groundmath

Fence post concrete calculator

Enter your hole size and post count for the concrete volume and the bag count in every size sold. Two subtractions most charts skip are done here and shown: the post displaces concrete, and the top 3–4 inches are backfilled with soil rather than filled. If you do not know your hole size yet, work out the depth first — the calculator below links straight to it.

✓ Formula v1.0 · Data v2026.13 · 2 sources · verified Aug 12, 2026
Your holes

Do not know your hole size yet? The fence post depth calculator works it out from the fence height and your frost depth, and hands you both numbers.

You should buy
3 × Concrete Mix, 80 lb bag
1.52 ft³ total · 1.52 ft³ per post · 0.06 cu yd
How we got this
  1. Concrete stops 3 in short of grade, so it fills 30 − 3 = 27 in
  2. Hole: π × (6 in)² × 27 in = 1.767 ft³
  3. Less the post standing in it, 4 × 4 in = 1.517 ft³ per post
  4. × 1 post ÷ 0.6 ft³ per bag = 3 bags, rounded up
✓ QUIKRETE — yields cited under sources →
Every bag size, compared
QUIKRETE Fast-Setting, 50 lb bag5 bags0.375 ft³ each · 5 per post if you buy per hole
QUIKRETE Fast-Setting, 60 lb bag4 bags0.45 ft³ each · 4 per post if you buy per hole
QUIKRETE Concrete Mix, 40 lb bag6 bags0.3 ft³ each · 6 per post if you buy per hole
QUIKRETE Concrete Mix, 50 lb bag5 bags0.375 ft³ each · 5 per post if you buy per hole
QUIKRETE Concrete Mix, 60 lb bag4 bags0.45 ft³ each · 4 per post if you buy per hole
QUIKRETE Concrete Mix, 80 lb bag3 bags0.6 ft³ each · 3 per post if you buy per hole
QUIKRETE Concrete Mix, 90 lb bag3 bags0.675 ft³ each · 3 per post if you buy per hole
Ready-mix instead0.06 cu yda full cubic yard is 45 of the 80 lb bags. Suppliers set their own minimum load and short-load fee, so the crossover is a question for your local plant, not a number we can publish

How this is calculated

  1. Filled depth = hole depth − 3 in, because the published method stops the concrete short of grade and backfills the rest with soil.
  2. Hole volume = π × (diameter ÷ 2)² × filled depth.
  3. Concrete per post = hole volume − post volume. The post is standing in the hole and takes up room.
  4. Bags = total volume ÷ the bag's published yield, rounded up per bag size.

Data used, with sources

ValueUsed asSource
0.375 ft³ per 50 lb bag published yield for QUIKRETE Fast-Setting — 50 lb bag. Poured dry into the hole and soaked; final set in 20 to 40 minutes. The QUIKRETE Companies
0.45 ft³ per 60 lb bag published yield for QUIKRETE Fast-Setting — 60 lb bag The QUIKRETE Companies
0.3 ft³ per 40 lb bag published yield for QUIKRETE Concrete Mix — 40 lb bag The QUIKRETE Companies
0.375 ft³ per 50 lb bag published yield for QUIKRETE Concrete Mix — 50 lb bag The QUIKRETE Companies
0.45 ft³ per 60 lb bag published yield for QUIKRETE Concrete Mix — 60 lb bag The QUIKRETE Companies
0.6 ft³ per 80 lb bag published yield for QUIKRETE Concrete Mix — 80 lb bag The QUIKRETE Companies
0.675 ft³ per 90 lb bag published yield for QUIKRETE Concrete Mix — 90 lb bag. Regional availability. The QUIKRETE Companies
3 × the post's width the published hole diameter for a post set in concrete: "Start by digging the post hole about 3 times the diameter of the post." A 4 in post therefore wants a 12 in hole, which is what makes the concrete volume grow so fast with post size. The QUIKRETE Companies
3–4 in left unfilled the instruction to stop the concrete short of grade: "Pour dry mix into the hole until it is approximately 3 in to 4 in from the top of the hole", then "Fill the upper 3 in to 4 in of the hole with sod or with the soil that was removed." Ignoring it over-orders concrete on every post. The QUIKRETE Companies

The two subtractions, and why they matter

Most bag charts compute the hole and stop. Both of the things they leave out push the same way — they make you buy more concrete than the job needs — and together they are not marginal.

The post displaces concrete. A 4×4 post is 0.11 square feet of cross-section; a 10-inch hole is 0.55. So the post is about a fifth of the hole, and on a narrow hole around a chunky 6×6 it can be over a third. Subtracting it is not a refinement, it is the difference between three bags and four.

The top of the hole is not concrete. The method published on the bag says to pour "until it is approximately 3 in to 4 in from the top of the hole", then backfill that with the soil you dug out. On a 30-inch hole that is a tenth of the depth. This page subtracts the shallow end of that range, which is the conservative choice — assuming less unfilled depth means ordering slightly more, and running short mid-pour is the expensive mistake.

