The case: a square patio, 12 ft by 12 ft, on compacted gravel next to the back door. It will be 4 in thick, which is the usual thickness for foot traffic, furniture and a grill. The owner will mix and place the concrete with two helpers and wants a materials list.
Short answer: the slab holds 48 cu ft. With a 10% allowance for spillage and an uneven base, that becomes 52.8 cu ft = 1.96 cubic yards (1.50 m³). In bags, that is 88 bags of 80 lb, 118 of 60 lb or 176 of 40 lb. At almost two yards, a ready-mix truck is the better choice unless the site cannot be reached.
1. Volume: length × width × thickness
Concrete is ordered by volume, and the only trap is units. The length and width are in feet, but the thickness is in inches, so divide it by 12:
12 ft × 12 ft × (4 in ÷ 12) = 48 cu ft
A cubic yard is 27 cu ft (3 × 3 × 3 ft), so the net volume is 1.78 yd³. Next comes the allowance. Forms are never perfectly level, the base has low spots, and some concrete stays in the wheelbarrow and the mixer. The calculator adds 10%:
48 × 1.10 = 52.8 cu ft ÷ 27 = 1.96 yd³
Order ready-mix rounded up to the next quarter yard. For bags, round up to whole bags and keep the receipt, since unopened bags can usually go back.
2. Bags: how much each size yields
Bagged concrete mix is sold by weight, but what matters is the volume it makes once mixed. The calculator uses the yields printed on most US bags:
| Bag size | Yield per bag | Bags for 52.8 cu ft | Total weight to carry |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 88 | 7,040 lb |
| 60 lb | 0.45 cu ft | 118 | 7,080 lb |
| 40 lb | 0.30 cu ft | 176 | 7,040 lb |
88 bags of 80 lb weigh about 3.2 metric tons, enough to overload most pickup trucks in one trip. Each bag also has to be mixed. With one small drum mixer taking two bags per batch, that is 44 batches. A crew of three can manage it, but the first panels start to set while the last ones are still being mixed. That is the main reason pros pour anything this size from a truck.
3. Thickness changes the order more than you think
Every extra inch adds a quarter to the volume of a 4 in slab:
| Thickness | Volume with 10% (cu ft) | Cubic yards | 80 lb bags | Typical use |
|---|---|---|---|---|
| 3.5 in | 46.2 | 1.71 | 77 | Walkways, light foot traffic |
| 4 in | 52.8 | 1.96 | 88 | Patios, sidewalks |
| 5 in | 66.0 | 2.44 | 110 | Hot tub pad, heavy planters |
| 6 in | 79.2 | 2.93 | 132 | Driveway aprons, vehicles |
If a hot tub will sit on the patio, check the tub maker's slab requirement before you order. A filled tub weighs far more than any patio table, and many manufacturers specify the thickness and reinforcement.
4. Other patio sizes at 4 inches
| Patio | Area (sq ft) | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 100 | 1.36 | 62 | 82 |
| 12 × 12 ft | 144 | 1.96 | 88 | 118 |
| 12 × 16 ft | 192 | 2.61 | 118 | 157 |
| 16 × 16 ft | 256 | 3.48 | 157 | 209 |
Below about one cubic yard, bags make sense. Above it, ready-mix saves labor, and it also gives you one consistent batch and a uniform finish.
5. The base: gravel under the slab
The slab is only as good as what it sits on. Strip the topsoil and organic material, compact the subgrade, then place and compact a granular base. A 4 in layer of crushed stone under the 12x12 ft patio is 1.78 yd³ before compaction. Order a little more, because gravel compacts. Base and slab together mean digging about 8 in below the finished height.
Watch the form boards too. A 2x4 is really 3.5 in wide, so a slab formed with 2x4s set on the base comes out 3.5 in thick at the edges unless the boards are set half an inch lower. Many crews use 2x6 forms for a 4 in patio and simply fill to a line. If you screed off the top of 2x4s anyway, the order drops to the 77 bags in the 3.5 in row of the table above.
6. Joints, slope and curing
Control joints. A common rule of thumb from concrete industry guidance is to space joints, in feet, no more than 2 to 3 times the slab thickness in inches. For 4 in, that is 8 to 12 ft. On a 12 ft square, one joint each way at 6 ft makes four 6x6 ft panels. Cut or tool the joints to about one quarter of the thickness, which is 1 in here.
Slope. Pitch the slab away from the house so water drains off. A fall of 1/8 in per foot drops 1.5 in over 12 ft, and 1/4 in per foot drops 3.0 in. Set the forms for this before the pour, not after.
