How Many Bags of Concrete Do You Need for a 12 x 12 Slab?
A 12 by 12 foot slab poured 4 inches thick is 48.0 cubic feet, or 1.78 cubic yards. With 10% extra you need about 88 bags of 80 lb mix, 118 bags of 60 lb or 176 bags of 40 lb. At 5 or 6 inches thick the count rises to 110 and 132 bags of 80 lb.
The quick math for a 12 x 12 slab
A 12 × 12 slab suits a larger patio, a gazebo base or a small workshop floor. A 12 × 12 ft slab covers 144 square feet. Concrete is measured by volume, so you also need the thickness. Convert the thickness to feet, multiply by the area for cubic feet, then divide by the cubic feet one bag makes.
bags = cubic feet × 1.10 ÷ yield per bag
A 4-inch slab is 12 × 12 × 0.333 = 48.0 cubic feet. Adding 10% for uneven ground and spillage gives 52.8 cubic feet. An 80 lb bag of premix makes about 0.6 cubic feet, so you need 88.0 bags, which rounds up to 88.
12x12 slab concrete chart
| Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 4 in | 48.0 | 1.78 | 88 | 118 | 176 |
| 5 in | 60.0 | 2.22 | 110 | 147 | 220 |
| 6 in | 72.0 | 2.67 | 132 | 176 | 264 |
Bag counts include 10% extra and are rounded up. Yields used: 0.6 cu ft for 80 lb, 0.45 cu ft for 60 lb and 0.3 cu ft for 40 lb. Always check the yield printed on the bag you buy, because brands differ slightly.
Which thickness should you pick?
A patio is usually 4 inches; a gazebo or workshop that holds heavier loads is often 5 to 6 inches with reinforcement. A 4-inch slab is the usual minimum for foot traffic and light loads. Going to 5 inches adds 25% more concrete and 6 inches adds 50%, which is why the bag counts above climb quickly. Local code and your soil decide the real requirement, so ask the building department for anything structural.
Bags or a ready-mix truck?
One cubic yard of concrete is about 45 bags of 80 lb mix. This slab needs 1.96 cubic yards at 4 inches, which is a lot of bags to mix by hand, so get a ready-mix quote as well. Trucks often charge a short-load fee on small orders, so compare the total. Mixing 88 bags is hard work and the concrete begins to set within an hour or two, so plan to pour in one session.
Prep checklist before the first bag
- Excavate and compact 4 inches of gravel base so the slab has even support.
- Build square, level forms; check the diagonals are equal.
- Add wire mesh or rebar if the plan calls for it, lifted off the ground on chairs.
- Have a screed board, float, edger and a source of water ready before you start mixing.
To try other sizes or add your own waste factor, use the concrete calculator, or convert any volume with the cubic yard calculator.
Common mistakes
- Adding too much water to make the mix easier to pour; it weakens the slab and makes it crack.
- Mixing in too-small batches so the first pour sets before the next is ready.
- Pouring on a soft or uneven base, which makes the slab thickness vary.
- Forgetting control joints in a larger slab.
Other sizes
How Many Bags of Concrete for a 10x10 Slab · How Many Bags of Concrete for a 8x10 Slab
Calculators for this guide
Frequently asked questions
How many 80 lb bags of concrete for a 12x12 slab?
88 bags for a 4-inch slab, 110 for 5 inches and 132 for 6 inches, including 10% extra.
How many cubic yards is a 12x12 slab?
1.78 cubic yards at 4 inches, 2.22 at 5 inches and 2.67 at 6 inches, before waste.
How much does an 80 lb bag of concrete cover?
One bag makes about 0.6 cubic feet, which covers roughly 1.8 square feet at 4 inches thick.
Should I add extra concrete?
Yes. Add 5 to 10 percent for an uneven base, spillage and measuring error. Running short mid-pour is worse than having a little left over.
Can I pour a slab in more than one day?
You can, but the joint between pours is a weak point. For a slab this size it is better to pour it all at once, or add a planned control joint where the pour stops.
Sources and further reading
- Concrete driveways and slabs — American Cement Association
- National Ready Mixed Concrete Association — NRMCA: ready-mix industry resources
- NIST Special Publication 811: conversion factors — National Institute of Standards and Technology
More guides
Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.