Post Hole Concrete Calculator
Calculate how many bags of concrete you need for fence posts, deck posts, and mailbox posts. Get accurate estimates for Quikrete, Sakrete, and other bagged concrete.
Last updated: January 15, 2026
Enter Your Measurements
Automatically sets recommended hole depth and post size
Affects recommended hole diameter
Results
Total Bags Needed: 9 × 80lb
Number of Posts:10
Bags per Hole:1
Concrete per Hole:0.528 cu ft
Total Concrete:5.28 cu ft (0.2 cu yd)
Total Weight:720 lbs
Recommended: Buy 10 bags (+10% for spillage)
720 lbs is a lot to haul!
That's about 1 trips with 10 bags each. Consider store delivery ($50-100) for orders over 500 lbs.
Post diameter: 3.5" — Hole diameter: 8"
How to Use This Calculator
- Select your project type — Choose fence, deck, mailbox, or pergola. This sets recommended hole depth and post size.
- Enter the number of posts — Count how many post holes you need to fill with concrete.
- Choose your post size — 4×4 is standard for fences, 6×6 for decks and pergolas. The calculator adjusts hole diameter automatically.
- Set hole dimensions — Enter diameter and depth. Rule of thumb: hole diameter should be 3× the post width.
- Select bag size — 80lb bags are the best value, but 60lb or 40lb bags are easier to handle.
How We Calculate
Hole Volume = π × (diameter/2)² × depth ÷ 1728
Concrete Needed = Hole Volume − Post Volume
Bags Needed = Concrete Volume ÷ Bag Yield (rounded up)
Example Calculation:
- • Hole: 8" diameter × 24" deep
- • Post: 4×4 (3.5" actual)
- • Hole volume: π × 4² × 24 ÷ 1728 = 0.70 cu ft
- • Post volume: 3.5² × 24 ÷ 1728 = 0.17 cu ft
- • Concrete needed: 0.70 − 0.17 = 0.53 cu ft
- • 80lb bags: 0.53 ÷ 0.60 = 1 bag per hole
Concrete Bag Yields:
40 lb
0.30 cu ft
50 lb
0.375 cu ft
60 lb
0.45 cu ft
80 lb
0.60 cu ft
Bag yields based on Quikrete and Sakrete product specifications. Learn more