Concrete Post Hole Calculator Guide - Fence Post Bags & Depth

A concrete post hole estimate starts with a round hole. Measure the hole diameter and depth, calculate the cylinder volume, multiply by the number of holes, then add waste before buying bags or comparing delivery options.

Use the Concrete Post Hole Calculator for round fence, deck, gate, mailbox, and sign post holes. Enter hole diameter, concrete-filled depth, gravel depth, number of holes, waste factor, bag size, and bag price to estimate cubic feet, cubic yards, 40/60/80 lb bags, and material-only cost. If the search is phrased more broadly as a hole calculator, use the Concrete Holes Calculator Guide to compare post holes, pier holes, and larger round holes. For structural pier pricing around diameter, depth, forms, rebar, and post bases, use the Concrete Pier Cost Calculator Guide.

For a fence-specific shortcut with 4x4 posts, 6x6 posts, gate posts, and bags per post, use the Fence Post Concrete Calculator Guide.

For bag yields by size, see the Concrete Bags per Yard Guide. If you already know the total cubic feet and only need bags, use the Concrete Bag Calculator. For a large fence run, compare bagged concrete and delivery in the Ready-Mix vs Bags Calculator.

If the project uses continuous rectangular footings instead of round holes, see the Concrete Footing Calculator Guide.

Quick answer

For a round post hole, calculate cylinder volume from hole diameter and depth, multiply by the number of holes, then add waste. A 12 in diameter hole that is 36 in deep needs about 2.36 ft3 before waste, or 2.59 ft3 with a 10% waste factor.

With a common 0.60 ft3 yield for an 80 lb bag, that same hole rounds up to about 5 bags. The exact number can change with the bag yield, over-digging, soil collapse, gravel depth, and how full the hole is after the post is set.

The calculator is for concrete quantity only. It does not decide post size, embedment depth, frost depth, gate load, wind exposure, deck code, or permit requirements.

Start with these common post-hole calculator setups if you need a fast bag count before entering custom dimensions. All examples use 10% waste and common bag yields of 0.60 ft3 for 80 lb bags, 0.45 ft3 for 60 lb bags, and 0.30 ft3 for 40 lb bags.

Suggested citation: eight 10 in diameter by 30 in deep fence post holes need about 11.97 ft3 of concrete with 10% waste, or roughly 20 80 lb bags, 27 60 lb bags, or 40 40 lb bags using common bag yields.

If the post line has corner posts, gate posts, deck piers, gravel at the bottom, or over-dug holes, calculate each group separately before adding the totals. That keeps larger high-load holes from getting hidden inside an average bag count.

Post hole formula

For one round hole:

cubic feet = 3.1416 x radius ft x radius ft x depth ft

If you know diameter in inches:

radius ft = diameter in / 24

Then multiply by the number of holes and add waste:

total cubic feet = cubic feet per hole x number of holes
concrete to buy = total cubic feet x (1 + waste percentage)

If the hole contains gravel at the bottom, use only the concrete-filled depth in the calculation. If the hole sides are uneven, measure the actual dug diameter instead of the planned auger size.

Best calculator inputs for post-hole planning

Use inputs that match the actual holes you will pour, not only the ideal fence layout. Post-hole estimates can move quickly when gate posts, corner posts, deck posts, gravel layers, or uneven soil are grouped into one average number.

Run one estimate for the regular post holes, then a second estimate for gate, corner, deck, or sign posts. That keeps high-load holes visible instead of hiding them inside an average bag count.

Post displacement and hole-shape adjustment

Some calculators subtract the volume of the post from the hole. That can be useful for tight professional estimates, but field estimates often skip the deduction because real holes are rarely perfect cylinders.

For a material buffer, it is usually safer to calculate from the actual dug diameter and concrete-filled depth, then round up to whole bags.

Common post hole examples

These examples use one round hole and 10% waste. They are material planning numbers, not post design, frost-depth, or code approval.

