Clear 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.
Formula & worked example
Formula: Volume = length × width × thickness, all in feet (thickness in inches is divided by 12); cubic yards = ft³ ÷ 27. Bag counts divide the waste-adjusted volume by each bag's yield — 0.60 ft³ for an 80-lb bag, 0.45 ft³ for a 60-lb bag — and round up.
Worked example: A 10 ft × 10 ft slab, 4 in thick, is 33.33 ft³ = 1.23 yd³. Adding 10% waste gives 36.67 ft³ — that is 62 × 80-lb bags, or 82 × 60-lb bags.
When to use this
- A slab, patio, driveway or shed base needs a yardage figure a supplier will respect.
- You are choosing between ready-mix delivery and bagging it yourself — and want the bag count before the store trip.
- Footings, tubes or steps with non-slab shapes that a flat area estimate gets wrong.
- You need the cost both ways to decide where the mixer truck stops making sense.
How to use it, step by step
- Pick the shape: slab, footing, tube, steps or curb.
- Enter dimensions in the units shown — thickness in inches for slabs.
- Set waste: 10% is the honest default for real ground.
- Choose a bag size if bagging, and read yards, bags and both costs.
- Order to the waste-adjusted figure; a short load costs more than a spare bag.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Length × width | The pour footprint | Patios 10×10 ft and up |
| Thickness | Slab depth | 4 in typical; 6 in for driveways |
| Waste % | Extra for uneven ground, spillage, over-dig | 10% standard; 15% for rough sites |
| Bag size | Pre-mix bag weight and yield | 80-lb yields 0.60 ft³; 60-lb 0.45 ft³ |
Another worked example
A 20 × 12 ft driveway apron, 6 inches thick: volume is 120 ft³ = 4.44 yd³, and with 10% waste you should order about 4.89 yd³ of ready-mix. Bagging it instead means 220 × 80-lb bags — the clearest possible argument for the truck on anything past a yard or two.
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.
How many bags equals a cubic yard?
About 45 × 80-lb bags (each yields 0.60 ft³ and a yard is 27 ft³). Past roughly 1–1.5 yards, delivery is usually cheaper than bags once your time and vehicle trips count — this tool prices both so you can see the crossover on your own numbers.
Why does the slab thickness matter so much?
Volume scales linearly with thickness: going from 4 in to 6 in adds 50% more concrete. It is the easiest dimension to under-measure on sloped ground, where the low edge quietly eats the difference.