Concrete Bag Calculator
This concrete bag calculator tells you how many 40, 50, 60 or 80 lb bags you need for a slab, fence post holes, a Sonotube column or any volume you already know. It shows the bag count for every size, the weight you will lift, the cost, and whether ready-mix would be cheaper. Tap the i button beside any field for an explanation.
Quick answer: bags = volume (cu ft) × 1.10 ÷ yield per bag, rounded up. An 80 lb bag makes about 0.60 cubic feet, so a 10 × 10 ft slab that is 4 in thick needs about 62 bags. One cubic yard needs about 45 bags of 80 lb mix.
Calculate how many bags of concrete you need
Example values are loaded so you can see a result straight away. Change any field and the numbers update.
1. Choose your units
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2. What are you pouring?
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3. Bag size, extra allowance and prices
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4. Ready-mix comparison (optional)
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Bags needed by bag size
The same job in each common bag size. Your selected size is the orange bar.
Bags at different extra allowances
How many bags the extra allowance adds. Your allowance is the orange bar.
Bags or ready-mix?
Estimates for planning only. Prices shown are examples you can edit, not quotes. Confirm measurements on site, check the yield on your bag and follow your local building code. See our Disclaimer.
How the concrete bag calculator works
A concrete bag calculator answers one practical question: how many bags should go in the cart? Every bag of mix has a printed yield, which is the volume of wet concrete it makes. Divide the volume you need by that yield, round up to whole bags, and you have your number.
This calculator does that for four kinds of job: a slab, walkway, pad or step; fence post holes; a round column or Sonotube-style form; and a volume you already know. It also shows the bag count for every common size, the weight you will have to lift, what the bags cost, and how that compares with a ready-mix truck.
How to calculate how many bags of concrete you need
If you want to check the answer by hand, the method takes four steps:
- Find the volume in cubic feet. For a slab, multiply length × width × thickness, with the thickness converted from inches to feet.
- Add an allowance, usually 5% to 10%, for uneven ground and spillage.
- Divide by the yield of one bag. The yield is printed on the bag. An 80 lb bag typically makes 0.60 cubic feet.
- Round up to the next whole bag.
Worked example: a 10 × 10 ft slab, 4 in thick
The slab holds 10 × 10 × 0.333 = 33.3 cubic feet. Adding 10% gives 36.7 cubic feet. Divided by 0.60, that is 61.1 bags, which rounds up to 62 bags of 80 lb mix. The same slab needs 82 bags of 60 lb mix, 98 bags of 50 lb mix or 123 bags of 40 lb mix. The bag count changes with the size, but the total weight of dry mix is almost the same, so a bigger bag saves trips, not material.
How many bags of concrete are in a yard?
A cubic yard is 27 cubic feet, so the bags per yard are 27 divided by the yield. The table also gives the figure per cubic metre, for readers who buy by metric volume.
| Bag size | Typical yield (cu ft) | Bags per cubic yard | Bags per cubic metre |
|---|---|---|---|
| 40 lb bag | 0.30 | 90 | about 118 |
| 50 lb bag | 0.375 | 72 | about 95 |
| 60 lb bag | 0.45 | 60 | about 79 |
| 80 lb bag | 0.6 | 45 | about 59 |
Use the table the other way round if you already know the yards. Multiply your volume by the bags per yard for your bag size, then add your allowance. If you are starting from the dimensions of a slab, footing or wall, our concrete yard calculator gives the volume in cubic yards first.
Which bag size should you buy?
All four sizes make the same concrete, so the choice is about handling and price. The table shows the 10 × 10 ft example in each size.
| Bag size | Yield (cu ft) | Bags for the 10 × 10 ft slab | Dry mix to lift | Example price | Price per cu ft |
|---|---|---|---|---|---|
| 40 lb | 0.30 | 123 | 4,920 lb | $4 | $13.33 |
| 50 lb | 0.375 | 98 | 4,900 lb | $5 | $13.33 |
| 60 lb | 0.45 | 82 | 4,920 lb | $6 | $13.33 |
| 80 lb | 0.6 | 62 | 4,960 lb | $7 | $11.67 |
The 80 lb bag means fewer bags to carry and open, but each one is heavy and hard to lift into a mixer. The 40 lb and 50 lb bags are easier on the back and suit anyone working alone, but you will open more of them and mix more batches. Compare shelf prices by cost per cubic foot, not per bag. At the example prices used in the calculator, an 80 lb bag at $7 works out to $11.67 per cubic foot, while a 60 lb bag at $6 costs $13.33. Check your own store, since prices and promotions change.
How many bags for a slab, walkway or pad?
