Understanding your result
The headline is the volume of concrete to mix or order, including your extra allowance. Beneath it you get the same amount as a number of bags, and as a ready-mix order rounded up to the next quarter of a cubic yard.
One bag makes shows the yield the calculator assumed for your bag size. This is the volume of wet concrete after mixing, not the size of the bag. Yields are similar between brands but not identical, so check the figure printed on the bag you buy.
Weight of the bags is there because someone has to move them. Sixty bags of 80 pounds is 4,800 pounds, which is several trips in most pickups and an afternoon of lifting before any mixing starts.
For post holes, the table shows the concrete per hole and the bags per hole. Fast-setting mixes are usually poured dry into the hole and watered in, one hole at a time, so the per-hole figure is the practical one.
How we calculate this
Every dimension is converted to feet before multiplying, so inches are divided by 12. In metric, everything is converted to meters and the result is in cubic meters.
In post hole mode the post is treated as a square of its actual size. A “4 × 4” post is really 3.5 inches square and a “6 × 6” is 5.5 inches square. Subtracting the post makes a real difference: in a 10 inch hole, a 4 × 4 post takes up about 16% of the space.
The assumptions behind the numbers
| Bag size | Mixed volume | Bags per cubic yard (or m³) |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
| 25 kg | about 11.7 L | about 86 per m³ |
| 20 kg | about 9.4 L | about 107 per m³ |
The pound-bag yields are the figures printed on standard concrete mix in the US. The kilogram-bag yields are scaled from them by weight, so treat them as approximate and check your bag. Other assumptions:
| Assumption | Value used | Why |
|---|---|---|
| Extra allowance | 10% unless you change it | Uneven subgrade, bowed forms and spillage |
| Ready-mix rounding | Up to the next 0.25 cu yd or 0.25 m³ | Typical ordering increment. Suppliers also have a minimum load |
| Post size | Actual dimensions, 3.5 in or 5.5 in square | Nominal lumber sizes are larger than the real ones |
| Reinforcement | Not included | Rebar and mesh depend on the design and local code |
Assumptions last reviewed October 3, 2026.
This calculator gives quantities, not structural design. It will not tell you how deep a footing must be, whether a slab needs reinforcement, or how far below the frost line to dig. Those answers come from your local building department.
Two worked examples
A shed base
A slab for a garden shed is 12 feet by 8 feet and 4 inches thick.
- Volume: 12 × 8 × 4 ÷ 12 = 32 cu ft
- With 10% extra: 35.2 cu ft, which is 1.30 cu yd
- In 80 lb bags: 35.2 ÷ 0.6 = 58.7, so 59 bags
- Weight of the bags: 59 × 80 = 4,720 lb
- Ready-mix: 1.5 cu yd
Now compare the cost. Say bags are $6.50 each, and ready-mix is $165 a yard with a $150 short-load fee. Fifty-nine bags come to $383.50. A yard and a half delivered comes to $397.50. On price alone it is a draw.
On labor it is not. Fifty-nine bags is more than two tons of material to lift into a mixer, one bag at a time, while the first batches are already setting. At this size the short-load fee is buying a slab poured in one go, and for most people that is worth $14.
Setting fence posts
Twelve 4 × 4 posts are going into holes 10 inches across and 30 inches deep, set with 50 pound bags of fast-setting mix.
- Hole: 3.1416 × (5 ÷ 12)² × 2.5 = 1.36 cu ft
- Post: (3.5 ÷ 12)² × 2.5 = 0.21 cu ft
- Concrete per hole: 1.36 − 0.21 = 1.15 cu ft
- Twelve holes: 13.8 cu ft
- With 10% extra: 15.2 cu ft
- In 50 lb bags: 15.2 ÷ 0.375 = 40.5, so 41 bags
That is about three and a half bags a hole. Without subtracting the posts, the estimate would have been 48 bags, seven more than needed. The saving is small on one fence, but it is a full extra trip in a car.
Common mistakes
- Using nominal lumber sizes. A 4 × 4 is 3.5 inches square. Form boards are the same: a “2 × 4” form gives a 3.5 inch slab, not 4.
- Multiplying feet by inches. A 10 × 10 slab that is “4” thick is not 400 cubic feet. Divide the thickness by 12.
- Ordering exactly the calculated volume. There is no good way to add a wheelbarrow of concrete an hour later. Order the extra.
- Underestimating the labor of bags. A cubic yard is 45 heavy bags, every one of which has to be lifted, opened, mixed and poured before the first batch stiffens.
- Forgetting the ready-mix minimum. Many plants have a minimum order or charge a short-load fee below a few yards. Ask before you compare prices.
- Digging holes wider at the top. A hole shaped like a funnel uses far more concrete than a straight one, and it is weaker against frost. Keep the sides vertical.
- Skipping the base. A slab poured on soft or uneven ground uses more concrete in the low spots and cracks over the soft ones. Compact a gravel base first.
Printable shopping list
Print shopping list gives you the order on a single page: bags by size, or the ready-mix volume, with the dimensions you entered. If you are ordering ready-mix, the plant will ask for the volume, the strength and the slump. Tell them what you are pouring and they will advise on the mix. If you are buying bags, check the yield printed on the bag against the one in the table, and count how many trips your vehicle will need at its payload.
Questions people ask
- How many bags of concrete make a cubic yard?
- About 45 bags of 80 pounds, 60 bags of 60 pounds, 72 bags of 50 pounds, or 90 bags of 40 pounds. An 80 pound bag makes 0.6 cubic feet of mixed concrete, and a cubic yard is 27 cubic feet.
- How much concrete do I need for a 10 × 10 slab?
- At 4 inches thick, a 10 by 10 foot slab takes 33.3 cubic feet, which is 1.23 cubic yards. That is 56 bags of 80 pounds before any extra. With 10% added, plan on 62 bags or a ready-mix order of 1.5 cubic yards.
- How many bags of concrete do I need per fence post?
- For a 4 × 4 post in a hole 10 inches across and 30 inches deep, about 1.15 cubic feet of concrete, which is three 50 pound bags or two 80 pound bags. Wider or deeper holes need more. The post hole mode of the calculator subtracts the space the post itself takes up.
- How thick should a concrete slab be?
- Four inches is the standard for paths, patios and shed bases. Driveways for cars are usually 4 to 5 inches, and 5 to 6 inches where heavier vehicles will park. Anything thinner than about 3.5 inches is prone to cracking.
- When is ready-mix cheaper than bags?
- Ready-mix costs less per cubic yard, but small loads carry a short-load fee. The break-even is usually somewhere between 1 and 2 cubic yards. Enter your local prices and the calculator will show which is cheaper for your quantity.
- Why add 10% extra?
- The ground under a slab is never perfectly flat, forms bow outward slightly under the weight, and some concrete is always left in the mixer or the wheelbarrow. Running short in the middle of a pour is far worse than having a little left over.
Built and reviewed by Bertha
Last updated · How we work out our numbers