How much concrete do I need?
Tell us what you're building and roughly how big it is. We'll tell you how many cubic yards to order, how many bags it'd take if you're doing it yourself, and whether it's a job worth handing off.
Patio, pad or driveway
Extra options — waste allowance and price
Concrete, explained without the jargon
Concrete is sold by volume, not by weight or by area. That's the part that trips most people up. You don't order "a patio" — you order a number of cubic yards, which is just a way of measuring a 3-D chunk of space.
One cubic yard is a block 3 feet long, 3 feet wide and 3 feet tall. That's 27 cubic feet, and it weighs somewhere around two tons. So the math for a flat slab is:
length (ft) × width (ft) × thickness (ft) ÷ 27 = cubic yards
The only trick is that thickness is almost always given in inches, so you divide it by 12 first. A 4-inch slab is one-third of a foot. That's the whole secret — everything else on this page is just that same idea applied to different shapes.
How thick does it need to be?
Thickness is the number people guess at, and guessing thin is how slabs crack. These are common starting points, not code. Your building department and your soil have the final say.
| What you're pouring | Common thickness | Why |
|---|---|---|
| Walkway or sidewalk | 4 inches | Foot traffic only. |
| Patio | 4 inches | Go to 5 if you're putting a heavy grill, fire feature or hot tub on it later. |
| Driveway (cars) | 4–5 inches | 5–6 inches if trucks, RVs or trailers will park on it. |
| Garage or shop floor | 4–5 inches | Thicker at the edges is common. |
| Hot tub or spa pad | 6 inches | A full tub is thousands of pounds in a small footprint. |
| Shed or small outbuilding | 4 inches | Heavier buildings may need a thickened edge or piers. |
| AC or generator pad | 4 inches | Check the equipment manufacturer's spec. |
| Footings under a wall | Code-dependent | Do not guess this one. Width and depth come from your building department. |
Measuring without a tape measure disaster
- Weird shapes: break the area into rectangles and circles, run each one separately, then add the cubic yards together. An L-shaped patio is just two rectangles.
- Sloped ground: measure the thickness at the thinnest point and the thickest point, then use the average of the two. A slab that's 4 inches at one end and 6 at the other averages 5.
- Round to the outside: if you're between two numbers, take the bigger one. Running short mid-pour is far worse than having a little extra.
- Measure twice: pace it off, then actually pull a tape. Steps and stride guesses are routinely off by a foot or more, and a foot of width on a 20-foot driveway is most of a cubic yard.
Bags or a truck?
This is the real decision, and it comes down to volume.
- Under about a third of a yard — bagged concrete from the home center is fine. That's post holes, a small AC pad, a couple of steps.
- A third of a yard to about one yard — technically DIY-able with bags, but understand what you're signing up for: roughly 15 to 45 eighty-pound bags, mixed and placed before the first batch starts setting. It's a hard day with help, and a miserable one alone.
- One to about four yards — ready-mix territory. Be aware most suppliers charge a short-load fee below their minimum, so 1.5 yards can cost surprisingly close to 4.
- Four yards and up — a full truck and a crew. At this size the concrete is the cheap part; placing and finishing it correctly before it sets is the job.
One cubic yard is about 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags. Yields vary a little by brand, so check the bag.
Pouring in Arizona heat
Most concrete guidance online is written for mild weather and quietly assumes you'll never see 110°F. Here, heat is the main variable. Hot concrete sets faster, which shortens your working time and raises the risk of surface cracking if it dries before it cures. Summer pours in the valley commonly get scheduled early in the morning for exactly this reason, and there are established practices for hot-weather placement and curing. This is worth a conversation with your supplier or contractor rather than a rule of thumb from a website — including ours.
Common questions
What's the difference between concrete and cement?
Cement is one ingredient. Concrete is the finished mix: cement plus water plus sand and gravel. Asking for "cement" when you mean a slab is like asking for flour when you want a cake.
How much extra should I order?
5 to 10 percent over your calculated volume. It covers spillage, a subgrade that's slightly deeper than you thought, and the fact that nobody digs a perfectly flat hole. Ordering short means either a second delivery fee or a cold joint in your slab.
When can I walk on it? Park on it?
Walking, generally 24 to 48 hours. Driving, around 7 days. Concrete keeps gaining strength for weeks and is typically considered fully cured at 28 days. Heat and mix design shift all of these, so follow what your supplier tells you for your specific pour.
Does rebar or mesh change how much concrete I need?
No — reinforcement doesn't meaningfully change the volume. It changes how the slab behaves when the ground moves under it, which is a different and equally important question.
Why does my number look different from another calculator?
Usually the waste allowance. Some calculators give you the raw geometric volume with nothing added; we show you both the bare number and the number we'd actually order. Check whether the other tool included overage before assuming one of them is wrong.
Can I trust this for a footing or a foundation?
For budgeting, yes. For building, no. Footing and stem wall dimensions are set by your local building code, your soil, and the load above them. Use this to estimate cost, then get the actual dimensions from your building department or engineer.