These are common sizes at 4 in thick, with 10% extra and rounded up to whole bags.
| Slab size (4 in thick) | Area | Concrete (cu ft) | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 16 sq ft | 5.3 | 10 | 14 |
| 4 × 8 ft | 32 sq ft | 10.7 | 20 | 27 |
| 6 × 6 ft | 36 sq ft | 12.0 | 22 | 30 |
| 8 × 8 ft | 64 sq ft | 21.3 | 40 | 53 |
| 10 × 10 ft | 100 sq ft | 33.3 | 62 | 82 |
| 12 × 12 ft | 144 sq ft | 48.0 | 88 | 118 |
Thickness changes the count in proportion. A 6 in slab needs half as many bags again as a 4 in slab, so double-check the depth before you buy. Not sure of the area? The square footage calculator works it out for odd shapes, and you can enter the result in the calculator above as length × width.
How many bags of concrete for a fence post?
Post holes are where bag counts are most often wrong, because a post takes up room in the hole. A 4 × 4 post is really 3.5 × 3.5 inches, and a 6 × 6 is 5.5 × 5.5. The calculator subtracts the post from the hole, and it lets you leave room for gravel at the bottom. The table shows several common setups, using 80 lb bags.
| Post | Hole | Concrete per hole (cu ft) | 80 lb bags per hole (exact) | Bags for 10 posts (+10%) |
|---|---|---|---|---|
| 4 × 4 fence post | 10 in wide, 30 in deep | 1.15 | 1.92 | 22 |
| 4 × 4 fence post | 12 in wide, 36 in deep | 2.10 | 3.50 | 39 |
| 4 × 6 post | 12 in wide, 36 in deep | 1.96 | 3.26 | 36 |
| 6 × 6 post | 12 in wide, 36 in deep | 1.73 | 2.88 | 32 |
| 6 × 6 post | 16 in wide, 42 in deep | 4.15 | 6.92 | 77 |
| Round post, 4 in across | 10 in wide, 30 in deep | 1.15 | 1.91 | 21 |
Here is why this matters. For ten 4 × 4 posts in 10 in holes that are 30 in deep, the calculator gives 22 bags. If you ignored the post and treated each hole as solid concrete, you would buy 25.
The depth of the hole depends on the height of the fence, the soil and your local frost depth, so follow your fence plan and local rules for that. Once you know it, enter it above.
How many bags for a Sonotube or round column?
Cardboard forms are sold by inside diameter, and a round form holds 0.7854 × diameter² × height. The table gives the concrete in each size for every foot of height, and the bags per foot (80 lb and 60 lb, before any allowance).
| Tube diameter | Concrete per foot of height (cu ft) | 80 lb bags per foot | 60 lb bags per foot |
|---|---|---|---|
| 8 in | 0.35 | 0.58 | 0.78 |
| 10 in | 0.55 | 0.91 | 1.21 |
| 12 in | 0.79 | 1.31 | 1.75 |
| 14 in | 1.07 | 1.78 | 2.38 |
| 16 in | 1.40 | 2.33 | 3.10 |
| 18 in | 1.77 | 2.95 | 3.93 |
| 24 in | 3.14 | 5.24 | 6.98 |
To use it, multiply the bags per foot by the height of your column and the number of columns, then add your allowance. A 12 in form that is 4 ft tall holds 3.1 cubic feet. With 10% extra, that needs 6 bags of 80 lb mix. In the calculator, choose the tube option, pick the tube size and enter the height.
How much extra should you add when working in bags?
Bags round themselves up, so the allowance matters less on a tiny job and more on a large one. On a small pad, the spare concrete in the last bag may be all the cushion you need. On a big slab, 5% to 10% is the sensible range, because the ground is never perfectly level and some concrete always stays in the wheelbarrow.
The chart in the results shows how the 10 × 10 ft example reacts to the allowance. With none, the slab needs 56 bags. At 5% it needs 59, at 10% 62, at 15% 64 and at 20% 67. Running short mid-pour is worse than carrying one extra bag home, because a second batch poured after the first has started to set leaves a weak joint.
Bags or ready-mix: where the line falls
A truck is cheaper per cubic yard, but it adds delivery or short-load fees, so small jobs can favour bags. At the example prices, 45 bags of 80 lb mix cost $315 per yard, against $165 for ready-mix plus a $100 delivery fee. The two meet at about two-thirds of a cubic yard, around 30 bags.
The results box shows both costs for your job, using prices you can edit. Past that crossover, the truck usually wins on price and on effort. For a full slab budget including the gravel base, reinforcement and labour, use the concrete slab cost calculator.
Mixing and handling bagged concrete
A 10 × 10 ft slab is 62 bags of 80 lb mix, or roughly 2.5 tons of dry material. Plan how it gets from the car to the work area. A few habits make the pour go better:
- Mix whole bags only. Splitting a bag puts the cement and aggregate out of balance.
