From Tape Measure to Truck: How to Estimate Concrete Without Coming Up Short
By the Super Simple Digital Tools Team · Updated June 2026
Estimating concrete trips up beginners because the answer lives in two different units. Suppliers sell ready-mix by the cubic yard and bagged mix by the bag, and the same hole in the ground can be described both ways. The starting point is always volume: measure length, width, and depth, convert depth from inches to feet, and multiply. That product is your cubic footage. Dividing by 27 gives cubic yards, the figure a batch plant cares about, while bag yields turn the same number into a count of 60 lb or 80 lb sacks for a hardware-store run.
Depth is where most estimates quietly go wrong. A patio or shed floor is normally poured at 4 inches, but a driveway that carries a car or truck wants 5 to 6 inches, and a wall footing is often 12 inches deep and twice the width of the wall above it. Because volume scales directly with thickness, an inch of extra depth across a large slab can add a surprising amount of concrete. Measure the form depth in several places rather than trusting a single reading, since a subgrade that sags even slightly raises your true volume.
Once you have volume, bag counts follow from fixed yields. An 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Those numbers are why a cubic yard works out to roughly 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags. For a sense of scale, a 10 by 10 foot pad at 4 inches needs somewhere around 56 bags of 80 lb mix, which is heavy enough to make the bags-versus-truck question worth asking before you load the car.
That decision usually comes down to volume and your tolerance for mixing. Bags shine on small jobs under about a cubic yard, fence posts, a stoop, a small equipment pad, because there is no delivery minimum and you mix at your own pace. The trade-off is cost and labor: at roughly $6 a bag you can pay close to $270 per cubic yard, while a ready-mix truck might deliver for around $150 per yard. Trucks, though, often require a 1 to 3 yard minimum and charge short-load fees, so ready-mix tends to win only above 1.5 to 2 yards.
Whichever route you choose, build in a cushion and plan the pour. Adding 5 to 10 percent for waste covers spillage, an uneven base, and forms that are a touch deep, and it is far cheaper than halting a pour to fetch more material. After placing concrete, remember that it keeps gaining strength for weeks: light foot traffic is usually fine after 24 to 48 hours, but full design strength is reached at about 28 days, so keep heavy loads off a fresh slab until it has cured.
- Measure slab thickness in three or four spots and use the deepest reading, since a dipping subgrade adds volume you will pay for.
- Match depth to the job: 4 inches for patios and floors, 5 to 6 inches for driveways, and 12 inches deep for typical wall footings.
- Always round bag counts up and add 5 to 10 percent; a leftover bag is cheaper than a second supply run mid-pour.
- For fence posts, dig the hole about one-third the post length deep and roughly three times the post width across, then budget about 3 bags of 80 lb mix per standard hole.