Hole Volume Calculator
Calculate hole volume for post holes, pier holes, rectangular excavations, trenches, and tapered holes, then estimate fill material, bags, and cost.
Estimate excavation volume before you buy fill material
A hole volume calculator estimates the space inside a hole by using shape, width, length, diameter, and depth. Use it for concrete post holes, deck piers, fence posts, planting holes, utility trenches, and small excavation projects.
Calculate a cylindrical hole with pi x radius squared x depth. Calculate a rectangular hole with length x width x depth. The calculator converts the result into cubic feet, cubic yards, gallons, liters, bag count, and optional cost.
Actual material needs can change because of loose soil, overdigging, collapse, rock, water, compaction, form tubes, rebar, posts, and voids inside the hole.
Adjusted hole volume
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Cubic feet
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Cubic yards
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Useful for bulk soil, gravel, concrete, or disposal.
Bags needed
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Estimated cost
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Raw volume
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Gallons
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Liters
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Quantity
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Allowance
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Hole volume breakdown
Shows the selected shape, formula, raw volume, allowance, and material estimate.
| Step | Result | How it is used |
|---|---|---|
| Run the calculator to see the hole volume breakdown. | ||
Planning note: If the hole will contain a post, pipe, rebar cage, form tube, rock, or drainage layer, subtract or adjust for that occupied space before ordering material.
How to use the hole volume calculator
- Select the hole shape: Choose round, rectangular, trench, or tapered round based on the excavation.
- Enter measured dimensions: Use diameter and depth for a round post hole, or length, width, and depth for rectangular holes and trenches.
- Add quantity and allowance: Quantity multiplies the result, while allowance covers overdigging, soil sloughing, and material waste.
- Use bag yield if helpful: Enter the cubic feet produced by one bag of concrete mix, soil, gravel, or backfill material.
- Review multiple units: Cubic feet are useful for bagged material, cubic yards for bulk orders, and gallons or liters for water or slurry comparisons.
Utility marking reference: 811 Before You Dig - Before You Dig.
Hole volume formulas
Hole volume is calculated from the shape of the empty space. Convert all dimensions to the same unit first, then apply the matching formula for a cylindrical, rectangular, trench, or tapered hole.
Round hole = pi x radius squared x depth
Rectangular hole = length x width x depth
Tapered hole = pi x depth / 3 x (r1 squared + r1 x r2 + r2 squared)
Example: a 12-inch round hole that is 36 inches deep has a raw volume of about 2.36 cubic feet. With a 10% allowance, the estimate becomes about 2.59 cubic feet, which rounds to 5 bags if each bag yields 0.6 cubic feet.
What can change the actual fill volume?
The calculated volume is the clean geometric volume. Real jobsite volume can change when the hole shape changes after digging or when objects occupy space inside the hole.
Overdigging
Hand digging and augers can make holes wider than planned, especially in loose or rocky soil.
Occupied space
Posts, pipes, tubes, rocks, rebar, and drainage layers reduce how much concrete, soil, or gravel will fit.
Compaction
Backfill, gravel, and soil can settle or compact, so loose material volume may be higher than finished volume.
Common hole volume examples
These examples use raw geometric volume before extra allowance. Use the calculator for exact quantities, repeated holes, and bag counts.
| Hole type | Dimensions | Raw volume | Typical use |
|---|---|---|---|
| Round post hole | 12 in diameter x 36 in deep | 2.36 ft3 | Fence post, deck post, small pier. |
| Large round pier | 24 in diameter x 48 in deep | 12.57 ft3 | Deck pier, footing, sign post. |
| Rectangular hole | 4 ft x 3 ft x 2 ft | 24.00 ft3 | Small footing, planting pit, utility access. |
| Trench | 20 ft x 12 in x 18 in | 30.00 ft3 | Drain line, conduit, edging, irrigation. |
Choose the right material for the hole
The same hole volume can mean different ordering decisions depending on whether the project uses concrete, gravel, soil, sand, or water. Match the calculator result to the material behavior before buying bags or bulk delivery.
Concrete post holes
Use cubic feet and bag yield. Subtract large posts or form tubes if they displace enough volume to change the bag count.
Gravel or drainage stone
Use cubic yards for bulk ordering and remember that compacted depth may be lower than loose delivered volume.
Planting soil or backfill
Use extra allowance for settlement, root balls, amendments, and soil that fluffs up when removed and then compacts after watering.
Water, slurry, or liquid fill
Use gallons or liters when checking water capacity, slurry fill, leak tests, or temporary ballast in a clean excavation.