Approximate bag counts for those single-hole examples:

• 8 in x 24 in: about 2 80 lb bags, 2 60 lb bags, or 3 40 lb bags.
• 10 in x 30 in: about 3 80 lb bags, 4 60 lb bags, or 5 40 lb bags.
• 12 in x 36 in: about 5 80 lb bags, 6 60 lb bags, or 9 40 lb bags.
• 14 in x 42 in: about 7 80 lb bags, 10 60 lb bags, or 14 40 lb bags.
• 16 in x 48 in: about 11 80 lb bags, 14 60 lb bags, or 21 40 lb bags.

The bag counts assume common yields of 0.60 ft3 for 80 lb bags, 0.45 ft3 for 60 lb bags, and 0.30 ft3 for 40 lb bags. Always use the yield printed on the product you plan to buy.

Common post-hole project scenarios

Different post projects can use the same volume formula but very different hole assumptions.

For continuous rectangular footings, use the Concrete Footing Calculator. For round holes and piers, use the Concrete Post Hole Calculator.

Example: one 12 in by 36 in post hole

Assume:

• Diameter: 12 in
• Radius: 0.5 ft
• Depth: 36 in, or 3 ft
• Waste: 10%

Base volume:

3.1416 x 0.5 x 0.5 x 3 = 2.36 ft3

With waste:

2.36 x 1.10 = 2.60 ft3

Using 80 lb bags at a common 0.60 ft3 yield:

2.60 / 0.60 = 4.33 bags
round up to 5 bags

Planning estimate: about 5 bags of 80 lb concrete mix for one hole.

Example: eight fence post holes

Assume each hole is 10 in diameter and 30 in deep.

Radius:

10 / 24 = 0.4167 ft

Volume per hole:

3.1416 x 0.4167 x 0.4167 x 2.5 = 1.36 ft3

For eight holes with 10% waste:

1.36 x 8 = 10.88 ft3
10.88 x 1.10 = 11.97 ft3

With 60 lb bags at a common 0.45 ft3 yield:

11.97 / 0.45 = 26.6 bags
round up to 27 bags

With 80 lb bags at 0.60 ft3 yield:

11.97 / 0.60 = 19.95 bags
round up to 20 bags

Always check the actual yield printed on the bag you plan to buy.

Example: twelve 12 in by 36 in fence posts

A heavier fence or gate line may use larger holes. If each hole is 12 in diameter and 36 in deep:

3.1416 x 0.5 x 0.5 x 3 = 2.36 ft3 per hole
2.36 x 12 holes = 28.32 ft3
28.32 x 1.10 = 31.15 ft3 with waste

That is about 1.15 cubic yards of concrete.

Bag check:

• 80 lb bags at 0.60 ft3 yield: about 52 bags.
• 60 lb bags at 0.45 ft3 yield: about 70 bags.
• 40 lb bags at 0.30 ft3 yield: about 104 bags.

That job is still possible with bags, but it is no longer a small pickup load. At this scale, compare material price, delivery, mixing time, set time, and crew pace before buying.

Example: fence line with regular and gate posts

Assume a fence needs:

• 18 regular holes at 10 in diameter and 30 in deep.
• 2 gate holes at 12 in diameter and 36 in deep.
• 10% waste.

Regular holes:

10 in x 30 in hole = 1.36 ft3
1.36 x 18 = 24.48 ft3

Gate holes:

12 in x 36 in hole = 2.36 ft3
2.36 x 2 = 4.72 ft3

Total with waste:

24.48 + 4.72 = 29.20 ft3
29.20 x 1.10 = 32.12 ft3

Bag check:

This is the kind of project where the cheapest shelf price per bag may not be the best plan. Delivery, pallet handling, helpers, and pace can matter.

Example: deck pier quantity check

Assume six round deck pier holes, each 12 in diameter and 48 in deep, with 10% waste.

