Hole Volume Calculator
Find the volume of post holes, footing holes and planting holes — and what it takes to fill them. Enter the hole size, depth and count, subtract the post, add a gravel base, pick a fill material, and get the hole volume, bags of fill, the dirt you'll dig out and the cost — with by-width and by-depth tables and a downloadable PDF.
| Width | Per hole | Total | Fill bags |
|---|---|---|---|
| Enter a depth to compare widths | |||
| Depth | Per hole | Total | Fill bags |
|---|---|---|---|
| Enter a width to compare depths | |||
How to use the hole volume calculator
Four quick steps turn your holes into the fill to buy and the dirt to move — volume, bags, loose spoil, and cost — with by-width and by-depth tables and a downloadable report.
Enter the hole
Pick round, square, or tapered, give the width and depth, and the number of holes.
Subtract post & base
Choose the post that sits in the hole and add a gravel drainage base if you’re using one.
Pick the fill
Concrete, gravel, soil, or sand, with a bag size and a waste allowance.
Read results & save a PDF
Get volume, bags, dug-out dirt, and cost — then download a report.
How the volume is calculated
Hole volume by shape, minus the post and gravel base, gives the fill; the dug-out soil swells as it loosens. Here is the exact math the calculator runs.
Worked example — 10 holes, 10 in round, 36 in deep, 4×4 post, 4 in gravel, concrete, 10% waste
One hole: π × 5² × 36 / 1728 ≈ 1.64 ft³ · Gravel base ≈ 0.18 ft³ · Post ≈ 0.28 ft³
Fill each: 1.64 − 0.18 − 0.28 ≈ 1.18 ft³ → with waste ≈ 1.30 ft³ → ceil(1.30/0.60) = 3 bags
Ten holes: ~26 bags · Dug out: 16.4 × 1.25 ≈ 20.5 ft³ loose (0.76 yd³)
≈ 3 bags per hole, ~26 for all ten, plus ~0.76 yd³ of loose dirt. Leave prices blank for typical US figures.
Hole & post reference charts
Handy lookups for round-hole volume, fill materials, and post sizing. Call 811 before you dig, and go below the frost line in cold climates.
| Diameter | Per foot of depth | At 36 in deep |
|---|---|---|
| 8 in | 0.35 ft³ | ~1.05 ft³ |
| 10 in | 0.55 ft³ | ~1.64 ft³ |
| 12 in | 0.79 ft³ | ~2.36 ft³ |
| 16 in | 1.40 ft³ | ~4.19 ft³ |
| 24 in | 3.14 ft³ | ~9.42 ft³ |
Doubling the diameter roughly quadruples the volume. Subtract the post and gravel base for the fill.
| Material | Density | Typical use |
|---|---|---|
| Concrete | 145 lb/ft³ | Setting posts, footings |
| Gravel | 105 lb/ft³ | Drainage, free-draining post set |
| Sand | 100 lb/ft³ | Backfill, bedding |
| Soil | 80 lb/ft³ | Planting holes, backfill |
An 80 lb concrete bag yields ~0.60 ft³; gravel and sand often come in 0.5 ft³ bags.
| Post | Actual size | Hole width (~3×) |
|---|---|---|
| 4×4 wood | 3.5 in | 10–12 in |
| 4×6 wood | 3.5 × 5.5 in | 12–14 in |
| 6×6 wood | 5.5 in | 16–18 in |
| 4 in round | 4 in | 12 in |
Set posts a third to half their above-ground height, deeper for gates and corners and below frost.
Everything the calculator works out
A hole size and a fill give you what goes in and what comes out — volume, bags, loose spoil, and cost — with width and depth comparisons.
Key figures behind the estimate
Built for any hole you dig
From a fence line to a footing to a planting bed, the same calculator turns holes into fill and spoil.
Setting a run of posts and wants the concrete bags per hole and for the whole line.
- Enter diameter & depth
- Pick the post & base
- Read bags per hole
Digging footings or planting holes and needs to know how much spoil comes out after swell.
- Set swell %
- Read loose yd³
- Add a haul-away price
Filling planting holes with soil or sand and wants the bags across many holes at once.
- Choose soil or sand
- Compare widths
- Attach the PDF to the quote
7 things to know about digging holes
A few details keep a hole estimate accurate and the dig safe.
Hole volume calculator FAQ
The volume, post, and dirt questions people ask most about digging and filling holes.
For a round hole, the volume is pi times the radius squared times the depth, where the radius is half the diameter. For a square or rectangular hole it is length times width times depth. Work in consistent units and convert to cubic feet by dividing cubic inches by 1,728, or get cubic yards by dividing cubic feet by 27.
