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Post Hole Concrete Calculator

Calculate exact bags of concrete for your post holes

Bags of Concrete Needed

24 bags

Per Hole

1.396 ft³

Total Vol

13.96 ft³

Cost

$156.00

in
in
in
$
Per Hole

1.396 ft³

Total Volume

13.96 ft³

Bags Needed

24

Total Weight

1,920 lbs

Water Needed

12.0 gal

Total Cost

$156.00

Cost Summary

24 × 80 lb bags$156
Cost per post$15.60
Bags per post2.4

Frequently Asked Questions

Q

How deep should a fence post hole be?

A fence post hole should be 1/3 the total post length plus 6 inches for a gravel base. For a standard 8-foot fence post, dig 32 inches deep (2 feet 8 inches). In cold climates, the hole must extend below the frost line, which ranges from 12 to 72 inches depending on location.

  • 6-ft fence post: dig 24 inches deep (1/3 of 72 in) + 6 in gravel = 30 inches total
  • 8-ft fence post: dig 32 inches deep (1/3 of 96 in) + 6 in gravel = 38 inches total
  • Frost line depth: 12 in (southern US) to 72 in (northern Minnesota/Montana)
  • Add 4–6 inches of gravel at the bottom for drainage to prevent frost heave
  • Check local code — many municipalities require post holes below frost line regardless of fence height
Q

How many bags of concrete do I need per fence post?

Most standard fence posts (4x4 post, 12-inch hole, 24-inch depth) require 2 to 3 bags of 80 lb concrete mix. Larger posts or deeper holes need more. Use this calculator to get an exact count based on your dimensions.

  • 4×4 post in 10-inch hole, 24 in deep: ~1.5 bags of 80-lb concrete per post
  • 4×4 post in 12-inch hole, 24 in deep: ~2.3 bags of 80-lb concrete per post
  • 6×6 post in 18-inch hole, 36 in deep: ~7.9 bags of 80-lb concrete per post
  • Always round up and buy 10% extra to account for uneven holes and spillage
  • A 50-post fence with 4×4 posts needs roughly 100–120 bags of 80-lb concrete
Bag SizeWeightYield (ft³)Avg Price
40 lb40 lbs0.30 ft³$3.50–$4.50
50 lb50 lbs0.375 ft³$4.50–$5.50
60 lb60 lbs0.45 ft³$5.00–$6.00
80 lb80 lbs0.60 ft³$6.00–$7.50
Q

How long does post hole concrete take to cure?

Fast-setting concrete sets in 20 to 40 minutes and reaches full strength in 24 hours. Standard concrete mix takes 24 to 48 hours to set and reaches full strength (around 3,000 PSI) in 28 days. Avoid putting heavy load on posts for at least 24 hours.

  • Fast-setting (Quikrete Fast-Setting): 20–40 min initial set, full strength in 4 hours
  • Standard concrete: 24–48 hours to set, reaches 3,000 PSI at 28 days
  • Keep concrete moist for 48 hours after pouring for optimal curing in hot weather
  • Temperature below 40°F slows curing significantly — use cold-weather formula if needed
  • Do not hang gates or attach heavy panels until at least 24 hours after pouring
Concrete TypeInitial SetFull StrengthBest For
Fast-Setting20–40 min4 hoursSame-day fence builds
Standard Mix24–48 hours28 daysBudget projects, heavy loads
High-Strength10–12 hours28 daysDeck posts, structural loads
Q

What size hole should I dig for a 4x4 post?

For a 4x4 post (3.5 inches actual width), dig a hole 10 to 12 inches in diameter. The general rule is the hole diameter should be 3 times the post width. This provides enough room for concrete to surround the post and anchor it securely.

  • 4×4 post (3.5 in actual): dig a 10–12 inch diameter hole
  • 6×6 post (5.5 in actual): dig a 16–18 inch diameter hole
  • Use a post hole digger for holes up to 12 in; rent a power auger for larger/deeper holes
  • Power auger rental: $50–$80/day, digs a 12-inch hole in under a minute
  • Keep sides vertical — a flared or bell-shaped hole wastes concrete
Q

Should I use fast-setting or regular concrete for post holes?

