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Post Hole Concrete Calculator

Ready to calculate
Post Volume Subtracted.
60 & 80 lb Bags.
Cheaper Option Flagged.
100% Free.
Privacy Secure.

How it Works

01Post Count

Enter number of posts — fence, deck, or sign post count.

02Hole Size

Hole diameter and depth in inches. Typical: 10-12 in × 30-36 in deep.

03Post Diameter

Post dia in inches. 4×4 = 3.5", 6×6 = 5.5". Subtracted from hole volume.

04Bags + Cost

Total bags and cheapest bag size (60 vs 80 lb) with 10% waste.

What is a Post Hole Concrete Calculator?

A post hole concrete calculator — also known as a fence post concrete calculator, deck footing calculator, or Quikrete post hole calculator — tells you exactly how many bags of concrete you need to set fence posts, deck footings, mailbox posts, sign posts, and any structural post in the ground. Every fence project starts with the same question: "How many bags for 20 posts?" Guess wrong and you're either making a second trip to Home Depot in the middle of a pour (with concrete setting in the first holes while you drive), or returning half a pallet of unopened Quikrete.

This tool uses the precise cylinder volume formula V = π × r² × h per hole, automatically subtracts the post's own volume (since the post displaces concrete equal to its cross-sectional area times the buried depth), multiplies by post count, adds a 10% waste factor, and outputs bag counts for both 60 lb and 80 lb Quikrete — flagging the cheaper option. The post displacement step is what most DIYers miss — it's why they consistently over-order by 20-30%.

Works for wood fence posts (4×4 = 3.5 in actual, 6×6 = 5.5 in actual), metal fence posts (2-3 in diameter typical for chain-link and ornamental iron), mailbox posts (4 in wood, 2-3 in galvanized), deck footings (6×6 actual or 8-inch Sonotube), sign posts (real estate, highway, commercial), and pergola and arbor posts.

Default values reflect industry-standard sizes: 10 inch hole diameter × 30 inch depth for a standard wood fence post. Hole depth should be 1/3 of post height above ground and below frost line (0-12 inches in warm states, 36-48 inches in northern states). The tool doesn't enforce these — you enter what matches your project and frost depth.

Great for DIY fence builders installing chain-link, wood privacy, vinyl, or split-rail fencing; deck contractors setting Sonotube-style footings below frost line; sign installers (real estate, highway, commercial); landscape designers installing pergolas, arbors, and pavilion structures; and municipal crews replacing guardrail and light posts.

How It Works

Cylinder volume per hole: V = π × (diameter_ft / 2)² × depth_ft. Hole diameter 10 inches × 30 inches deep = 1.36 ft³ per hole.
Subtract post volume: The post displaces concrete. For a 4×4 wood post (3.5 inch dia) 30 inches into the hole: 0.20 ft³ displacement per hole.
Multiply by post count × 1.10 waste factor. 10% over-order is standard because some spills, some sticks to the trowel, and holes are never perfectly cylindrical.
Convert to bags: 80 lb bag yields 0.6 ft³, 60 lb bag yields 0.45 ft³. Ceiling both to whole bags.
Flag cheaper option: 80 lb bags typically win on cost per ft³ ($6.50 ÷ 0.6 = $10.83/ft³ vs $5.25 ÷ 0.45 = $11.67/ft³).

Post Hole Formula

Cylinder math with post displacement:

Hole_vol = π × (HD_in / 24)² × (HL_in / 12)
Post_vol = π × (PD_in / 24)² × (HL_in / 12)
Volume_per_hole = Hole_vol - Post_vol
Total_volume = Volume_per_hole × N_posts × 1.10
80 lb bags = ceil(Total_volume / 0.6)
60 lb bags = ceil(Total_volume / 0.45)
HD = hole diameter (in), HL = hole depth (in)
PD = post diameter (in), N = number of posts

The /24 in the radius formula is because we need radius in feet: diameter (in) ÷ 2 ÷ 12 = diameter ÷ 24.

Real-World Example

Worked Example

A 6-post fence, 10 inch diameter holes × 30 inches deep, 4×4 wood posts (3.5 inch):

  • Hole vol per hole = π × (10/24)² × (30/12) = π × 0.174 × 2.5 = 1.36 ft³
  • Post vol = π × (3.5/24)² × (30/12) = π × 0.021 × 2.5 = 0.167 ft³
  • Net per hole = 1.36 - 0.17 = 1.19 ft³
  • Total × 6 posts × 1.10 waste = 7.85 ft³
  • 80 lb bags = ceil(7.85 / 0.6) = 14 bags at $91
  • 60 lb bags = ceil(7.85 / 0.45) = 18 bags at $94.50
  • 80 lb wins by $3.50. Get 14 bags and you're set.

