Post hole concrete calculator
Ver el procedimiento — con sus números
This gives the concrete that fills the hole you describe. How deep the hole goes, how wide it is and whether the post needs a footing at all are questions for your plans, your frost depth and your local authority — the tool takes those numbers, it does not set them.
Type the hole, the post and how many holes, and you get the net concrete per hole and the whole bags to buy for the run. The hole is a cylinder — π × (diameter ÷ 2)² × depth — the post standing in it is taken off, the total is multiplied by your waste factor and divided by the yield of one bag, then rounded up. Change the bag yield to match what your yard sells.
Ejemplo resuelto
A 10-inch hole, 30 inches deep, with a 4×4 (3.5 in square) in it. The hole is π × 5² × 30 = 2,356 in³, which is 2,356 ÷ 1,728 = 1.36 ft³. The post takes 3.5 × 3.5 × 30 = 367.5 in³, or 0.21 ft³. Net concrete per hole is 1.36 − 0.21 = 1.15 ft³. Fourteen holes make 16.1 ft³; with 10% waste that is 17.7 ft³, and at 0.6 ft³ per 80 lb bag you need 29.5 — so buy 30 bags (17.7 ft³ is 0.50 m³, or 0.66 yd³).
The same run in metric: a 250 mm hole, 750 mm deep, with a 90 × 90 mm post. The hole is π × 125² × 750 = 36.8 million mm³, or 0.0368 m³; the post takes 90 × 90 × 750 = 6.08 million mm³, 0.0061 m³. Net per hole is 0.0307 m³. Fourteen holes make 0.430 m³; with 10% waste that is 0.473 m³, and at 0.014 m³ per 30 kg bag you need 33.8 — so buy 34 bags.
En la obra
Bags or a truck. Thirty bags is a pickup load and an afternoon of mixing; the line where a short-load truck starts to make sense depends on your plant's minimum charge and the price of your back. Put the total cubic feet (or cubic metres) from the result into the concrete calculator's ready-mix comparer to see where the break-even sits for your prices.
Dry-pour is a different count. Some crews pour the mix in dry and water it in the hole. The volume of the hole does not change, so the bag count does not either — but a dry pour packs a little looser, so lean on the waste factor rather than the bag maths if that is how you set posts.
The post goes in first. The tool subtracts the post because it is standing in the hole while you pour. If you are setting a bracket on a cured pier and bolting the post on top, there is no post in the concrete — set the post size to 0 and the hole fills solid.
What went wrong?
Post-hole takeoffs slip in the same three places: cubic inches that never become cubic feet, a post that never gets subtracted, and a bag count rounded the wrong way.
Quick referencefor post holes
- Cubic inches in a cubic foot
- 1,728 (12 × 12 × 12)
- Hole volume
- π × (diameter ÷ 2)² × depth
- 4×4 · 6×6 actual
- 3.5 in · 5.5 in square
- 10 in hole × 30 in, 4×4 in it
- about 1.15 ft³ net — 2 bags at 0.6 ft³
- 12 in hole × 36 in, 6×6 in it
- about 1.73 ft³ net — 3 bags at 0.6 ft³
- Common bag yields
- 80 lb ≈ 0.6 ft³ · 60 lb ≈ 0.45 ft³ · 50 lb ≈ 0.375 ft³ — read your bag
- Métrico
- 1 m³ = 35.3 ft³ · 30 kg bag ≈ 0.014 m³
- Cubic millimetres in a cubic metre
- 1,000,000,000 (1,000 × 1,000 × 1,000)
- 250 mm hole × 750 mm, 90 mm post in it
- about 0.031 m³ net — 3 bags at 0.014 m³
- 300 mm hole × 900 mm, 140 mm post in it
- about 0.046 m³ net — 4 bags at 0.014 m³
Waste & overagewhat to add, and why
- Fence crews mix 10–15% over on post holes because an augered hole in soft ground is never the diameter of the auger — it bells out at the bottom and slumps at the top. The waste field starts at 10%.
- Buy bags in whole numbers per hole, not per job: twelve holes at 1.6 bags each round to 20 bags on the sum but 24 in the field if you mix a hole at a time.