Hole width matters more than you think

The published rule is a hole about 3 times the post's width, and the reason it drives cost so hard is that volume grows with the square of the diameter while you only measured across it. Widening a hole from 10 to 12 inches — two inches, barely visible — adds 44 percent more concrete. From 10 to 16 inches is more than double.

This is worth knowing before you rent an auger, because auger bits come in fixed sizes and it is tempting to take the bigger one. It is also why a gate post, which genuinely wants a larger hole, costs disproportionately more to set than the line posts either side of it. If you are pricing a long run, the cubic yard calculator converts the total into the unit bulk suppliers actually quote in.

Common mistakes

  • Trusting the bag's own rule of thumb. QUIKRETE's data sheet says two 50 lb bags set a 3-inch post in a 9-inch hole 30 inches deep. Work that hole through the sheet's own yield and its own top-gap instruction and it needs about 2.2 bags — so the example rounds down. The yields are sound; the shortcut is not.
  • Forgetting the gravel you put in the bottom. It displaces concrete. Subtract its depth from the hole depth you enter.
  • Buying exactly the calculated count. Spillage is real and no source publishes an allowance for it, so we do not invent one — buy a spare bag instead.
  • Mixing fast-setting concrete like regular mix. It is designed to go in dry and be soaked in place, and it sets in 20 to 40 minutes. Mixing a barrow of it is a race you lose.
  • Loading the fence too soon. The sheet says to wait 4 hours before the post takes any strain — and that is the fast-setting product.

Get the depth right before the volume

This page answers how much concrete fills a hole. It does not decide how deep that hole should be, and that is the question with the consequences: too shallow and the post leans or heaves, and no quantity of concrete fixes it. Frost depth and setbacks are local-code matters, so the fence post depth calculator — linked from the calculator above and in the footer — handles them and carries the state-by-state picture, including the states that publish no frost depth at all.

If you are setting posts in compacted gravel rather than concrete, the crushed stone calculator sizes that instead, and applies the compaction allowance that makes loose stone arrive as more than its finished volume.

Frequently asked

How many bags of concrete do I need per fence post?
It depends entirely on the hole, which is why a single number is the wrong answer — and why the charts that print one are wrong more often than they are right. Work it as a volume: the hole, less the post standing in it, less the top few inches you backfill with soil. A 12-inch hole 30 inches deep around a 4×4 is about 1.6 cubic feet of concrete, which is three 80 lb bags. Widen that hole to 16 inches and it is nearly six. Volume grows with the square of the diameter, so hole width matters far more than most people expect.
Does the concrete fill the hole all the way to the top?
No, and this is the instruction most often skipped. QUIKRETE's own data sheet says to pour "until it is approximately 3 in to 4 in from the top of the hole", then "fill the upper 3 in to 4 in of the hole with sod or with the soil that was removed". Two reasons: it hides the concrete, and it lets you grow grass over it rather than mow around a disc. This calculator subtracts that gap, which is why it asks for hole depth rather than concrete depth.
Do I subtract the post from the hole volume?
Yes. The post occupies part of the hole, and on a narrow hole it is not a rounding error — a 4×4 in a 10-inch hole displaces about 15 percent of the volume. Many charts ignore it and quietly over-sell you. We subtract it, and show the subtraction as its own step so you can check it. The one place to be generous instead is spillage: buy a spare bag.
How many bags of concrete make a cubic yard?
45 of the 80 lb bags, from the published yield of 0.6 cubic feet each against 27 cubic feet in a yard. That number is the practical crossover point to think about: once a job approaches a yard, mixing it bag by bag becomes a day's work. Whether ready-mix is actually cheaper depends on your supplier's minimum load and short-load fee, which vary too much regionally for us to publish a figure.
Is fast-setting concrete different from regular concrete mix?
For the quantity, barely — both yield 0.0075 cubic feet per pound, so a 50 lb bag of either is 0.375 cubic feet. What differs is the method and the timing. Fast-setting mix is designed to be poured into the hole dry and soaked with water, reaching final set in 20 to 40 minutes, with the post left unstrained for 4 hours. Regular mix is mixed first and gives you working time to plumb the post. Choose on how many posts you are setting and how confident you are getting them plumb, not on volume.
Should I put gravel in the bottom of the hole?
It is common practice, for drainage under the post end, and it changes the arithmetic: gravel displaces concrete, so subtract its depth from the hole depth you enter here. The trade-off is real rather than settled — gravel drains, but it also gives a frost-susceptible material a path under the post. The cited horse-fencing guidance recommends a concrete collar specifically for sandy or chronically wet soils, which suggests the answer depends on your ground rather than on a universal rule.

Sources

Quantities on this page are planning estimates, not engineering advice or a purchase guarantee — product sizes vary by producer, so confirm coverage with your supplier before ordering. This sizes concrete, not the fence. Hole depth is governed by local code — frost depth and setbacks are set by your city or county — and this page assumes the depth you enter is already correct. Confirm it, and the boundary, with your building department, and call 811 before you dig.