Concrete Calculator for Fence Posts
Bags of 40, 50, 60 or 80 lb concrete per fence post by height, post size, hole diameter and frost depth.
Built by Toolsque Team · last updated September 15, 2026 · checked against our testing process
Fence Specifications
Concrete & Gravel Needed
Total concrete 11.0 cu ft (0.41 cu yd). Buy one extra bag for every 10 posts to cover wide or uneven holes.
How to use the fence post concrete calculator
- Enter fence height and number of posts: standard residential fence runs use 6 or 8 foot spacing between posts.
- Select post type and dimensions: choose 4x4 or 6x6 square wood, 5x5 vinyl, or round steel terminal and line posts.
- Select concrete bag size: compare 80 lb, 60 lb, 50 lb, or 40 lb pre-mixed bags.
- Enter optional frost depth: post embedment depth automatically scales below the frost line to prevent winter ground heaving.
- Review bags and base gravel: order the calculated bags plus 6 inches of crushed gravel per hole for base drainage.
How much concrete per fence post?
A standard 4x4 wood post in a 10 inch hole with 24 inches of concrete takes about 1.5 bags of 80 lb concrete (0.92 cubic feet or 123 lb). A heavy 6x6 post in a 16 inch hole with 36 inches of concrete takes about 6 bags of 80 lb concrete (3.6 cubic feet or 475 lb). The table below gives exact bag counts across common post and hole combinations.
| Post Type | Hole Size (width × concrete depth) | Concrete (cu ft) | 50 lb Bags | 60 lb Bags | 80 lb Bags |
|---|---|---|---|---|---|
| 4x4 wood | 10 in × 24 in | 0.92 | 2.5 | 2.0 | 1.5 |
| 4x4 wood | 10 in × 30 in | 1.15 | 3.1 | 2.6 | 1.9 |
| 4x4 wood | 12 in × 36 in | 2.10 | 5.6 | 4.7 | 3.5 |
| 6x6 wood | 16 in × 36 in | 3.56 | 9.5 | 7.9 | 5.9 |
| 6x6 wood | 16 in × 42 in | 4.15 | 11.1 | 9.2 | 6.9 |
| 5x5 vinyl | 12 in × 30 in | 1.53 | 4.1 | 3.4 | 2.5 |
| Round metal, 2 3/8 in | 8 in × 24 in | 0.64 | 1.7 | 1.4 | 1.1 |
| Round metal, 4 in | 12 in × 36 in | 2.09 | 5.6 | 4.7 | 3.5 |
Concrete depth here represents the portion filled with concrete. The hole itself is 6 inches deeper to accommodate base drainage gravel. Round up per post when purchasing materials to account for uneven hole digging.
How deep should a fence post be?
Bury approximately one third of the post height above ground, never less than 24 inches, plus 6 inches deeper for base gravel. A 6 foot fence requires 24 inches of post in the ground and a 30 inch total hole depth. In freezing climates, the base of the footing must sit below the local frost depth to prevent freeze-thaw ground heaving.
| Fence Height | Post in Ground | Total Hole Depth (with gravel) |
|---|---|---|
| 3 ft | 24 in | 30 in |
| 4 ft | 24 in | 30 in |
| 5 ft | 24 in | 30 in |
| 6 ft | 24 in | 30 in |
| 7 ft | 28 in | 34 in |
| 8 ft | 32 in | 38 in |
Gate and corner posts carry higher dynamic loads than line posts: set them 6 inches deeper than standard line posts, or deeper than the local frost line if greater.
What size hole for a 4x4 or 6x6 post?
Dig the hole diameter approximately three times the width of the post. A 4x4 post measures 3.5 inches actual width, requiring a 10 inch hole, or 12 inches in loose sandy soils. A 6x6 post measures 5.5 inches actual and requires a 16 inch hole. These dimensions match standard rental auger diameters.
Fast-setting vs standard concrete mix
Fast-setting concrete pours dry directly into the hole, takes water poured on top, and sets in 20 to 40 minutes without temporary bracing. Standard mix requires mechanical mixing in a wheelbarrow or mixer but delivers higher ultimate compressive strength once cured, making standard mix ideal for heavy gate posts.
How long before the post can take weight
Fast-setting mix holds a post upright within 30 minutes, but wait 24 to 48 hours before attaching heavy rails or fence panels. Standard concrete requires bracing for 24 hours and reaches full design strength after 28 days.
How concrete volume is calculated
Hole volume is calculated as cylinder volume minus post displacement. For square posts, post area is width squared; for round posts, area is circular. Net volume in cubic feet equals net area multiplied by concrete depth divided by 1,728. Pre-mixed bag yields are 0.30 cu ft (40 lb), 0.375 cu ft (50 lb), 0.45 cu ft (60 lb), and 0.60 cu ft (80 lb).
Before you dig
In the United States, dial 811 or visit your state 811 portal several days prior to digging. Utility locating is free and prevents strikes on buried gas, electric, or fiber lines.
Related tools
Pricing a complete fence installation? Combine with the job costing calculator and contractor markup calculator. Pouring footings or paths? Check the gravel calculator and construction cost estimator.
What this calculator does not cover
This calculator estimates concrete and gravel volumes for standard residential post fences. It does not design commercial retaining walls, structural deck piers, pergola foundations, or high-wind commercial signposts, which require site-specific structural engineering.
Frequently Asked Questions
How many bags of concrete for a 4x4 fence post?
About 1.5 bags of 80 lb concrete, 2 bags of 60 lb or 2.5 bags of 50 lb for a 4x4 in a 10 in hole with 24 in of concrete. That is about 0.92 cubic feet or 123 lb. Deeper or wider holes add up quickly, so enter your exact dimensions in the calculator.
How deep should a fence post be?
About one third of the fence height, with a minimum of 24 in in the ground, plus 6 in of gravel below. A 6 ft fence needs a 30 in hole. In cold areas the post must also reach below the local frost depth.
What size hole do I need for a 4x4 fence post?
About three times the post width. A 4x4 is 3.5 in wide, so a 10 in hole is standard and 12 in is better in sandy or loose soil. A 10 in auger is the usual rental for 4x4 posts.
How many pounds of concrete per fence post?
About 120 to 125 lb for a 4x4 in a 10 in hole with 24 in of concrete, and about 475 lb for a 6x6 in a 16 in hole with 36 in of concrete. Bags yield roughly 0.0075 cubic feet per pound, so an 80 lb bag makes 0.60 cubic feet.
How much concrete for a 6x6 post?
A 6x6 in a 16 in hole with 36 in of concrete needs about 3.6 cubic feet, which is 5.9 bags of 80 lb or 7.9 bags of 60 lb. Round up to 6 or 8 bags per post.
Should I use fast-setting concrete for fence posts?
Yes for line posts: pour it dry, add water, and it sets in 20 to 40 minutes without bracing. Use standard mix for gate posts and heavy 6x6 posts, where the extra long term strength matters.
Do I put gravel at the bottom of a fence post hole?
Yes. About 6 in of gravel under the post lets water drain away from the bottom of the wood and gives the post a firm base. Set the post on the gravel, plumb it, then fill around it with concrete.
Do I pour concrete before or after placing the post?
Post first. Add the gravel, set and plumb the post, then fill around it. Slope the top of the concrete away from the post so rain runs off instead of pooling against the wood.