Per hole:

3.1416 x 0.5 x 0.5 x 4 = 3.14 ft3

Six holes with waste:

3.14 x 6 = 18.84 ft3
18.84 x 1.10 = 20.72 ft3

Using 80 lb bags:

20.72 / 0.60 = 34.5 bags
round up to 35 bags

This is still a material estimate, not deck footing approval. Confirm diameter, depth, base shape, reinforcement, inspection timing, and hardware with the approved plan or local authority before ordering.

How deep should a post hole be?

Hole depth is a design and site question, not only a material calculation. Post height, soil, frost depth, wind exposure, gate load, local code, and the post manufacturer's instructions can all change the required depth.

Use the calculator for material quantity only. Confirm post depth and diameter before buying concrete. If a code, plan, permit, or manufacturer specification requires a different hole size, follow that requirement instead of a generic example.

Hole diameter and concrete fill

Two holes with the same depth can use very different amounts of concrete if the diameter changes. A 12 in hole has more than twice the concrete volume of an 8 in hole at the same depth because the radius is squared in the cylinder formula.

Also check what volume the post itself displaces. Most field estimates ignore post displacement because over-digging and uneven hole sides often use that margin quickly. For tight professional estimates, measure the actual hole and confirm the placement method.

Gravel depth and concrete-filled depth

If gravel goes under the post, do not count the gravel layer as concrete depth. Use the depth actually filled with concrete.

Example:

total hole depth = 36 in
gravel depth = 6 in
concrete-filled depth = 30 in

That change matters. A 10 in diameter hole at 36 in deep is about 1.64 ft3 before waste, while the same hole with only 30 in of concrete fill is about 1.36 ft3 before waste.

For drainage and post-setting practice, follow the post manufacturer, project plan, and local requirements. This page only estimates concrete volume.

Bagged concrete or ready-mix?

Post holes are usually practical with bags because the volume per hole is small and the work is spread across separate holes. Ready-mix may make sense for a large fence, many posts, or a job where a crew can place concrete quickly.

If the total bag count is high, compare the material total with ready-mix delivery using the Ready-Mix vs Bags Calculator.

For small repairs, bagged concrete is often simpler. For a long fence with dozens of holes, ready-mix can become attractive if access, timing, and placement logistics work. The calculator helps you price the material side, but the practical decision also depends on how quickly the crew can set posts and place concrete.

Fast-setting vs standard concrete mix

Post-hole projects often use either fast-setting concrete or standard concrete mix. The calculator math is similar, but the workflow is different.

Do not choose a product only from bag count. Product instructions, local code, post material, hardware, wind exposure, frost depth, and inspection requirements can all affect the correct method.

When ready-mix might make sense

Ready-mix is not automatically better for post holes because the holes are separate and often spread along a fence line. It can make sense when several conditions line up:

• The total volume is near or above a supplier's practical minimum.
• A crew can set and brace posts before the truck arrives.
• The truck, buggy, or pump can reach the holes efficiently.
• The pour can be placed before the truck wait-time clock becomes expensive.
• The delivered total beats the bagged price plus handling and mixing time.

If the job is only a few holes, bags are usually simpler. If the job needs 50, 70, or 100+ bags, compare ready-mix delivery, pallet delivery, rental mixer, and labor before buying.

For the broader buying comparison, use Ready-Mix vs Bags Cost.

Material cost estimate

For bagged concrete:

bagged total = rounded bag count x price per bag

If eight holes require 20 bags of 80 lb mix at $5.50 per bag:

20 x $5.50 = $110.00

This is a material-only estimate. It does not include posts, gravel, bracing, tools, delivery, tax, labor, cleanup, or any hardware.

For ready-mix:

ready-mix total = cubic yards after waste x price per yard + delivery fees

If twelve larger holes require about 1.15 yd3, a $165 per yd3 ready-mix price, and a $125 delivery or short-load fee:

1.15 x $165 = $189.75
$189.75 + $125 = $314.75

That is still material and delivery only. It does not decide whether delivery is practical for a fence line with separate holes.

Bag price table for post holes

Use this table after you know total bag count.

