🕳️ Post Hole Concrete Calculator
Calculate concrete needed for post holes
Common: 6" (fence posts), 8" (decks)
Typical: 2-3 feet
How to Use This Calculator
Select Calculation Type
Choose to calculate for hole only (full volume) or hole minus post (concrete only around post). Hole minus post gives more accurate concrete quantity.
Enter Hole Dimensions
Input hole diameter in inches (typically 6" for fence posts, 8" for deck posts) and hole depth in feet (typically 2-3 feet).
Enter Post Information
If calculating hole minus post, enter post diameter in inches (must be less than hole diameter). Enter number of posts if calculating for multiple posts.
Calculate and Review
Click "Calculate Post Hole Concrete" to see concrete volume in cubic yards and cubic feet, number of bags needed (80 lb and 60 lb), and total weight. Use this for ordering concrete.
Formula
Hole Volume = π × r² × h
Concrete Volume = Hole Volume - Post Volume (if post included)
Total Volume = Concrete Volume × Number of Posts
Concrete (cubic yards) = Total Volume (cubic feet) / 27
Example 1: 6" diameter, 2 ft depth, 1 post, hole only
Step 1: Radius = 6 / 2 / 12 = 0.25 ft
Step 2: Volume = π × 0.25² × 2 = π × 0.0625 × 2 = 0.393 cu ft
Step 3: Concrete = 0.393 / 27 = 0.0146 cubic yards
Step 4: Bags (80 lb) = 0.393 / 0.6 = 1 bag (round up)
Example 2: 6" hole, 4" post, 2 ft depth, 10 posts
Step 1: Hole Volume = π × (0.25)² × 2 = 0.393 cu ft
Step 2: Post Volume = π × (0.167)² × 2 = 0.175 cu ft
Step 3: Concrete per Post = 0.393 - 0.175 = 0.218 cu ft
Step 4: Total = 0.218 × 10 = 2.18 cu ft = 0.081 cubic yards
About Post Hole Concrete Calculator
The Post Hole Concrete Calculator is an essential tool for contractors, DIY enthusiasts, and fence builders who need to calculate concrete volume and bags needed for post holes. This calculator implements geometric formulas to determine concrete volume for cylindrical post holes, accounting for post size if needed.
When to Use This Calculator
- Post Hole Planning: Calculate concrete needed for fence post holes
- Material Ordering: Determine number of concrete bags to order
- Cost Estimation: Estimate concrete costs for post installation
- Project Planning: Plan concrete quantities for multiple posts
- Educational Use: Learn and understand post hole concrete calculations
Why Use Our Calculator?
- ✅ Dual Calculation Modes: Supports hole-only and hole-minus-post calculations
- ✅ Multiple Posts: Calculates for single or multiple posts
- ✅ Bag Calculations: Shows number of 80 lb and 60 lb bags needed
- ✅ Accurate Formulas: Uses geometric equations for precise calculations
- ✅ Time Savings: Instant calculations eliminate manual math
Understanding Post Hole Concrete
Basic Principle: Post hole volume is calculated as cylinder volume: Volume = π × r² × h, where r is hole radius and h is hole depth. For hole-minus-post calculation, subtract post volume from hole volume: Concrete Volume = Hole Volume - Post Volume. This gives accurate concrete quantity around the post.
Hole Sizes: Typical hole diameters: 6" for fence posts (4×4 posts), 8" for deck posts (6×6 posts), 10-12" for large posts. Typical depths: 2-3 feet for fence posts, 3-4 feet for deck posts. Deeper holes provide more stability but require more concrete.
Concrete Bags: Standard concrete bags: 80 lb bags (~0.6 cu ft per bag), 60 lb bags (~0.45 cu ft per bag). For small quantities, bags are convenient. For large quantities, ready-mix concrete (cubic yards) is more economical.
Tips for Best Results
- Accurate Dimensions: Measure hole diameter and depth accurately
- Account for Post: Use hole-minus-post calculation for accurate concrete quantity
- Add Safety Margin: Add 10-20% extra for spillage and variations
- Round Up Bags: Always round up number of bags (can't use partial bags)
- Consider Ready-Mix: For large quantities, consider ready-mix concrete
Frequently Asked Questions
How much concrete do I need for a post hole?
Concrete needed = Hole Volume / 27 (cubic yards). For example, 6" diameter, 2 ft depth: Volume = π × (0.25)² × 2 = 0.393 cu ft = 0.0146 cubic yards = 1 bag (80 lb). For multiple posts, multiply per-post volume by number of posts. The calculator does this automatically.
Should I calculate hole only or hole minus post?
Hole minus post gives more accurate concrete quantity because it accounts for post volume. For example, 6" hole, 4" post, 2 ft depth: Hole Volume = 0.393 cu ft, Post Volume = 0.175 cu ft, Concrete = 0.218 cu ft (45% less). Use hole minus post for accurate quantities.
How many bags of concrete do I need per post?
For 6" diameter, 2 ft depth hole: Volume = 0.393 cu ft = 1 bag (80 lb). For 8" diameter, 3 ft depth: Volume = 1.047 cu ft = 2 bags (80 lb). For 6" hole with 4" post: Concrete = 0.218 cu ft = 1 bag (80 lb). Always round up to whole bags.
What size hole should I dig for a fence post?
Typical hole sizes: 6" diameter for 4×4 posts, 8" diameter for 6×6 posts. Typical depth: 2-3 feet for fence posts (1/3 to 1/2 of post height above ground). Deeper holes provide more stability. Check local building codes for specific requirements.
Do I need to add extra concrete for waste?
Yes, add 10-20% extra concrete for spillage, variations in hole size, and uneven ground. For example, if calculator shows 0.015 cubic yards, order 0.018-0.020 cubic yards. For bags, round up to next whole bag (already accounted for in calculator).