For example, a 10-inch diameter hole 36 inches deep is pi times 5 squared times 36, which is about 2,827 cubic inches, or roughly 1.64 cubic feet.
A tapered hole that is wider at one end uses the cone-frustum formula, and multiplying by the number of holes gives the total. This calculator handles all three shapes and the hole count for you.
Work out the hole volume, subtract the space the post takes up, and that is the concrete you need. A common 10-inch-wide hole 36 inches deep holds about 1.6 cubic feet; a 4×4 post occupies roughly 0.3 cubic feet of that, leaving about 1.3 cubic feet of concrete per hole.
Since an 80-pound bag of concrete yields about 0.6 cubic foot, that hole takes two to three bags. Multiply by the number of posts and add about 10 percent for waste and oversized holes.
A rule of thumb is to size the hole about three times the post width and set it one-third to one-half of the post’s above-ground height, so a 6-foot fence post is often set about 2 feet deep.
A common guideline is to bury one-third to one-half of the post’s above-ground height, so a 6-foot-tall fence section usually needs a hole 2 to 3 feet deep. Corner and gate posts, which take more load, are set deeper.
In cold climates the hole must also extend below the frost line so the post does not heave when the ground freezes; frost depth is set by your local building department and can be 36 to 48 inches or more.
Hole width is typically about three times the post width — roughly 10 to 12 inches for a standard 4×4. Deeper and wider holes use more concrete, which this calculator works out for each depth so you can compare.
A good rule of thumb is to make the hole about three times the width of the post. For a standard 4×4 (which is actually 3.5 inches), that means a hole around 10 to 12 inches in diameter; a 6×6 post wants a hole closer to 16 to 18 inches.
A wider hole gives the concrete more surrounding soil to grip and makes the post more stable, especially for gates, corners and tall fences. Too narrow a hole leaves only a thin collar of concrete that can crack or let the post wobble.
Keep in mind that doubling the hole diameter roughly quadruples the concrete needed, so there is a balance between stability and material cost. This calculator lets you compare widths side by side.
The soil you dig out equals the hole volume when it is in the ground, but loose soil takes up more room than packed soil. This expansion, called swell, is typically 20 to 30 percent for ordinary soil and can reach 40 percent or more for clay.
So a hole that measures 3 cubic feet in the ground produces roughly 3.6 to 3.9 cubic feet of loose spoil to move, pile or haul away. Over many holes this adds up quickly.
This matters when you plan where to put the dirt or how many trips it takes to haul it off. The calculator shows both the in-place volume and the loose volume after swell, plus an approximate weight.
A few inches of gravel at the bottom of a post hole is commonly recommended for wood posts because it lets water drain away from the base rather than pooling against the end grain, which helps the post last longer. Four to six inches is typical.
The post sits on the gravel, and the concrete or backfill goes around and above it. Because the gravel base fills part of the hole, it reduces the amount of concrete you need, which this calculator accounts for.
For metal posts or in very wet ground, practices vary, and some situations call for a full concrete footing instead. Follow local practice and, for structural posts, your engineer or building code.
A tapered hole — wider at one end than the other — is a cone frustum, so its volume is pi times the depth, divided by 3, times the sum of the top radius squared, the bottom radius squared, and the two radii multiplied together.
A belled hole, flared wider at the bottom, resists frost heave and uplift because the soil above the bell holds the footing down. You enter the top and bottom diameters and the depth, and the calculator works out the frustum volume.
For a straight hole the top and bottom diameters are equal and the formula reduces to the simple cylinder volume. Either way, subtract any post and gravel base to get the fill you need.
For a typical fence post in a 10-inch-wide hole about 2 feet deep, plan on one to two 80-pound bags of concrete per post; a deeper 3-foot hole or a wider hole pushes that to two to three bags.
The exact number depends on the hole size, the post that displaces some of the volume, and any gravel base. Divide the net fill volume per hole by the bag yield — about 0.6 cubic foot for an 80-pound bag — and round up.
Buy a little extra so you do not run short mid-project, especially with fast-setting mix that you pour dry into the hole. This calculator gives the bags per hole and the total for all your posts at once.
Estimate only — call 811 before you dig. This calculator finds hole volume and fill in US units, using π × (dia ÷ 2)² × depth for round holes, length × width × depth for square, and a cone-frustum for tapered holes; fill is the hole minus the post and gravel base, plus waste, and dug-out soil swells about 20–30% as it loosens. Bag yields, densities, and typical prices are rough US guides that vary by product and region. Always call 811 (or your local utility locate service) to have underground lines marked before digging, and set posts and footings below the local frost line and to your building code — for structural footings, follow an engineer’s design. For planning purposes only.