Fence Post Hole Concrete Calculator: How Many Bags You Need
Reviewed by the OnlineFree.app team ยท Updated
Key points
- A 9 in by 30 in hole around a 4x4 post holds about 0.89 cubic feet - roughly 3 bags of 50 lb mix.
- The Fence Post Hole Concrete Calculator subtracts the submerged post volume before it converts cubic feet to bags.
- Bag yields used by the tool: 40 lb = 0.30 ft3, 50 lb = 0.375 ft3, 60 lb = 0.45 ft3, 80 lb = 0.60 ft3.
- A 10% waste margin absorbs spillage; the default run of 10 posts needs 27 bags of 50 lb mix.
- Holes shallower than 32 inches trigger a frost-depth warning, so check local code before you dig.
How many bags of concrete per fence post?
A typical 9 in x 30 in post hole around a 4x4 post needs about 0.89 cubic feet of concrete, which is roughly 3 bags of 50 lb fast-setting mix once a 10% waste margin is added.
That per-hole number is what most builders actually walk into the hardware store with. The Fence Post Hole Concrete Calculator scales it to the whole run: you enter the number of posts (1-500), hole diameter (4-18 in), hole depth (12-60 in), post size, bag size and a waste percentage, and the result card returns a bag count rounded up to whole bags plus the total volume in cubic feet and cubic yards.
It also shows a per-post breakdown - hole volume minus the volume of the post standing in it - and a comparison table of bags needed at 40, 50, 60 and 80 lb, so you can buy whatever is on the shelf rather than what you assumed would be there.
Who benefits most from a fence post concrete estimate
The people who get the most out of this are the ones buying concrete in a single trip. A homeowner setting eight posts on a Saturday wants a number before leaving the house, not a second drive to the store after running short at post six.
Small contractors and deck builders use it the same way, but for quoting. Run the dimensions for a job, get cubic yards and a bag count, and price the concrete line with something better than a guess based on last summer's project.
Gate posts and pergola footings are another good fit, because those holes are usually wider than a standard fence line and the bag count climbs fast - a 12 in hole holds well over twice the concrete of a 9 in hole at the same depth. If you want a plain calculator set for other jobs, the other free online tools on the site cover similar everyday math.
The math behind the bag count
Every hole is treated as a cylinder: volume = pi x radius squared x depth. At the default 9 in diameter and 30 in depth, the radius is 4.5 in, which gives 1.10 cubic feet of gross hole volume.
The post takes up part of that space. A "4x4" post actually measures 3.5 in by 3.5 in, so a 30 in submerged length displaces about 0.21 cubic feet, leaving roughly 0.89 cubic feet of concrete per hole. Ten posts is 8.9 cubic feet; add the default 10% waste margin and it becomes 9.8 cubic feet, or 0.36 cubic yards.
Bag count is that volume divided by the yield of the bag you pick. The yield figures used by the tool match the mixes sold at retail: a 40 lb bag yields about 0.30 ft3, 50 lb about 0.375 ft3, 60 lb about 0.45 ft3, and 80 lb about 0.60 ft3. At 0.375 ft3 per 50 lb bag, 9.8 ft3 rounds up to 27 bags. The Portland Cement Association is a useful reference for how bagged mix yields are defined.
Where a fence post concrete estimate goes wrong
Depth is the biggest variable the tool cannot check for you. It warns when hole depth drops below 32 inches, because codes generally require the footing to sit below the local frost line - the International Code Council publishes the model codes most jurisdictions adopt, and your local building department has the depth that applies to your address.
Holes are also rarely perfect cylinders. Clay soils get over-dug, augers bell out at the bottom, and a hole that started at 9 inches can be closer to 11 at the base. That is what the waste margin is for: 10% covers ordinary spillage, and 15-20% is a safer hedge in rocky ground where the sides crumble.
Nominal versus actual post size is a quiet source of error too. A "6x6" post is 5.5 in square, not 6 in, and plugging in 6 in overstates the displaced volume. Bagged mix also yields less than the label if it is over-watered; the printed yield assumes a correctly mixed batch, so follow the water quantity on the bag.
Bag sizes, waste margin, and one-trip shopping
The comparison table is the part people use at the shelf. The same 9.8 cubic feet takes 33 bags of 40 lb, 27 of 50 lb, 22 of 60 lb, or 17 of 80 lb. Total weight lands near 1,300 lb either way, so the choice is mostly about how much you want to lift per bag, not about saving material.
The waste margin slider runs from 0% to 30% and defaults to 10%. Zero gives the theoretical minimum, which is useful if you have already over-dug and want to know the floor. Twenty percent is a reasonable hedge for a first fence in unfamiliar soil.
Buy at least one spare bag on a run this size, and keep unopened bags dry. If you are pricing several fence runs side by side, a free spreadsheet can sit alongside the calculator for the bookkeeping. Treat the bag count as a starting point you confirm against your own dig - hole dimensions vary more than any estimate can see.
Frequently asked questions
How many bags of concrete do I need for 10 fence posts?
Using the Fence Post Hole Concrete Calculator defaults - 10 posts, 9 in diameter, 30 in deep, 4x4 posts, 50 lb bags, 10% waste - you need 27 bags, which is about 0.36 cubic yards. Each hole takes roughly 0.89 cubic feet of concrete once the submerged post volume is removed. Switch to 80 lb bags and the same job fits in 17 bags.
Does the Fence Post Hole Concrete Calculator subtract the post itself?
Yes. The per-post breakdown shows hole volume minus the volume of the post standing in it. A 4x4 post actually measures 3.5 in by 3.5 in, so in a 30 in deep hole it displaces about 0.21 cubic feet. That leaves roughly 0.89 cubic feet of concrete to pour per hole.
Is the Fence Post Hole Concrete Calculator free, and does it store my data?
It is free and runs in your browser on OnlineFree.app with no account and no file upload. You type the post count and dimensions, and it returns bags, cubic feet and cubic yards immediately, with two-way updates as you adjust numbers. The only record of your fence dimensions is whatever you keep yourself.
How deep should fence post holes be?
Most fence posts go into holes 30 to 36 inches deep, and the Fence Post Hole Concrete Calculator flags any depth below 32 inches. Building codes typically require the footing to reach below the local frost depth, which varies widely by region, so confirm the number that applies to your address with your local building department before digging.
Can I use expanding foam instead of concrete for fence posts?
Expanding polyurethane foam is faster, lighter and often cheaper, and it works for light fences in stable soil. Concrete remains the safer choice for gate posts, corner posts and anything taking wind load or vehicle traffic. The Fence Post Hole Concrete Calculator sizes concrete only, so it will not estimate a foam kit for you.