shapes and a whole rooms list — handy before you size a slab.