Fast-setting concrete is ideal for post holes because you can pour it dry and add water on top. It sets in 20-40 minutes so you can attach fence panels the same day. Regular concrete is cheaper and works fine if you can wait 24-48 hours before loading the post.

  • Fast-setting: $7–$8 per 50-lb bag; pour dry into hole, add water — no mixing needed
  • Regular mix: $5–$6.50 per 80-lb bag; requires mixing in a wheelbarrow or bucket
  • Fast-setting saves 2+ hours per project since you can brace and hang panels immediately
  • For 50+ post projects, regular mix saves $100–$200 in material costs
  • Both types reach the same final strength (~3,000–4,000 PSI) when fully cured

Example Calculations

110 Fence Posts (4" post, 12" hole, 24" deep)

Inputs

Post Diameter4 in
Hole Diameter12 in
Hole Depth24 in
Number of Posts10
Bag Size80 lb
Price per Bag$6.50

Result

Bags Needed24 bags
Volume per Hole1.396 ft³
Total Volume13.96 ft³
Total Weight1,920 lbs
Water Needed12.0 gal
Total Cost$156.00

R = 6/12 = 0.5 ft, r = 2/12 = 0.167 ft, D = 24/12 = 2 ft. V per hole = π × (0.25 - 0.028) × 2 = 1.396 ft³. Total = 1.396 × 10 = 13.96 ft³. Bags = ceil(13.96 / 0.60) = 24.

220 Deck Posts (6" post, 18" hole, 36" deep)

Inputs

Post Diameter6 in
Hole Diameter18 in
Hole Depth36 in
Number of Posts20
Bag Size80 lb
Price per Bag$6.50

Result

Bags Needed158 bags
Volume per Hole4.712 ft³
Total Volume94.25 ft³
Total Weight12,640 lbs
Water Needed79.0 gal
Total Cost$1,027.00

R = 9/12 = 0.75 ft, r = 3/12 = 0.25 ft, D = 36/12 = 3 ft. V per hole = π × (0.5625 - 0.0625) × 3 = 4.712 ft³. Total = 4.712 × 20 = 94.25 ft³. Bags = ceil(94.25 / 0.60) = 158.

Formulas Used

Volume Per Hole

V = π × (R² - r²) × D

Calculates the concrete volume for one post hole by subtracting the post cylinder from the hole cylinder.

Where:

R= Radius of the hole (hole diameter / 2), converted to feet
r= Radius of the post (post diameter / 2), converted to feet
D= Depth of the hole in feet

Total Bags Needed

Bags = ceil(V × N / coverage)

Total bags rounded up. An 80 lb bag yields 0.60 cubic feet of mixed concrete.

Where:

V= Volume per hole in cubic feet
N= Number of post holes
coverage= Cubic feet per bag (40 lb = 0.30, 50 lb = 0.375, 60 lb = 0.45, 80 lb = 0.60)

Water Needed

Water (gal) = bags × 0.5 × (bag weight / 80)

Approximate water needed for mixing. Based on 0.5 gallons per 80 lb bag, scaled proportionally for other bag sizes.

Where:

bags= Total number of bags
bag weight= Weight of each bag in pounds

How to Calculate Concrete for Post Holes

Setting posts in concrete requires knowing the exact volume of the annular space between the post and the hole wall. This calculator uses the cylinder volume formula adjusted for the post displacement to give you precise bag counts.

The most common mistake is underestimating concrete needs. Each post hole is essentially a cylinder with a smaller cylinder (the post) removed from the center. The volume equals PI times the difference of the squared radii times the depth.

Always buy 10% extra bags to account for uneven holes, spillage, and the gravel drainage base. Store unused bags in a dry location for future projects.

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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