Curing. Keep the surface moist, or cover it with plastic or a curing compound, for the first week. Concrete that dries too fast loses surface strength and dusts or scales, especially in climates with freeze-thaw cycles.
7. Reading the bag before you buy
The bags on the shelf can look identical but contain different products. Check four things on the label:
- Yield. Look for the cubic feet per bag, usually 0.60 for 80 lb. A "high-strength" or "fast-setting" mix may yield a little less, so recheck the count if you switch products.
- Product type. Use concrete mix (sand, cement and coarse gravel) for a slab. Mortar mix and sand mix have no coarse aggregate and are not made for slabs.
- Compressive strength. The 28-day rating, often 4,000 psi or more for general-purpose mixes, is plenty for a patio.
- Water per bag. Follow it. The label gives a range; stay near the low end for strength and add water only until the mix is workable.
Ready-mix tickets carry the same information in a different form: the volume delivered, the specified strength and the time the drum was batched. Ask for air-entrained concrete if your winters freeze.
8. Signs of a problem after the pour
- Fine map cracking (crazing) in the first weeks usually means the surface dried too fast or was finished while bleed water was still on top. It is mostly cosmetic.
- Flaking or scaling after the first winter points to weak surface paste, too much water, or deicing salt used on a young slab.
- A crack running away from a joint suggests the joint was too shallow or cut too late.
- One panel sinking almost always means the base was soft or poorly compacted under that corner.
9. Pour-day checklist
- [ ] Forms set, square (diagonals equal), staked and pitched away from the house
- [ ] Base compacted; plastic or reinforcement placed as planned
- [ ] 88 bags of 80 lb on site (or 1.96 yd³ ordered, rounded up)
- [ ] Mixer, two wheelbarrows, screed board, bull float, edger and groover
- [ ] Water hose and curing cover ready
- [ ] Forecast checked: no rain or frost for the first day
10. Common mistakes
- Forgetting to divide inches by 12. Writing 12 × 12 × 4 gives 576 "cubic feet", twelve times too much.
- Ordering the exact volume. A slab poured with too little concrete ends with a thin, weak corner. Keep the 10%.
- Adding water to make it flow. Extra water weakens the concrete and causes shrinkage cracks. Mix to the bag's instructions.
- Skipping joints on a small slab. A 12 ft panel at 4 in will crack. Without joints, it picks where.
FAQ
How many 80 lb bags for a 12x12 patio? 88 bags at 4 in thick, including 10% waste. Without the waste allowance it is 80.
How many yards of concrete for a 12x12 slab 6 inches thick? 2.93 cubic yards with waste, or 132 bags of 80 lb.
Is 3.5 inches enough? For foot traffic on a well-compacted base, many builders pour 3.5 in walkways. It saves 0.24 yd³ here, but leaves less margin for soft spots. For a patio, 4 in is the safer default.
Should I use 60 lb bags because they are easier to lift? They are, but you will carry 118 of them instead of 88. The total weight is about the same either way.
Do I need rebar or mesh? Many patios on good soil are poured without it, using joints to control cracking. Reinforcement helps hold cracked pieces together and level. It is worth adding on clay soils or under heavy loads.
Case summary
| Item | Result |
|---|---|
| Patio | 12 × 12 ft, 4 in thick |
| Net volume | 48 cu ft (1.78 yd³) |
| With 10% waste | 52.8 cu ft = 1.96 yd³ (1.50 m³) |
| Bags | 88 × 80 lb, 118 × 60 lb or 176 × 40 lb |
| Gravel base (4 in) | 1.78 yd³ |
| Control joints | One each way at 6 ft, 1 in deep |
Different size, thickness or waste factor? Enter your dimensions in the concrete calculator to get cubic yards and bag counts for your patio.
Try it with your own numbersConcrete Calculator→Sources and references
- Portland Cement Association (PCA) slab thickness, curing and finishing practice for residential flatwork, described in words
- American Concrete Institute (ACI) the joint-spacing rule of thumb (2 to 3 times the slab thickness) and joint depth of one quarter of the thickness
- SizeBench concrete calculator the volume formula, 10% waste allowance and bag yields (0.60, 0.45 and 0.30 cu ft) used in this guide
How we made this guide
Every number in this guide is calculated by the same code as our calculators, from the standards listed above, and the page is tested before publication. The SizeBench team researched, wrote and reviewed it; we use automation and AI tools to help draft, never to produce the figures. Our methodology →
Revision history
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