Concrete Bag Calculator (40, 60 and 80 lb Bags)
Work out how many 40 lb, 60 lb, 80 lb or 25 kg bags of concrete mix you need for a slab, post hole, column or step. Includes a waste allowance and cubic yard totals.
Leave blank for 1.
Spillage and uneven ground. Blank counts as 0.
In your own currency.
Enter the dimensions to see the volume.
About the concrete bag calculator
The Concrete Bag Calculator answers a simple question: how many bags of premixed concrete do I need? Enter the size of the pour, and it returns the count for 40 lb, 60 lb and 80 lb bags and for 25 kg bags, always rounded up to whole bags. The same calculation also shows the volume in cubic yards, cubic feet, cubic metres and litres, so you can compare the bag route with ordering ready-mix.
Bagged concrete suits small jobs: setting a fence or mailbox post, a deck footing, a small pad under an air conditioner, a step or a repair. It is sold by weight, but what matters for planning is how much space each bag fills once it is mixed. That space is called the yield.
How to calculate bags
Steps
- Select the shape of the pour. A rectangle covers a pad, a circle covers a post hole or a round footing, and a tube covers a hollow ring.
- Enter the dimensions and choose a unit for each one. The depth field is labelled Thickness or height.
- Use Quantity when you have several identical holes or pads, for example 6 fence posts.
- Keep the waste allowance at 10 percent, or raise it for rough holes.
- Read the bag counts for each bag size and, if you enter a price per bag, the estimated cost.
Formulas
Yield per bag
The calculator uses these yields, which are typical for premixed concrete.
- 40 lb bag: 0.30 ft³
- 60 lb bag: 0.45 ft³
- 80 lb bag: 0.60 ft³
- 25 kg bag: 0.0125 m³
A cubic yard is 3 ft x 3 ft x 3 ft, or 27 ft³, so one cubic yard is 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb at these yields. Brands differ a little, so read the label on the bag you buy and, if the printed yield is different, scale the count to match.
Worked example: a 10 ft by 10 ft slab
A slab 10 ft by 10 ft and 4 in thick has a net volume of 33.33 ft³ (1.23 yd³). With a 10 percent allowance the order is 36.67 ft³, which is 1.36 yd³, 1.04 m³ or 1,038.3 litres. That works out to 123 bags of 40 lb, 82 of 60 lb, 62 of 80 lb or 84 of 25 kg. At 6.50 per 80 lb bag the 62 bags cost 403.00.
Worked example: round columns or post holes
Take a round column 2 ft in diameter and 3 ft tall. Each one holds 9.42 ft³, and four of them are 37.70 ft³ before waste. For a single column with 10 percent added, the order is about 10.37 ft³, which is 18 bags of 80 lb or 24 bags of 60 lb. Small post holes follow the same logic: enter the hole diameter and depth as a Circle, set the quantity to the number of posts, and the tool multiplies for you. Remember that the post itself takes up space in the hole, so you will often need slightly fewer bags than the full hole volume suggests. Leaving the 10 percent allowance in is a safe way to cover the difference.
Which bag size to buy
Heavier bags mean fewer bags to open and stack, but an 80 lb bag is awkward to lift and mix in a barrow. Lighter 40 lb or 60 lb bags are easier to handle. Compare the unit price: a bag that looks cheaper may cost more per cubic foot of concrete. The Unit Price Calculator makes that comparison quick. If you have a weight in pounds and need kilograms, use the Mass Converter.
Bags versus ready-mix
Bags are practical for small volumes because you mix only what you need and there is no delivery. As the volume grows, the work and the cost climb quickly. At 150 per yd³ for ready-mix, the 1.36 yd³ in the example above comes to about 203.70, compared with 403.00 for the 62 bags of 80 lb at 6.50 each. Prices vary by region and supplier, so enter your own numbers. As a rough guide, many people switch to a delivery once a job passes a cubic yard or so, and certainly when it needs more than a few dozen bags. Ready-mix suppliers often have a minimum load and a short-load fee, so ask before ordering a small amount.
Why round up and add waste
Bags are sold whole, so the count is rounded up. The waste allowance covers uneven ground, holes dug a little larger than planned and spillage. Having a spare bag on site is cheaper than a second trip to the store with a half-set pour waiting.
Accuracy and limits
The counts are estimates based on typical yields and the measurements you enter. They do not account for reinforcement, water added, or the exact product. For structural work such as footings, posts that carry load or anything under code, confirm quantities and mix with your supplier and an engineer.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
For a flat pour with thickness guidance, see the Concrete Slab Calculator. The Concrete Calculator covers stairs, triangles and other shapes, and the Volume Converter switches between cubic yards, feet, metres and litres.
Frequently asked questions
How much does one bag of concrete yield?
The calculator uses 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag, 0.60 ft³ for an 80 lb bag and 0.0125 m³ for a 25 kg bag. These are typical figures, so check the yield printed on the bag you buy.
How many bags of concrete make a cubic yard?
A cubic yard is 27 ft³, which is 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb at these yields, before any waste allowance.
How many bags do I need for a 10 ft by 10 ft slab at 4 in?
With a 10 percent allowance you need 123 bags of 40 lb, 82 of 60 lb, 62 of 80 lb or 84 of 25 kg. The order volume is 36.67 ft³, or about 1.36 yd³.
How many bags do I need for fence post holes?
Enter each hole as a Circle with its diameter and depth, then set the quantity to the number of posts. The post takes up space in the hole, so the result is a safe upper estimate.
When is ready-mix better than bags?
Once a job passes roughly a cubic yard, or needs dozens of bags, delivered ready-mix is usually easier and often cheaper. Enter your bag price and your per-yard price to compare, and ask about minimum loads and short-load fees.
Why does the bag count round up?
Bags are sold whole, so the calculator takes the order volume, including waste, divides by the bag yield and rounds up. Buying one spare bag is cheaper than stopping a pour to fetch more.