How Much Concrete Do I Need? (2026 Slab Calculator + Bag Chart)
Ordering concrete feels like guesswork the first time, and guessing wrong is expensive. Too little leaves a weak cold joint halfway through the pour; too much means paying for concrete you must dispose of. This guide covers the calculation, a bag chart for common sizes, and the bags versus ready-mix comparison. For your exact slab, use the free concrete calculator before calling a supplier.
How much concrete do I need for a slab?
Multiply the slab’s length in feet by its width in feet by its thickness in inches divided by 12. That gives cubic feet. Divide by 27 for cubic yards, the unit ready-mix suppliers use. A 10 by 10 foot slab at 4 inches thick needs 33.33 cubic feet, or 1.23 cubic yards.
How do you calculate concrete volume?
A slab is a rectangular box: length times width times thickness. The trap is units: length and width are in feet, but thickness is usually in inches, so divide it by 12 first. Then divide cubic feet by 27 for cubic yards, the unit ready-mix suppliers use.
Knowing the formula lets you sanity-check any supplier quote. The concrete calculator adds the bag count and cost comparison.
How many bags of concrete do I need?
Divide your slab’s cubic feet by the yield printed on the bag. Standard yields are 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag. Always round up, since you cannot buy part of a bag.
For example, a 10 by 10 slab at 4 inches needs 56 bags of 80 lb mix, 75 of 60 lb, or 112 of 40 lb.
| Slab size | Thickness | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 8 x 8 ft | 4 in | 0.79 | 36 | 48 |
| 10 x 10 ft | 4 in | 1.23 | 56 | 75 |
| 10 x 12 ft | 4 in | 1.48 | 67 | 89 |
| 12 x 12 ft | 4 in | 1.78 | 80 | 107 |
| 10 x 10 ft | 6 in | 1.85 | 84 | 112 |
| 20 x 12 ft | 6 in | 4.44 | 200 | 267 |
These are exact volumes, rounded up to bags, and exclude the 10% waste allowance below. Add that.
How many cubic yards is a 10×10 slab?
A 10 by 10 foot slab at 4 inches thick is 1.23 cubic yards. At 6 inches thick, the same slab is 1.85 cubic yards, a 50% increase from just two extra inches of depth. This is the number your ready-mix supplier wants, so quote it in cubic yards, not bags or square feet.
Is it cheaper to buy bags or ready-mix concrete?
Ready-mix is usually cheaper once a slab passes about one cubic yard. A 10 by 10 slab at 4 inches costs about $308 in 80 lb bags at $5.50 each, versus about $185 in ready-mix at $150 per yard. Bags win only for tiny jobs, repairs, and sites a truck cannot reach.
Enter the quotes you actually get into the concrete calculator instead of the example prices here, since local prices and delivery fees change the answer.
How thick should a concrete slab be?
Four inches suits walkways, patios, and shed bases carrying only foot traffic and light loads. Driveways for ordinary cars need 5 to 6 inches, and heavy trucks or RVs need 6 to 8 inches. Garage floors are usually 5 to 6 inches.
Thickness drives volume most: going from 4 to 6 inches raises it 50% for the same slab size.
Should I order extra concrete?
Yes, order about 10% more than the calculated volume. Forms flex outward under wet concrete, the ground is never perfectly level, and some material is always lost to spillage. Running short mid-pour is far worse than a small surplus, because fresh concrete against partly set concrete forms a weak cold joint.
Note the calculator shows the exact volume, and the 10% allowance is not included in its results, so add it when ordering. For the example slab, 10% extra means about 1.36 cubic yards, or 62 bags.
What about footings, steps, and odd shapes?
Slab math only covers flat rectangles. Footings are long, narrow trenches with their own depth and width, steps are a stack of boxes of different sizes, and round pads need circle math. Calculate each shape separately and add the volumes together.
For a footing, multiply length by width by depth, all in feet. For steps, work out each step as its own box and add them up. Mixing these shapes into slab math is the most common reason a pour comes up short.
How much does a concrete slab cost in 2026?
Material cost depends on your route. Bagged concrete runs roughly $5 to $7 per bag, while ready-mix typically costs $125 to $175 per cubic yard before delivery, fuel, and short-load fees. A 10 by 10 slab at 4 inches therefore costs roughly $185 to $216 in ready-mix, or $280 to $392 in bags.
Labor, reinforcement, forming, and finishing cost extra. If the slab is part of a bigger home project, the down payment calculator and closing costs estimator cover the purchase side if you are buying the property you are improving.
What are the most common concrete questions?
How long does concrete take to cure?
Concrete sets within hours but cures over weeks. Walk on it after 24 to 48 hours, drive light vehicles after about 7 days, and expect most of its strength at 28 days. Keep it damp the first week.
Can I pour concrete in cold weather?
Yes, with precautions. Below about 40 degrees Fahrenheit, concrete sets slowly and can freeze before gaining strength. Use insulating blankets, avoid frozen ground, and never let fresh concrete freeze in its first 24 hours.
Do I need reinforcement in a slab?
For patios and walkways, wire mesh or fiber reinforcement controls cracking well enough. Driveways and garage floors benefit from rebar, especially on soft soil. Reinforcement does not change the volume, so bag and yard counts stay the same.
How do I avoid cracks in a new slab?
Cut control joints every 8 to 12 feet so cracks follow the joints instead of wandering. Cure properly, avoid adding extra water on site, and compact and level the base before the pour.
Disclaimer: This article is general information only, not professional construction advice. Volumes are estimates for exact rectangular dimensions; real pours vary with form flex, ground level, and spillage, so order about 10% extra. Prices are examples, not quotes. For structural or load-bearing work, consult a qualified contractor or engineer.
