MaterialMath, Calculators for DIY Warriors
Tool · F-01

Fence post calculator

Posts, gravel, and concrete sized to your run. Frost-line aware set your hole depth to local code and it does the rest.

Quick answer

How many fence posts and concrete bags do I need?

Posts = ceil(run length ÷ spacing) + 1. With 8 ft on-center spacing, a 100 ft fence needs 14 posts. Each 4×4 post in a 10″ × 36″ hole takes about 1.5 bags of 60 lb fast-set concrete plus 4 inches of gravel at the bottom for drainage.

Intro

When to use this calculator

Fence post math has three parts: how many posts you need, how big each hole should be, and how much concrete to mix per hole. This calculator handles all three.

Enter your fence length, post spacing, post diameter, and frost depth. The calculator returns total posts, gravel for the bottom of each hole, and concrete bags needed per post, plus a total for the whole project.

The calculator uses 8-foot post spacing as the default. That's the standard for most residential wood and vinyl fences. If you're using 6-foot spacing for higher-wind areas, change it.

How to use it

How to use this calculator

  1. Enter total fence length in feet (perimeter for an enclosed yard, single distance for a straight run).
  2. Pick your post spacing. 8 feet is standard, 6 feet for high-wind or heavy gates.
  3. Enter post diameter (4×4 = 4 inches, 6×6 = 6 inches, round = whatever the post is).
  4. Enter your local frost depth from your county building department, or use our state guide.
  5. The calculator returns post count, gravel per hole, and total concrete bags.
Inputs
Run length
ft
Post spacing
ft
Post size
Hole diameter
in
Hole depth
in
Gravel base
in
Concrete bag size
Waste factor10%
Concrete bags per fence post (10″ hole × 36″ deep, 4″ gravel base)
Post size60 lb bags / post80 lb bags / post
4×41.51.0
6×61.30.9
Round metal (3″ OD)1.71.2

Reading the output

Understanding your result

The big number is total posts. Below it: total concrete bags, bags per post, gravel weight for the bases, and adjusted volume with waste factor.

What the bag count assumes: the calculator subtracts the post's own volume from the hole, then adds 10% waste. If you're pouring on a slope or in soft soil where the hole walls slough in, bump the waste to 15 to 20%.

When to round up vs round down: always round up. Buy one extra bag beyond what the calculator says. Mid-pour shortages are the worst kind of shortage because fast-setting concrete starts curing in the hole within 20 minutes.

When to bump the depth: always go below frost line, even if the one-third rule says shallower. A post above frost line will heave out of the ground in the first winter, no matter how much concrete you used.

Common mistakes (and how to avoid them)

✗ Calculating fence length without subtracting gates.

Fix: A 100-foot fence with two 4-foot gates only has 92 feet of fence panels. Posts at gates need to be sized differently.

✗ Using a 24-inch hole depth in a 36-inch frost zone.

Fix: The post will heave in the first winter. Always go below frost depth, even if the rule of thirds says shallower is fine.

✗ Forgetting the gravel base.

Fix: A 6-inch gravel layer at the bottom of the hole counts as part of the depth, dig 6 inches deeper than the frost line.

✗ Skipping the corner posts upgrade.

Fix: Corner posts and gate posts carry more lateral force. Use 6×6 instead of 4×4 at corners and gates.

✗ Pouring concrete with the post not plumb.

Fix: Once concrete sets, you can't adjust. Brace every post with two 1×4 stakes before pouring.

Frequently asked questions

How deep should a fence post hole be?
One-third of the post above ground plus your local frost depth, whichever is greater. For a 6-foot fence in a 36-inch frost zone, that's a 36-inch hole (deeper than the 24-inch one-third rule). For a 6-foot fence in Florida (no frost), 24 inches is enough.
How wide should the hole be?
Three times the post diameter. A 4×4 post (3.5 inch actual) needs a 10 to 12 inch hole. A 6×6 post (5.5 inch actual) needs a 15 to 18 inch hole. Wider holes give the concrete more lateral stability.
Can I set fence posts without concrete?
Yes, with gravel-only setting. Tamp gravel around the post in 6-inch lifts. Works for sandy or well-drained soils. Doesn't work in clay or heavy moisture areas, concrete is more forgiving.
How many bags of concrete per post?
Depends on hole size. A 12×36 inch hole takes about 2.4 cubic feet of concrete = 4 bags of 80 lb. A 10×30 inch hole takes about 1.4 cubic feet = 2.5 bags. The calculator returns the exact number.
Should I use fast-setting concrete for fence posts?
Yes if you want to set the post and back-fill in the same day. Fast-setting reaches usable strength in 4 hours. Standard concrete needs 24 hours before you can let go of the post bracing.
More from MaterialMath

Related guides

Frost line depth by state

State-by-state table from adopted IRC building code values, plus how to find your county's exact number.

Open

Bags of concrete per fence post

Frost-line aware bag math for 4×4, 6×6, and round posts.

Open
Commerce, optional

Where to buy your materials

We've linked to common products at Home Depot and Lowe's below. These are affiliate links, meaning we earn a small commission if you buy through them, at no cost to you. We've used these specific products on projects ourselves. If your local independent yard has a better price, take it.

Where to buy

×14

4x4 pressure-treated pine, 8 ft.

Ground-contact rated lumber lasts 15–20 yrs in the soil. Skip non-ground-contact stock.

Check price atThe Home DepotCheck price atLowe's

Affiliate links, same price for you, helps keep the calculators free.

Where to buy

×43

Quikrete fast-setting concrete, 50 lb bags.

Sets in ~40 minutes, pour dry into the hole, add water on top. No mixing tub needed.

Check price atThe Home DepotCheck price atLowe's

Affiliate links, same price for you, helps keep the calculators free.

Want a printable version?

We'll email a one-page PDF summary of this page's math plus a calling script for local supply yards. Everything we know is right here on the page, the PDF just makes it portable.

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