How Many Bags of Concrete for Fence Posts?

Quick charts, sizing rules, and setting methods for every fence type

Written by M., a general contractor in Richmond, VA · Last updated

A 4x4 fence post in a 12 x 24 inch hole takes 4 bags of 50 lb concrete mix or 3 bags of 80 lb. Larger posts, deeper holes and gate posts take more; the chart below covers the common sizes.

For an exact calculation tailored to your project, try our Post Hole Calculator.

Bags-Per-Post Quick Reference Chart

Counts assume a round hole, subtract the volume of the post, and are rounded up to whole bags.

Post Size Hole Diameter Hole Depth 50-lb Bags 80-lb Bags
4x4 12" 24" 4 3
4x4 12" 36" 6 4
4x4 12" 42" 7 5
6x6 (gate post) 16" 36" 10 6
6x6 (gate post) 16" 42" 12 7
4x6 (corner/end) 14" 36" 8 5

Quick math behind the chart:

A 50-lb bag of concrete yields roughly 0.375 cubic feet of mixed concrete. An 80-lb bag yields about 0.6 cubic feet. To find the volume of your hole, use the cylinder formula (pi x radius squared x depth), then subtract the volume of the post itself. Divide the remaining volume by the bag yield.

How much the post displaces:

A 4x4 (3.5" x 3.5" actual) takes up 0.085 cubic feet per foot of depth: 0.17 cubic feet in a 24" hole, about half a 50-lb bag. A 6x6 (5.5" x 5.5") takes up 0.21 cubic feet per foot: 0.63 cubic feet in a 36" hole, about 1.7 bags of 50-lb mix. Skip the subtraction and you over-buy by that much per post. A 4" round post displaces about the same as a 4x4, and a 6" round post about the same as a 6x6, so use those rows for round posts in the same size hole.

Total Bags for Your Whole Fence

This table assumes standard 4x4 posts at 8-foot spacing with 12" x 24" holes (the depth for a 4-ft fence; a 6-ft fence needs 36-inch holes, 6 bags of 50 lb per post from the chart above), filled to grade around the post: 4 bags of 50-lb mix per post, or the same 1.5 cubic feet from 80-lb bags (0.60 cu ft each), which is about 2.5 bags of 80 lb per post when you split bags between holes. Posts are the fence length divided by 8, rounded up, plus one for the end. Cost is figured on 80-lb bags at $5 to $8 each. Add 1-2 extra bags for gate posts and corners.

Fence Length Total Posts 50-lb Bags or 80-lb Bags Cost (80-lb bags)
25 ft 5 20 13 $65 to $104
50 ft 8 32 20 $100 to $160
75 ft 11 44 28 $140 to $224
100 ft 14 56 35 $175 to $280
150 ft 20 80 50 $250 to $400
200 ft 26 104 65 $325 to $520

Add 10% to the bag count for waste and holes dug wider than planned. For bag yields and pallet counts, see the complete concrete bags guide.

Hole Sizing Rules

Hole size sets the bag count. Too small and the post has too little concrete around it to stay put; too big and you pay for bags you did not need.

The Three Rules:

  • Diameter = 3x the post width. A 4x4 post (actual 3.5") needs a 10-12" diameter hole. A 6x6 post (actual 5.5") needs a 16-18" hole. This gives enough room for concrete on all sides.
  • Depth = one-third of the post's total length. Bury about one-third of the post's total length (the Quikrete and Sakrete instructions), which is half its height above ground: 36 in for a 6-ft fence on a 9-ft post, 24 in for a 4-ft fence on a 6-ft post; go deeper where the frost line requires it.
  • In frost areas, dig below the frost line. If your frost line is 36" and the one-third rule says 24", go to 36". Frost heave will push your posts out of the ground otherwise. Check your local frost line depth at our frost line guide.

Most post hole diggers (manual or auger) come in standard sizes: 6", 8", 10", and 12". For 4x4 posts, a 10" or 12" auger works perfectly. For 6x6 gate posts, you may need a 14" or 16" auger, or dig by hand.

Dry Pour vs Wet Mix vs Fast-Setting

There are three common methods for setting fence posts in concrete. Each has trade-offs, and the "best" method depends on your situation.

1. Fast-Setting Dry Pour (Most Popular DIY Method)

Pour the dry fast-setting mix (like Quikrete Fast-Setting) directly into the hole around the post, then add water on top. The mix absorbs the water and sets in 20-40 minutes. No mixing required.

Pros: Fastest, easiest, no wheelbarrow or mixing. Cons: Slightly weaker than wet mix, harder to ensure full saturation at the bottom of deep holes.

2. Traditional Wet Mix (Strongest)

Pre-mix the concrete in a wheelbarrow or mixing tub with water, then pour the wet concrete around the post. This is how professionals usually do it for structural posts.

Pros: Strongest bond, complete and even cure, best for gate posts. Cons: More labor, needs wheelbarrow and tools, limited working time once mixed.