This table is material-only. It does not include delivery, tax, mixer rental, post hardware, digging labor, or cleanup.

Bag count pressure table

Use the final bag count as a logistics signal, not only a shopping list.

For many separate holes, timing matters differently than a slab. You may need posts set, aligned, and braced before concrete is placed, especially if a truck or hired crew is involved.

DIY, pallet delivery, or post contractor?

Use the final bag count and site conditions to choose the next estimate. The right answer is often different for a mailbox, a short fence section, a gate opening, and a deck pier layout.

For a small mailbox or sign post, bags are usually simple. For a long fence line, large gate posts, or deck pier work, a contractor quote may be valuable because digging, alignment, bracing, removal, and inspection risk are outside the material calculator.

Material cost vs project cost

Use this broader planning formula for a fence-post or deck-post conversation:

project total =
  concrete material
  + posts, gravel, hardware, and bracing
  + digging, layout, string lines, and cleanup
  + delivery, tax, equipment rental, permits, and labor

Layout, utility, and property-line checks

Post holes create non-material risks that a calculator cannot solve. Before digging or accepting a quote, confirm:

• Utility marking or local call-before-you-dig requirements.
• Property line, easement, fence setback, HOA, and permit rules.
• Gate swing, latch clearance, corner bracing, and end-post loads.
• Post material, hardware, brackets, and manufacturer instructions.
• Spoil pile removal and old concrete/post removal.

These checks do not change the cylinder formula, but they can change whether the project is allowed, where posts go, and whether larger holes are needed.

Post hole checklist

• Confirm post spacing, line layout, gate locations, and corner posts first.
• Measure hole diameter and concrete-filled depth after digging.
• Confirm post depth, frost depth, wind load, and local requirements.
• Subtract gravel depth if gravel occupies the bottom of the hole.
• Add waste for over-digging, loose soil, collapsed sides, and spillage.
• Use the bag yield printed on the product, not only the bag weight.
• Round up to whole bags and keep a small buffer for uneven holes.
• Keep posts plumb, aligned, and braced while the concrete sets.
• Keep material-only cost separate from the full installed project cost.
• Do not use a material calculator as structural, code, or permit approval.

Quote checklist for fence or post work

Before accepting a bid, ask what is included in writing:

• Number of posts, post spacing, post size, and post material.
• Hole diameter, hole depth, gravel depth, and concrete-filled depth.
• Concrete product or mix, bag count, or ready-mix delivery plan.
• Corner, end, gate, sign, or deck posts that need larger holes.
• Cleanup, spoil removal, old post removal, and haul-off.
• Hardware, brackets, rails, panels, gates, latch hardware, and caps.
• Bracing method, cure time, warranty, schedule, and payment terms.
• Permit, utility marking, property line, and code responsibilities.

That checklist makes the calculator result easier to compare against a real contractor quote.

Bid comparison table for post work

If two fence, deck, or post bids are far apart, compare the same details.

The useful bid is the one that explains hole sizes, post types, concrete method, cleanup, exclusions, and who handles layout or code requirements.

Post-hole bid red flags

These red flags are reasons to ask for missing scope before comparing prices.

A strong post-work quote should name the post count, hole groups, concrete method, removal scope, layout responsibility, and cleanup. If the bid only says "set posts in concrete," use the calculator result to ask for the missing details.

Common estimating mistakes

• Using planned auger size instead of the actual dug hole size.
• Forgetting that diameter changes volume faster than depth does.
• Counting gravel depth as concrete depth.
• Forgetting corner, gate, deck, or sign posts that need larger holes.
• Ignoring over-digging, loose soil, collapsed sides, and spillage.
• Using bag weight without checking yield in cubic feet.
• Treating concrete material cost as the full fence or post project cost.
• Buying a large number of bags before comparing delivery and access.
• Averaging regular post holes with larger gate, corner, or deck posts.
• Counting gravel depth as concrete-filled depth.
• Forgetting old post removal, utility marking, and spoil cleanup in quotes.

