Concrete for Fence & Deck Posts
Last reviewed: April 2026
Calculate concrete needed for fence and deck post holes. Enter hole diameter, depth, and number of posts to get bags of concrete or cubic yards. This calculator runs entirely in your browser — your data stays private, and no account is required.
Setting fence and deck posts in concrete provides stability and prevents leaning, heaving, and rot at ground level. The amount of concrete depends on hole diameter, hole depth, and post size. This calculator accounts for the volume displaced by the post itself — you're filling the space around the post, not the entire hole.
Fence posts (4×4): 8–10″ diameter hole, 24–36″ deep (minimum 1/3 of total post length underground). Gate posts and corner posts: 10–12″ hole, 36″ deep for extra stability. Deck posts (6×6): 12″ diameter hole, depth to local frost line (36–48″ in northern states, 12–24″ in southern states). Mailbox posts: 8″ hole, 18–24″ deep. Always check local building codes — many jurisdictions specify minimum post hole depth and concrete requirements.
50-lb bag: Yields ~0.375 cubic feet. Good for small fence post jobs. 60-lb bag: Yields ~0.45 cubic feet. The most common bag for post holes. 80-lb bag: Yields ~0.6 cubic feet. Best value per cubic foot but heavy to handle. For most fence posts (8″ hole, 30″ deep, 4×4 post), expect 1–2 bags of 60-lb concrete per post. For deck posts (12″ hole, 42″ deep, 6×6 post), expect 3–4 bags.
Fast-setting (Quikrete Fast-Setting): Sets in 20–40 minutes. Pour dry mix into the hole, add water on top. No mixing needed. Great for fence posts — you can attach rails the same day. Regular concrete: Must be mixed with water before pouring. Takes 24–48 hours to set. Slightly stronger when fully cured. Better for structural posts (decks, pergolas).
1. Add 4–6″ of gravel to the bottom of each hole for drainage. 2. Set the post plumb (use a level on two sides). 3. Crown the concrete slightly above grade so water drains away from the post. 4. For wood posts, the concrete should not extend above the soil line — trapped moisture at the concrete-wood interface accelerates rot. 5. Brace posts until concrete cures. 6. In frost-prone areas, dig below the frost line to prevent heaving.
Foam post-setting compounds (like Sika PostFix) expand to fill the hole and set in 3 minutes. One can replaces 1–2 bags of concrete at similar cost but is dramatically easier to use. Compacted gravel alone works for non-structural fence posts in well-drained soil. Ground-contact pressure-treated posts can be set directly in compacted soil for temporary fencing.
| Hole Diameter | Depth | Cubic Feet | 60-lb Bags |
|---|---|---|---|
| 8 in | 24 in | 0.58 | 1 |
| 10 in | 30 in | 1.14 | 2 |
| 12 in | 36 in | 1.96 | 3–4 |
| 12 in | 42 in | 2.29 | 4 |
Proper post hole dimensions are the foundation of every fence, deck, mailbox, and sign installation. Getting the diameter, depth, and concrete fill right ensures structural integrity for decades, while undersized holes lead to leaning posts, frost heave, and premature failure that costs far more to fix than to do correctly the first time.
The standard guideline for post hole diameter is three times the post width. A 4×4 post (actual 3.5") needs an 10–12 inch diameter hole. A 6×6 post (actual 5.5") needs a 16–18 inch hole. This provides adequate room for concrete to surround the post and distribute loads into the surrounding soil. Depth should be one-third of the total post length plus 6 inches for a gravel drainage base. A 6-foot fence with an 8-foot post needs a hole 32 inches deep (8 ÷ 3 = 2.67 feet = 32 inches, plus 6 inches of gravel = 38 inches total). In cold climates, holes must extend below the frost line — the depth at which the ground freezes. Frost line depths range from 12 inches in the southern U.S. to 60+ inches in northern states. Posts set above the frost line will heave upward as frozen soil expands, causing the structure to tilt and eventually fail.
