How Many Bags of Concrete Do I Need? (Slab & Post Chart)
The short answer: about 45 bags of 80 lb concrete make one cubic yard. The longer answer depends on the bag size, how thick the slab is, and how much extra you add for the realities of a job site. This guide gives you the numbers, the formula, and charts for common sizes, so you can work out your own project in a couple of minutes.
How many bags in a cubic yard?
A cubic yard is 27 cubic feet. Each bag of concrete mix produces a fixed volume of finished concrete, called its yield. Divide 27 by the yield and you get the bag count:
| Bag size | Yield per bag | Bags per cubic yard | Bags per cubic foot |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 3.3 |
| 50 lb | 0.375 ft³ | 72 | 2.7 |
| 60 lb | 0.45 ft³ | 60 | 2.2 |
| 80 lb | 0.60 ft³ | 45 | 1.7 |
These are typical yields printed on common brands. Always check the bag, since specialty mixes (fast-setting, high-strength, countertop) can differ.
The formula, in three steps
- Find the volume in cubic feet. Length × width × thickness, with the thickness converted from inches to feet (divide by 12). A 10 × 12 ft slab that is 4 in thick is 10 × 12 × 0.333 = 40 ft³.
- Add waste. Multiply by 1.05 to 1.10 for 5 to 10% extra. For 10%, 40 ft³ becomes 44 ft³.
- Divide by the bag yield and round up. 44 ÷ 0.60 = 73.3, so buy 74 bags of 80 lb.
The concrete calculator does all three steps and works for circles and any units.
Bags needed for a 4 inch slab
These figures are for a flat slab 4 inches thick, with no waste added. Four inches is the usual thickness for patios, walkways, shed floors and most pads.
| Slab size | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.20 | 9 | 12 | 18 |
| 6 × 6 ft | 0.44 | 20 | 27 | 40 |
| 8 × 8 ft | 0.79 | 36 | 48 | 72 |
| 10 × 10 ft | 1.23 | 56 | 75 | 112 |
| 10 × 12 ft | 1.48 | 67 | 89 | 134 |
| 12 × 12 ft | 1.78 | 80 | 107 | 160 |
| 12 × 16 ft | 2.37 | 107 | 143 | 214 |
| 16 × 16 ft | 3.16 | 143 | 190 | 285 |
| 20 × 20 ft | 4.94 | 223 | 297 | 445 |
Bags needed for a 6 inch slab
Driveways, garage floors and anything that carries vehicles are typically poured 5 to 6 inches thick.
| Slab size | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 1.85 | 84 | 112 | 167 |
| 12 × 12 ft | 2.67 | 120 | 160 | 240 |
| 12 × 20 ft | 4.44 | 200 | 267 | 400 |
| 20 × 20 ft | 7.41 | 334 | 445 | 667 |
| 24 × 24 ft | 10.67 | 480 | 640 | 960 |
Notice how quickly the numbers climb. At 120 bags (nearly five tons of bagged mix), you are mixing, pouring and finishing by hand against the clock, and it is worth seriously pricing ready-mix.
Fence posts, deck piers and footings
Post holes are small, so the numbers are small, but they add up when you have a dozen. The volume of concrete is the hole's volume minus the post's own volume. For a typical 4×4 post in a 10 inch hole, 24 inches of concrete depth (30 inch hole with a 6 inch gravel base), you need about 0.92 ft³ of concrete per post. That is roughly 1½ bags of 80 lb or about 2½ bags of 50 lb, before waste.
Use the fence post concrete calculator to enter your own hole size and depth. For long strip footings, use the footing calculator.
Bagged concrete or ready-mix?
Bagged concrete is convenient. You buy exactly what you need, no truck has to reach the site, and you work at your own pace. But bag for bag it is expensive, and the labor is brutal. As a rough guide:
- Under about half a cubic yard (roughly 20 bags of 80 lb): bags are the obvious choice, such as for post holes, steps or a small pad.
- Half a yard to about two yards: it depends on your labor, your access, and the delivery price. Many suppliers charge a short-load fee for small orders, so call and ask.
- Over two yards: ready-mix is almost always cheaper per yard and is far less work. You will also get more consistent strength, since a plant mixes it, not a tired person with a hoe.
Prices vary by region and change often, so take two or three quotes. For ready-mix, compare the price per yard, the delivery charge, the minimum order and the cost of any short-load fee.
Mistakes that cost people money
- Forgetting waste. Subgrades are uneven and forms bow slightly. Running short mid-pour is a disaster, since the first batch starts to set while you rush to buy more.
- Measuring the thickness once. If the ground is rough or sloped, the slab may be much thicker in places. Measure at several points and average them.
- Using the wrong bag yield in the formula. An 80 lb bag yields about 0.60 ft³, but a 60 lb bag yields 0.45 ft³ and a 40 lb bag only 0.30 ft³. Use the yield printed on the bag you are actually buying.
- Over-watering the mix. Extra water makes it easier to pour and weaker once cured. Follow the bag directions.
- No reinforcement or control joints. Concrete cracks. Reinforcement (see the rebar calculator) and cut or tooled joints decide where.
Frequently asked
How much does a cubic yard of concrete weigh? About 4,000 lb (roughly 2 tons). At 150 lb per ft³ it works out to 4,050 lb.
How long before I can walk on it? Standard concrete is usually walkable after 24 to 48 hours, and reaches most of its strength in 7 days and full design strength at 28 days. Keep it damp for the first few days. Rapid-setting mixes are faster, so follow their instructions.
Can I pour concrete in the cold? Below about 40°F, concrete cures slowly and can be damaged by freezing in the first days. Use cold-weather methods or wait for warmer weather.
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.