- One spare bag on the truck covers the hole that hit a rock and had to be dug wider.
Mistakes to catchbefore you order
- Typing the auger's nominal size when the hole is wider — measure a couple of the holes; an 8 in auger in sandy ground often leaves a 10 in hole.
- Forgetting the post: a 4 × 4 in a 10 in hole takes up about a sixth of the HOLE — 3.5² ÷ (π × 5²) = 16% — which is about 19% on top of the concrete that actually goes in, since what you pour is the other five sixths. The tool subtracts it only if you type the post size.
- Measuring hole depth from the top of the post instead of from grade to the bottom of the hole.
What an inspector looks forthe building inspector, for a deck or a fence needing a permit
On this part of the job
- Hole depth measured from finished grade, against whatever frost depth your jurisdiction uses — the inspector will have the number, so have your own book open to the same page.
- Diameter at the bottom, and whether the drawing wanted a bell or a pad rather than a straight auger hole.
- Bearing at the bottom of the hole: undisturbed, not backfill, not mud, no standing water.
- How the post is fixed to the concrete. A wood post cast into a pier is treated differently from a post on a saddle above it, and which one you used is visible for about ten minutes.
- Position: the hole centres against the layout, and the setback from any lot line.
Have this ready
- The deck or fence plan with post spacing and pier size.
- Your own code book open at the frost-depth and footing sections.
- Holes open, clean and covered — they are inspected empty.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La fórmula
per hole = π × (D ÷ 2)² × depth − post area × depth
Work in one unit — inches or millimetres — for the whole thing, then convert the answer once: cubic inches ÷ 1,728 for cubic feet, cubic millimetres ÷ 1,000,000,000 for cubic metres. Post area is side² for a square post or π × (diameter ÷ 2)² for a round one. Bags are the total, with waste, divided by the yield of one bag and rounded up.
What this tool assumes
That the hole is a straight cylinder from grade to the bottom, that the post stands the full depth of it, and that the bag yield you typed is what the bag delivers. The default yields (0.6 ft³ for an 80 lb bag, 0.014 m³ for 30 kg) are common figures, not a promise — brands differ, so read yours. Nothing here says how deep or how wide the hole should be.
Preguntas
- How many bags of concrete do I need per fence post?
- Work out the hole as a cylinder — π × (diameter ÷ 2)² × depth — take off the part of the post that sits in it, and divide by the yield of one bag. A 10-inch hole 30 inches deep with a 4×4 in it holds about 1.15 cubic feet net, which is two 80 lb bags at 0.6 ft³ each. In metric, a 250 mm hole 750 mm deep around a 90 mm square post holds about 0.031 m³ net — three 30 kg bags at 0.014 m³. Type your hole, your post and your bag and the tool does that for every hole on the run.
- Do I subtract the post from the concrete?
- Yes — the post takes up room in the hole, and on a long fence it adds up. A 4×4 (3.5 inches square) in a 30-inch hole displaces about 0.21 cubic feet (0.006 m³) — 3.5² ÷ (π × 5²), about a sixth of the hole. Over fourteen holes that is nearly five bags you would otherwise buy and not use. The tool knocks the post off automatically; set the post size to zero if the hole is being filled solid, as for a pier or a bollard.
- How deep and how wide should the hole be?
- That is a decision for your plans, your frost line and your local code, not this page. As a rule of thumb the trade digs a hole about three times the post width and sets a fair share of the post below grade, but the numbers that apply to your job come from your authority or your engineer. Type whatever your job calls for and the tool gives you the concrete for it.
- Which bag size is the default?
- An 80 lb bag at 0.6 cubic feet of yield in imperial, a 30 kg bag at 0.014 cubic metres in metric — the same common defaults the concrete calculator uses. Yield varies by brand and mix, so read the bag and change the field. A 60 lb bag is usually about 0.45 ft³; a 50 lb bag nearer 0.375 ft³.
- Why does the tool round bags up?
- Because you cannot buy half a bag and a short hole is a loose post. The bag count is the total concrete divided by the yield, rounded up to the next whole bag. The waste factor on top covers the hole that caves a little, the one the auger wandered on, and the bag that splits in the truck.
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