Who Uses This Calculator?

1
DIY fence builders — chain-link, wood privacy, vinyl, split-rail
2
Deck contractors setting Sonotube-style deck footings
3
Sign installers — real estate posts, highway signs, mailbox posts
4
Landscape designers installing pergola and arbor posts
5
Municipal crews replacing guardrail and light posts

Technical Reference

Quikrete bag yields — 80 lb bag yields 0.60 ft³ (typical), 60 lb yields 0.45 ft³, 40 lb yields 0.30 ft³. Published yields vary ±5% with slump and water content.

IRC R403.1.1 — frost line depth. Footings for deck and structural posts must extend below frost line: 0-12 inches in Gulf states, 36-48 inches in northern states.

Post spacing (fences) — typical 6-8 ft on-center for wood privacy, 8-10 ft for chain-link. Affects post count, not hole size.

Key Takeaways

Most DIYers over-order post concrete by 30-50% because they forget to subtract the post volume taking up space inside the hole. A 4×4 wood post (3.5 in actual) occupies 0.17 ft³ per 30-inch depth — about 15% of a 10-inch hole. This calculator subtracts it automatically. The 10% waste factor is right for post holes — concrete slumps out of the collar, some sets on your trowel, holes are never perfectly cylindrical, and the bottom 3-4 inches is often loose soil that compresses under the pour. Fast-setting Quikrete ($1-2 more per bag) sets in 20-40 minutes and is usually worth it for fences — you can set 20 posts in one weekend without waiting 24 hours between each for standard mix to set enough to backfill. For deck footings in frost-prone regions, hole depth must extend below local frost line per IRC R403 — check your AHJ before buying.

Frequently Asked Questions

How many bags of concrete for a fence post?
For a typical 10 inch × 30 inch hole with a 4×4 wood post, each post takes about 1.2 ft³ of concrete = 2 × 80 lb bags or 3 × 60 lb bags. A 6-post fence needs about 14 × 80 lb bags or 18 × 60 lb bags.
How deep should a fence post hole be?
1/3 of the post height above ground, minimum 24 inches, below frost line. A 6 ft tall fence needs 24-30 inch deep holes in warm climates, 36 inch in cold. Deeper = more stable but more concrete.
How wide should a fence post hole be?
3× the post width. A 4×4 post (3.5 inch) needs a 10-12 inch diameter hole. A 6×6 post needs 14-16 inch. Wider holes hold better and use more concrete — typical fence posts use 10 inch holes.
Do I really need to set fence posts in concrete?
Yes for privacy fences, wind-exposed runs, and gate posts. Compacted gravel works for split-rail and low decorative fences in stable soil. Concrete sets in 20-40 min with fast-setting mix, vs 24-48 hours with standard.
How much concrete for 20 fence posts?
About 23 ft³ total at 10 inch × 30 inch holes with 4×4 posts (incl. 10% waste). That's 39 × 80 lb bags or 52 × 60 lb bags. Budget $250-350 for bagged concrete; ready-mix under 1 yd³ rarely justifies delivery.
Is fast-setting concrete better for fence posts?
For DIYers yes — Quikrete Fast-Setting sets in 20-40 minutes, so you can plumb posts and move on without waiting. Standard Quikrete takes 24-48 hours to set enough to backfill. Costs $1-2 more per bag.
Should I dry-pack or wet-mix concrete for fence posts?
Wet mix is stronger long-term and is required for deck footings and anything structural. Dry-pack (dump in bag, add water on top) works for non-structural fence posts and is much faster for 10+ posts.
How do I keep a fence post plumb while concrete sets?
Tack a temporary brace to each post at 45° to the ground. Check plumb in both directions with a 2 ft level. Fast-setting concrete lets you remove braces in 30-40 min; standard needs overnight.
How much does it cost to set a fence post in concrete?
Material per post: 2 × 80 lb bags at $6.50 = $13 in concrete, plus $10-20 post. Labor if hiring: $30-50/post. Full 100 ft fence (13 posts at 8 ft OC) costs $300-400 in material, $500-700 if contracted.
Can I use Quikrete Fast-Setting for deck footings?
Check your local code. Some AHJs require 28-day rated 3,000+ psi concrete for deck footings (Fast-Setting is 4,000 psi at 28 days — meets spec). Others require inspection before backfill regardless of mix.

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Disclaimer

Educational reference. Frost-line depth varies by region. For deck footings, verify with your AHJ and footing schedule.