Calculator · Concrete
Concrete Calculator: How Many Bags or Yards Do I Need?
Pick your project, enter the measurements and get a shopping list: cubic yards, bags of every size and what bags cost next to a ready-mix truck. No email, no phone number: it all runs in your browser.
Materials ticket · Slab
Enter your measurements and your shopping list prints here.
How to calculate how much concrete you need
Concrete is sold by volume, so every project comes down to three steps:
- Measure in feet. Convert the thickness from inches to feet by dividing by 12 (4 in = 0.333 ft).
- Multiply to get cubic feet. Length × width × thickness for slabs and footings; π × radius² × height for round pads, sonotubes and post holes.
- Convert to yards or bags. Divide cubic feet by 27 for cubic yards, or by the bag yield printed on the bag (an 80 lb bag makes about 0.6 ft³).
Shortcut for 4-inch slabs: divide the square footage by 81 to get cubic yards. A 10×10 slab is 100 ft² ÷ 81 = 1.23 yd³.
Then add a margin. Forms bow, the ground is never perfectly flat and some mix stays in the wheelbarrow. We add 10% by default; use 15% on uneven ground.
Bags per cubic yard
Bag yields from the Quikrete Concrete Mix data sheet. Other brands are close, but check the label.
| Bag size | Yield per bag | Bags per yd³ | Bags for a 10×10×4" slab |
|---|---|---|---|
| 40 lb | 0.3 ft³ | 90 | 123 |
| 50 lb | 0.375 ft³ | 72 | 98 |
| 60 lb | 0.45 ft³ | 60 | 82 |
| 80 lb | 0.6 ft³ | 45 | 62 |
| 90 lb (regional) | 0.675 ft³ | 40 | 55 |
The last column includes 10% extra for waste.
Concrete for common slab sizes
Four inches thick, the minimum for a patio, walkway or shed base. The bag and truck columns include 10% extra.
| Slab (ft) | Cubic yards | 80 lb bags | Truck order |
|---|---|---|---|
| 4 × 4 | 0.20 | 10 | 0.25 yd³ |
| 8 × 8 | 0.79 | 40 | 1.00 yd³ |
| 10 × 10 | 1.23 | 62 | 1.50 yd³ |
| 10 × 12 | 1.48 | 74 | 1.75 yd³ |
| 12 × 12 | 1.78 | 88 | 2.00 yd³ |
| 10 × 20 | 2.47 | 123 | 2.75 yd³ |
| 12 × 20 | 2.96 | 147 | 3.50 yd³ |
| 20 × 20 | 4.94 | 245 | 5.50 yd³ |
| 24 × 24 | 7.11 | 352 | 8.00 yd³ |
How thick should the slab be?
Four inches is the usual minimum for patios and walkways, and Concrete Network puts the minimum for a driveway at 4 inches too. Going from 4 to 5 inches adds about 20% to the concrete cost but raises the load a driveway can carry by nearly 50%, which is why many contractors pour 5 inches where cars park. For hot tubs, RVs or anything structural, follow the manufacturer's instructions and your local code.
Concrete for fence and deck posts
Quikrete's rule for setting posts: dig the hole about 3 times as wide as the post, bury one third of the post's total length and put 6 inches of concrete under it. The calculator uses that rule when you leave the hole size blank. Per post, with 10% extra:
| Post | Hole (dia. × depth) | 80 lb bags | 50 lb bags |
|---|---|---|---|
| 4×4 wood post, 4 ft fence | 10.5" × 24" | 2 | 4 |
| 4×4 wood post, 6 ft fence | 10.5" × 36" | 3 | 5 |
| 6×6 wood post, 6 ft (pergola, deck) | 16.5" × 36" | 8 | 12 |
| 6×6 wood post, 8 ft | 16.5" × 48" | 10 | 16 |
In cold climates the hole usually has to reach below the frost line, which can be deeper than one third of the post. Check with your local building department.
Concrete for sonotubes and piers
Per tube, 4 ft tall, with 10% extra. Multiply by the number of tubes, or enter them in the calculator.
| Tube diameter | Cubic feet | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 in | 1.4 | 3 | 4 |
| 10 in | 2.18 | 4 | 6 |
| 12 in | 3.14 | 6 | 8 |
| 16 in | 5.59 | 11 | 14 |
Bags or a ready-mix truck?
One cubic yard takes 45 bags of 80 lb, which is 3,600 lb of dry mix to lift, haul and mix. At $6.47 a bag that is about $291, while ready-mix averaged $160–$195 per yard in 2026.
- Up to 1 yd³: bags. Rent a mixer (Quikrete suggests about 1 ft³ of drum per 80 lb bag you mix at once) or mix in a wheelbarrow.
- 1–3 yd³: compare. Many plants charge a short-load fee under their minimum order, so ask for the total delivered price. A tow-behind concrete trailer or a helper and a rented mixer are the other options.
- Over 3 yd³: order the truck. It is cheaper and the whole slab is poured at once, without cold joints between batches.
Tips before you pour
- Order a little extra. Running short in the middle of a pour leaves a weak joint. Leftover bags keep for the next project.
- Cut control joints. Quikrete recommends joints about 1 inch deep every 6 to 8 feet so the slab cracks where you want it to.
- Cure it. Keep the slab moist for 5 to 7 days and protect it from freezing for the first 48 hours.
- Call before you dig. Dial 811 a few days before digging post holes or footings so utility lines are marked for free.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
45 bags. A cubic yard is 27 cubic feet and an 80 lb bag of Quikrete Concrete Mix yields about 0.6 cubic feet. With 60 lb bags it takes 60, and with 40 lb bags, 90.
How many bags of concrete do I need for a 10×10 slab?
A 10×10 ft slab, 4 inches thick, is 1.23 cubic yards: 56 bags of 80 lb, or 62 with 10% extra for waste. At 6 inches thick you need 50% more.
Is it cheaper to buy bags or order a concrete truck?
For small jobs, bags. For large ones, the truck. One cubic yard in 80 lb bags costs about $291 (45 bags at $6.47), while ready-mix averages $160–$195 per yard in 2026. Many plants add a short-load fee on small orders, so under about 1 yard bags usually win, and above 3 yards the truck almost always does.
How much does a yard of concrete weigh?
About 3,780 lb, almost two tons. Fresh concrete weighs roughly 140 lb per cubic foot (Quikrete data sheet), and a yard is 27 cubic feet. Check your trailer and truck ratings before hauling bags or a U-cart.
How much water do I add to an 80 lb bag of concrete?
Start with about 6 pints (3 quarts) per 80 lb bag and add water until the mix is workable, up to about 9 pints. Too much water weakens the concrete.
How deep should a fence post hole be?
Quikrete recommends burying one third of the post and digging the hole about three times as wide as the post, with 6 inches of concrete under it. For a fence post that stands 6 ft above ground, that is a 9 ft post in a hole about 3 ft deep. Local codes and frost depth can require deeper holes.
How long does concrete take to cure?
Keep it moist for at least 5 days in warm weather (70 °F or more) and 7 days between 50 and 70 °F, and protect it from freezing for the first 48 hours. Wait at least 24 hours before putting any strain on a post.
Sources
- QUIKRETE Concrete Mix (No. 1101) product data sheet, rev. October 2022quikrete.com
- Concrete Network: Concrete prices 2026, cost per cubic yardconcretenetwork.com
- Concrete Network: Concrete driveway installation essentialsconcretenetwork.com
- The Home Depot: Quikrete 80 lb Concrete Mix (price seen through a price tracker)homedepot.com