Concrete yield reference: QUIKRETE - Product Calculator.
How to subtract posts, pipes, and form tubes
If something will remain inside the hole, the fill material does not occupy that space. Calculate the hole volume first, calculate the embedded object volume second, then subtract the object volume before adding waste allowance.
Net fill volume = hole volume - embedded object volume
Round post volume = pi x post radius squared x embedded depth
Square post volume = post width x post width x embedded depth
Example: a 4-by-4 post embedded 3 feet takes up about 0.33 cubic feet if the actual post is treated as 4 inches by 4 inches. In a 12-inch by 36-inch round hole, subtracting that post from the 2.36 cubic foot hole leaves about 2.03 cubic feet before allowance.
Spoil volume and cleanup planning
The volume you dig out is not always the volume you haul away. Soil expands when loosened, and some material may be reused as backfill while the rest becomes spoil that must be staged, loaded, or disposed of.
| Planning item | Why it matters | Practical check |
|---|---|---|
| Loose soil swell | Excavated soil can occupy more space than it did in the ground. | Leave staging room near the hole or plan extra haul capacity. |
| Backfill reuse | Some soil may go back into the hole around pipes, plants, or sleeves. | Separate clean reusable soil from rock, clay, roots, and debris. |
| Trench spoil | Long trenches create spoil piles that can block access or add load near the trench edge. | Keep spoil placement, access, and trench safety in the work plan. |
| Disposal units | Haulers may quote by cubic yard, truckload, ton, or container size. | Convert the calculator result to cubic yards before comparing quotes. |
Excavation safety reference: eCFR - 29 CFR Part 1926 Subpart P, Excavations.
Frequently Asked Questions
What is a hole volume calculator used for?
A hole volume calculator estimates the empty space inside a hole so you can plan concrete, soil, gravel, sand, water, slurry, or disposal volume. It is useful for post hole work, fence holes, deck piers, footing layout, trench excavation, planting holes, and small projects where the shape, depth, width, length, or diameter are known. The result helps compare material capacity in cubic foot, cubic yard, gallon, or liter units.
How do I calculate the volume of a round or cone-shaped hole?
For a straight round hole, use the cylinder formula: volume = pi x radius squared x depth. Divide the diameter by 2 to get the radius, make sure radius and depth use the same measurement unit, then convert the final result into cubic feet, cubic yards, gallons, or liters as needed. For a tapered hole that behaves more like a cone or frustum, use the top radius, bottom radius, and depth so the shape is not overestimated.
How much concrete do I need for a 12-inch by 36-inch post hole?
A 12-inch diameter post hole that is 36 inches deep has a raw volume of about 2.36 cubic feet. With a 10% allowance, it needs about 2.59 cubic feet of concrete fill. If one concrete bag yields 0.6 cubic feet, the calculator estimate rounds up to 5 bags before adjusting for post volume, form tubes, rebar, or gravel at the bottom of the hole.
Should I subtract the volume of a post, pipe, or form from the hole?
Yes, if the post, pipe, sleeve, or form tube takes up a meaningful amount of area inside the hole. Small posts may not change a bag estimate much, but larger columns, pipes, and embedded objects reduce the amount of concrete, sand, soil, or gravel backfill needed. Calculate the object volume from its own formula, then subtract it from the hole volume before adding project allowance.
Why should I add an allowance to hole volume?
An allowance helps cover real-world variation: auger wobble, uneven sides, loose soil, small cave-ins, rock removal, spills, and measurement error. It also helps when a trench, footing, or rectangular excavation has rough edges instead of a perfect length x width x depth shape. For clean form-tube work, a small allowance may be enough; for hand-dug or irregular holes, 10% to 20% is often more realistic.
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Disclaimer: This hole volume calculator is for general educational, estimating, landscaping, construction planning, gardening, and budgeting use only. It estimates geometric volume from user-entered hole shape, dimensions, units, quantity, allowance, bag yield, and optional bag price. It does not replace building-code requirements, structural engineering, geotechnical evaluation, excavation safety rules, utility locating, concrete specifications, manufacturer instructions, contractor judgment, or supplier measurements. Actual material needs can vary because of soil collapse, overdigging, rocks, roots, groundwater, frost depth, irregular sides, auger size, form tubes, posts, pipes, rebar, drainage layers, compaction, settlement, bag yield variation, moisture condition, spills, and measurement error. Before digging, confirm buried utilities, permits, footing requirements, safe excavation practices, soil conditions, and the correct material type for the project.
Last updated: June 9, 2026