Post and Footing Concrete Calculator: How Many Bags for Your Holes
Count the concrete bags and gravel for fence post or deck footing holes. Pick up your fence or deck plan, set hole size and depth, get a buy list and a drawing. Runs in your browser.
When to use this
- You have worked out how many posts or footings you need and want the number of concrete bags.
- You want the bag count to follow the hole size, depth and the post that sits in it, not a rule of thumb per post.
- You want the gravel for the bottom of the holes and one buy list that adds up with your fence or deck.
How to use
- If you used the fence or deck tool on this site in this browser, press the Import chip at the top to fill in the number of holes and sizes.
- Otherwise type the number of holes, the post size, the hole diameter and the hole depth.
- Choose your bag size, or type the yield printed on your bag.
- Press Calculate concrete. Read the bags to buy and check the drawing.
- Add the result to your project plan so the stain tool and the combined buy list can use it.
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Report a problem or suggest an improvement
Nothing is sent until you press Send. Your measurements are included only if you tick the box, and you can see the full report below first.
Examples
- 16 fence posts, 4x4, 10 in holes: 16 holes, 3.5 in post, 10 in wide, 36 in deep, 4 in of gravel, 60 lb bags (0.45 ft³ each), 5 percent spare: 46 bags of concrete and about 3 ft³ of gravel.
- Deck footings with tubes: 6 holes, 5.5 in post, 12 in wide, 42 in deep. Import the deck plan for the hole count and type the rest.
Understanding the result
Each hole is a round cylinder. The concrete is the volume of that cylinder, from the top of the gravel up to the top of the concrete, minus the square post that stands in it. Multiply by the number of holes, add the spare, and divide by the yield of one bag. The result is rounded up to whole bags. The gravel is the cylinder under the post. The drawing shows one hole to scale.
Limitations
- This estimates materials only. It is not a structural design and does not check soil, loads, slope, local codes or frost rules.
- One hole size per calculation. If your fence and your deck use different holes, run the tool once for each and add the bag counts.
- Round holes only. Square or belled footings, concrete tubes with flared bases and slabs are not modelled.
- Real holes are rarely perfect cylinders, so keep the spare. Wet mixes and uneven sides change how many bags you use.
FAQ
How many bags of concrete do I need per post?
It depends on the hole and the bag. A 10 in wide hole, 32 in of concrete and a 4x4 post needs about 1.2 ft³, which is about 2.7 bags of 60 lb mix. The tool does this for every hole and rounds up the total.
How deep should a fence post hole be?
A common starting point is about one third of the post length underground, and below the frost line where the ground freezes. Local rules and soil change this, so check them before you dig.
How wide should the hole be?
Many installation guides suggest a hole about three times as wide as the post. A very narrow hole leaves little concrete around the post, and the tool warns you when the hole is less than twice the post width.
Do I need gravel at the bottom of the hole?
A layer of gravel under the post lets water drain away instead of sitting under the post. It is commonly recommended, and the tool counts it separately from the concrete.
Where do I find the bag yield?
It is printed on the bag, often as cubic feet or litres per bag. Common 80 lb bags make about 0.6 ft³ and 60 lb bags about 0.45 ft³, but brands differ.
Does this tool store my measurements?
No. It runs in your browser. The share link keeps your numbers after the # sign, which is never sent to a server. Your project plan is stored only in your own browser if you press the button.
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