FAQ

What calculator should I use for post holes?

Use the Concrete Post Hole Calculator. Enter hole diameter, concrete-filled depth, gravel depth, number of holes, waste factor, bag yield, and bag price.

How many bags do I need for a post hole?

It depends on hole diameter, depth, waste factor, and bag yield. A 12 in diameter hole that is 36 in deep often needs about 5 bags of 80 lb mix after a 10% waste factor.

How much concrete is in a 10 in by 30 in post hole?

A 10 in diameter hole that is 30 in deep is about 1.36 ft3 before waste. With a 10% waste factor, plan around 1.50 ft3 before converting to bags.

How many bags for eight 10 in by 30 in fence posts?

Eight holes at 10 in diameter and 30 in deep need about 12.00 ft3 with 10% waste. That is about 20 80 lb bags, 27 60 lb bags, or 40 40 lb bags using common bag yields.

How many bags for a 4x4 fence post?

The post size alone is not enough. You need the hole diameter, concrete-filled depth, number of posts, waste factor, and bag yield. A 10 in diameter by 30 in deep hole often needs about 3 bags of 80 lb concrete after waste.

How many bags for a 6x6 post?

Many 6x6 posts use larger holes than light fence posts, but the exact size depends on load, soil, frost depth, and project requirements. Calculate from the actual hole diameter and concrete-filled depth, then round up to whole bags.

Should gate posts use the same hole as regular fence posts?

Not always. Gate posts can see more load and movement than regular line posts. Confirm gate-post size, hole size, bracing, and hardware requirements before using the calculator for material quantity.

Should I add gravel under a post?

That depends on the post type, drainage, soil, and local practice. If gravel changes the concrete depth, use only the concrete-filled depth in the calculation.

Does the post itself reduce the concrete amount?

The post displaces some volume, but many practical estimates ignore that small deduction because uneven holes, loose sides, and spillage often use the margin. For a tight estimate, measure the actual hole and account for placement details.

Are 40 lb, 60 lb, and 80 lb bags interchangeable?

They can all work if the concrete product is appropriate, but they do not cover the same volume. Use the printed yield on the bag and round up to whole bags.

Can I use the footing calculator for post holes?

Use the Concrete Post Hole Calculator for round post holes. The footing calculator is best for continuous rectangular footing runs.

Does the estimate include labor?

No. These calculations are material-only estimates. Digging, layout, bracing, posts, hardware, cleanup, permits, delivery, tax, and labor are separate.

When should I compare ready-mix for post holes?

Compare ready-mix when the total bag count is high, the crew can place concrete quickly, and delivery access is practical. For a few separate holes, bags are usually simpler.

Can I use fast-setting concrete for post holes?

Follow the product instructions and project requirements. Fast-setting products can be useful for some post work, but yield, placement method, water, bracing, and curing details still matter.

Should I subtract the post volume from the hole?

Usually only for tight estimates. The post displaces some concrete, but over-dug holes, uneven sides, loose soil, and spillage often use that margin. For repeated professional estimates, calculate the actual hole volume and deduct the post volume if the placement method is controlled.

Do corner and gate posts need separate calculations?

Often yes. Corner, end, gate, deck, pergola, and sign posts may use larger holes than regular line posts. Calculate each hole group separately instead of averaging all posts together.

Should I call for utility marking before digging post holes?

Yes when required by local rules or when underground utilities may be present. Utility marking, property lines, setbacks, easements, and permits are outside the calculator but can control where holes may be dug.

Next step

Open the Concrete Post Hole Calculator, enter your hole diameter, concrete-filled depth, gravel depth, and number of holes, then compare the 40 lb, 60 lb, and 80 lb bag counts before buying material.

For larger fence or deck-post jobs, compare material totals in the Concrete Cost Calculator and read Ready-Mix vs Bags Cost before ordering a large number of bags. If you only need the bag-count answer for one hole, use How Many Bags of Concrete for a Post Hole?.