- Follow the water amount printed on the bag, and measure it the same way every batch. Extra water makes the mix easier to move and weaker once it sets.
- Do not start more than you can place and finish. Concrete begins to set while you work, so stage the forms, tools and helpers first.
- Wear gloves, eye protection and a dust mask. Cement dust and wet concrete both irritate skin and eyes.
Does the brand matter?
Many people search for a QUIKRETE or Sakrete calculator because those are the bags on the shelf. The common sizes are the same across major brands, but the yield printed on each product can differ slightly, and specialty mixes such as fast-setting post mix or high-strength mix may not match the standard figures. If your bag differs, choose the custom bag option and enter the yield and weight from the label. Manufacturers publish their own calculators, so it is worth comparing the result with one for a bag you are about to buy.
Common mistakes when counting bags
- Ignoring the post. Treating a post hole as solid concrete overbuys by a noticeable margin.
- Mixing up diameter and radius. Hole and tube sizes are diameters. Halve them only in the formula.
- Using the wrong yield. The yield of a 60 lb bag is not that of an 80 lb bag. Pick the size you will actually buy.
- Rounding each post separately. Add up the concrete for every hole first, then round up once. Rounding each hole on its own over-orders.
- Mixing different products in one pour. Each product sets at its own rate.
This page is for planning only. For structural work, heavy loads or poor soil, ask a professional to specify the concrete.
Concrete bag calculator FAQ
How many bags of concrete do I need?
Work out the volume in cubic feet, add 5% to 10% extra, divide by the yield printed on the bag, and round up. An 80 lb bag typically makes 0.60 cubic feet, so a job that needs 30 cubic feet takes about 55 bags with a 10% allowance.
How many 80 lb bags of concrete for a 10 × 10 slab?
A 10 × 10 ft slab that is 4 inches thick holds 33.3 cubic feet. That needs about 56 bags of 80 lb mix with no extra, and 62 bags with a 10% allowance.
How many bags of concrete make a yard?
A cubic yard is 27 cubic feet, so divide 27 by the yield printed on your bag. That gives 45 bags at 0.60 cubic feet, 60 at 0.45, 72 at 0.375 and 90 at 0.30. The table above also gives the figure per cubic metre.
How many bags of concrete do I need for a fence post?
A 4 × 4 post in a 10 inch wide, 30 inch deep hole takes about 1.9 bags of 80 lb mix once the post itself is subtracted. In practice that means two bags per post, and the calculator gives the exact total for all your posts together.
How many bags of concrete for a Sonotube?
A 12 inch tube that is 4 feet tall holds 3.1 cubic feet, which needs about 6 bags of 80 lb mix with a 10% allowance. Use the tube option in the calculator for other sizes and heights.
How much does an 80 lb bag of concrete cover?
An 80 lb bag makes about 0.60 cubic feet, which covers roughly 1.8 square feet at 4 inches thick or 1.2 square feet at 6 inches. Check the yield on your own bag, since brands differ slightly.
How many bags do I need for a 4 × 8 slab?
A 4 × 8 ft slab that is 4 inches thick holds 10.7 cubic feet. With a 10% allowance that needs about 20 bags of 80 lb mix or 27 bags of 60 lb mix.
Is it cheaper to buy bags or order ready-mix?
Per cubic yard, ready-mix is cheaper, but trucks add delivery or short-load fees. With the example prices here, bags win below about two-thirds of a cubic yard, or roughly 30 bags of 80 lb mix. The calculator compares both for your job using prices you can edit.
Do all brands of concrete yield the same?
No. The common bag sizes are similar, but the yield printed on each product can differ, and specialty mixes such as fast-setting or high-strength mix may not match the standard figures. Use the custom bag option to enter the yield and weight from your label.
Should I round up the number of bags?
Yes. Bags are sold whole, and running short mid-pour is worse than having a bag left over. Round up the total for the whole job, not each post or section separately.
Sources and further reading
The volume formulas are standard geometry. Bag yields used here are the typical manufacturer figures, and each bag’s label is the final word. For product details and conversions, go to the source:
- QUIKRETE concrete calculator (a manufacturer’s own calculator, useful for a second opinion)
- Sakrete concrete calculators (product yields and project guidance from another major brand)
- NIST Special Publication 811, Guide for the Use of the International System of Units (unit conversion factors)
- American Cement Association (cement and concrete research and education)
Codes are adopted locally and change over time, so confirm the rules with your local building department.
Plan the rest of the project
For walls, the concrete block calculator counts the blocks you need. Browse the concrete calculators hub for more tools like this one.