Concrete set: The most common and strongest method. Mix quick-setting concrete (like Quikrete or Sakrete) directly in the hole — pour dry mix around the post, then add water. Each 50-lb bag fills approximately 0.375 cubic feet. A typical 10" diameter × 36" deep hole requires 3–4 bags. Concrete provides rigid, permanent support and prevents rotation. Gravel/crushed stone set: Increasingly popular for wood fence posts because it allows water to drain away from the post base, reducing rot. Compacted gravel provides excellent lateral support while extending post life by preventing the moisture-trapping contact that concrete creates against wood. Not suitable for gate posts or structural posts where lateral rigidity is critical. Foam set (polyurethane expanding foam): Products like Sika PostFix expand to fill the hole, setting in minutes. Each can replaces 1–2 bags of concrete. Lightweight, fast, and surprisingly strong. Costs more per post ($12–$18 per can vs. $5–$7 per concrete bag) but saves significant labor on large projects.
Clay soil: Difficult to dig when dry (extremely hard) and when wet (sticky, heavy). Requires larger diameter holes because clay provides less lateral resistance than sand or gravel. Clay also retains moisture against posts, accelerating wood rot — consider gravel backfill or treating the buried portion with wood preservative. Sandy soil: Easy to dig but collapses easily, requiring wider holes or post hole forms. Provides good drainage but less lateral support — use concrete set for structural posts in sand. Rocky soil: May require a rock bar, jackhammer, or professional augering. Rocks encountered mid-hole can be pried out or, if too large, the hole location may need to shift. High water table: If water fills the hole during digging, pump or bail it before pouring concrete, and ensure gravel drainage below the post base to prevent standing water rot.
Manual post hole digger (clamshell): Standard tool for small projects (1–10 holes). Effective in most soil types for holes up to 36 inches deep and 8–12 inches in diameter. Costs $30–$60. Manual auger: Screw-type tool that handles deeper holes and harder soil better than clamshell diggers. Requires more upper body strength. $40–$80. Power auger (one-person): Gas or electric powered, handles 10–30 holes efficiently. Available for rental at $50–$100/day. Use caution — power augers can torque violently when they hit rocks or roots. Two-person power auger: More powerful and stable than one-person models. Essential for holes deeper than 36 inches or in very hard soil. Rental: $75–$150/day. Tractor-mounted auger: For large-scale fencing projects (50+ holes), a PTO-driven auger on a compact tractor is the most efficient option. Rental: $150–$300/day plus tractor.
Before digging, always call 811 (the national utility locating service) at least 48 hours in advance. Underground gas, electric, water, sewer, cable, and fiber optic lines can be as shallow as 12 inches, and striking one can cause injury, service disruption, and significant repair liability.
After placing the post in the hole with 6 inches of gravel at the bottom, check plumb on two adjacent sides using a level before adding concrete or backfill. Temporary braces (2×4 stakes angled from the post to the ground) hold the post perfectly vertical while concrete sets — a critical step because adjusting a post after concrete hardens is virtually impossible without breaking it out and starting over. For fence runs, establish a straight line using string pulled taut between corner posts, then align intermediate posts to the string. Crown the concrete slightly above grade and slope it away from the post to shed water, preventing pooling at the base that accelerates post deterioration.
→ Always add 10–15% for waste. Materials get cut, broken, or mis-measured. Order extra to avoid a second trip to the supply store mid-project.
→ Measure twice, calculate once. Double-check your measurements before entering them. A small measurement error can mean ordering significantly too much or too little material.
→ Check local building codes. Requirements vary by jurisdiction. This calculator uses standard practices, but your local code may specify different minimums or methods.
→ Save your calculations. Use the share or print feature to keep a record of your material estimates for reference at the store or job site.
See also: Fence Calculator · Concrete Calculator · Deck Calculator · Gravel Calculator · Retaining Wall