3. Dry Pack (Controversial)

Pour regular (not fast-setting) dry concrete mix into the hole and let ground moisture hydrate it over time. Some old-school builders swear by it, but it is the least reliable method.

Pros: Zero water needed on site. Cons: Unpredictable cure, may never fully hydrate in dry climates, weakest option. Most manufacturers do not recommend this method.

Our recommendation: Use fast-setting dry pour for standard line posts and traditional wet mix for gate posts, corner posts, and any post that will bear significant load.

Gate Posts Need Special Treatment

Gate posts take more abuse than any other post in your fence. Every time the gate opens and closes, it puts lateral force on the hinges and the post. A standard 4x4 post with two bags of concrete is not enough.

Gate Post Best Practices:

  • - Use 6x6 posts instead of 4x4 for all gate locations
  • - Use 2-3x the concrete of a standard line post (10-12 bags of 50-lb mix for a 6x6 post in a 16-inch hole)
  • - Always use the wet-mix method for gate posts, never dry pour
  • - Dig the hole 16-18" in diameter and at least 36" deep (or below frost line)
  • - Let gate posts cure for at least 48 hours before hanging the gate
  • - For heavy gates (wood privacy, iron), consider adding a steel post bracket or concrete collar above grade

The #1 reason gates sag over time is an undersized gate post. A bigger post and a few more bags of concrete cost far less than rebuilding a sagging gate.

Step-by-Step: Setting a Fence Post

Quikrete Fast-Setting Method (Dry First, Then Water):

  1. Dig the hole to the correct diameter and depth
  2. Add 4-6" of compacted gravel to the bottom (see gravel section below)
  3. Set the post in the hole, brace it plumb with stakes and clamps
  4. Pour dry fast-setting concrete mix into the hole, filling to within 3-4" of ground level
  5. Add water according to the bag directions (typically 1 gallon per 50-lb bag)
  6. Let the water soak in for a few minutes, then add more if the surface still looks dry
  7. Mound the concrete slightly above grade and slope it away from the post for drainage
  8. Wait 20-40 minutes before attaching fence panels; 4 hours before heavy stress

Sakrete / Traditional Wet Mix Method (Water First, Then Mix):

  1. Dig the hole and add gravel base
  2. In a wheelbarrow, add about 3 quarts of water per 80-lb bag
  3. Slowly add the dry mix to the water while mixing with a hoe
  4. Mix until the consistency is like thick peanut butter (no dry pockets)
  5. Set the post and brace it plumb
  6. Shovel wet concrete around the post, tamping to remove air pockets
  7. Crown the top and slope away from the post
  8. Wait 24-48 hours before attaching fence panels or hanging gates

The Gravel Base: Don't Skip This Step

Before you pour any concrete, add 4-6 inches of compacted gravel to the bottom of every post hole. This is the most overlooked step in DIY fence building, and skipping it is one of the top reasons posts rot prematurely.

Why Gravel Matters:

  • - Drainage: Water collects at the bottom of the hole. Gravel lets it drain away from the post end grain, where wood absorbs moisture the fastest.
  • - Stability: Compacted gravel gives you a firm, level base to set the post on before pouring concrete.
  • - Frost protection: In cold climates, gravel gives water a place to expand without putting direct pressure on the concrete footing.

Use 3/4" crushed stone or pea gravel. Avoid sand (it holds water) and river rock (it doesn't compact well). Pour it in, tamp it flat with a 2x4 or tamping bar, and make sure it's level before setting the post.

Foam Alternatives: Sika PostFix and Similar Products

Expanding polyurethane foam is a newer alternative to concrete for setting fence posts. Products like Sika PostFix and Fast 2K come in two-part pouches that you mix and pour into the hole, where they expand and harden in minutes.

Foam vs Concrete:

  • - Cost: a standard 4x4 post takes 3 bags of 80-lb concrete, about $15 to $24. Foam usually costs more per post, and for large gate posts concrete is the stronger and cheaper choice.
  • - Speed: Foam sets in 3-5 minutes. Fastest option by far.
  • - Weight: One pouch replaces 1-2 bags of concrete. Much easier to transport and handle.
  • - Strength: Adequate for standard fence posts. Not recommended for structural applications or gate posts bearing heavy loads.
  • - Removal: Foam is much easier to remove if you ever need to replace a post. Concrete requires breaking it out.

Foam suits long runs of standard 4x4 line posts, especially when you work alone. Use concrete for gate posts, corner posts and anything structural.

Sources

Prices checked October 31, 2026. Bag prices are a national range from the cost guide below, so get a local quote; the post depth rule and bag yields are from the Quikrete and Sakrete data sheets.

  1. HomeGuide, Concrete Delivery Cost (updated May 1, 2026). Retrieved October 31, 2026.
  2. QUIKRETE Concrete Mix (No. 1101) data sheet (PDF) (revised October 2022). Retrieved October 31, 2026.
  3. Sakrete High Strength Concrete Mix technical data sheet (PDF) (January 2024). Retrieved October 31, 2026.
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