Fence Post Depth Calculator

The one third rule for fence post depth

The most repeated rule in fence building says to bury about one third of the post's height above ground, which leaves two thirds showing. A 4 foot fence wants roughly 16 inches of post in the hole, a 5 foot fence about 20 inches, and a 6 foot fence about 24 inches. The idea is simple physics: the buried third anchors the post so it can resist the sideways force that wind puts on everything above grade.

One caution before you dig: the one third rule is guidance from fence manufacturers and home inspection groups such as InterNACHI, not a line in the national model building codes. The International Code Council's residential code does not set a fence post depth, and it generally exempts fences under 7 feet from permits. So the rule is a starting point, and two other numbers can push the hole deeper: the frost line under your yard and the local rules where you live.

The rule also assumes average soil. Loose sand or soft clay holds a post worse than packed loam, so a shallow setting that works on a hillside lot can lean after the first wet spring. When in doubt, go deeper rather than shallower, because extra depth costs one more shovel of dirt and a little more concrete.

Worked example: a 6 foot fence where the frost line sits 30 inches down

Numbers make the two rules easy to compare. Take a 6 foot fence with posts that stand 6 feet above grade, in a yard where the frost line sits 30 inches down. The one third rule gives:

• 12 x 6 / 3 = 24 inches of post below grade, from the one third rule
• 30 + 6 = 36 inches below grade, from the frost rule, which adds 6 inches of clearance under the frost line
• = 36 inches is the dig depth, because you always use the larger of the two numbers

The frost rule wins here, and it usually does anywhere the ground freezes. The post you buy has to cover both the part you see and the part you bury: 6 feet above ground plus 36 inches below works out to 6 + 3 = 9 feet of post. Buying an 8 foot post for a 6 foot fence is the classic mistake this page exists to prevent.

Concrete comes next, after a gravel base. With a 10 inch diameter hole, 6 inches of gravel at the bottom, and a 36 inch hole, the concrete fills:

• 36 - 6 = 30 inches of concrete depth, once the gravel takes the bottom 6 inches
• (3.1416 x 5 x 5 x 30) / 1728 = 1.36 cubic feet of concrete per post, from the cylinder volume divided by 1,728 cubic inches per cubic foot
• 1.36 / 0.375 = 3.6 bags, so you buy 4 bags of the 50 pound mix

The 0.375 figure is the yield of a 50 pound bag of concrete mix, about 0.375 cubic feet. A 60 pound bag yields about 0.45 cubic feet, so the same hole takes 1.36 / 0.45 = 3.0 bags. Buy one more bag than the math says; runs come up short more often than they come up long.

The frost line: the number that can double your hole depth

Where the ground freezes in winter, frozen soil swells, and swelling soil grips anything buried in it and pushes it upward. That is frost heave, and it is why a fence set above the frost line slowly walks itself out of the ground over a few winters, tilting a little more each year. The standard fix is to set the bottom of the footing 6 inches below the frost line, so the frozen layer pushes on soil that cannot lift the concrete.

The depth that governs your hole is a local lookup, not a national figure. The model residential code prints its frost line table blank and tells each jurisdiction to fill in its own value, so two towns 50 miles apart can publish different numbers. Your city or county building department publishes the official depth for your address, and climate records from NOAA show how deep the ground freezes in a typical winter. Some published values give the range of what you might see:

In a frost free area the frost rule drops out and the one third rule stands on its own, so a 6 foot fence in Phoenix is back to a 24 inch hole. In Fargo the same fence digs to 48 + 6 = 54 inches, which is why fence posts in the northern plains look like they were planted for a bridge.

Post depth chart by fence height

The chart below applies the one third rule to common fence heights and adds the post length you would buy. It assumes no frost requirement; where the ground freezes, the dig depth is the larger of the one third number and your frost line plus 6 inches.

Frost changes the second and third columns together. A 6 foot fence where the frost line sits 42 inches down needs a 48 inch hole, so the post length grows to 6 + 3.5 = 9.5 feet. Tall fences in cold climates are the reason lumber yards stock 10 and 12 foot posts.

Two heights deserve extra care. Anything over 6 feet catches more wind, so lean to the deep end of the range. And a gate post takes moving loads every day it opens, so setting gate posts 6 inches deeper than the line posts on the same run is cheap insurance against the sag that shows up in year two.

How much concrete each post needs

Concrete is sold by the bag and priced by the pallet, so the useful number is bags per post, then bags times posts. The volume is a cylinder: 3.1416 times the radius squared, times the concrete depth, divided by 1,728 to reach cubic feet. A wider hole costs concrete faster than a deeper one, because the radius is squared.

The same 6 foot fence from the worked example, but with a 12 inch hole for a gate post, shows the jump:

• 36 - 6 = 30 inches of concrete depth, same as before
• (3.1416 x 6 x 6 x 30) / 1728 = 1.96 cubic feet, up from 1.36 at 10 inches wide
• 1.96 / 0.375 = 5.2 bags, so 6 of the 50 pound bags

On the small end, a 24 inch hole for a low garden fence with an 8 inch auger works out the other way: 24 - 6 = 18 inches of concrete, then (3.1416 x 4 x 4 x 18) / 1728 = 0.52 cubic feet, and 0.52 / 0.375 = 1.4 bags, so 2 bags. A 12 inch by 48 inch hole in a frost climate is the big one: (3.1416 x 6 x 6 x 42) / 1728 = 2.75 cubic feet, and 2.75 / 0.375 = 7.3 bags, so 8.

Quick mix reference: a 50 pound bag yields about 0.375 cubic feet, a 60 pound bag about 0.45, and an 80 pound bag about 0.6. Fewer big bags go further per dollar, but two people carrying 80 pound bags across a backyard may decide the smaller bag is worth it.

Hole width, gravel and drainage

Depth gets the attention, but three details around the hole decide whether the post stays put. The first is width: the standard is a hole about 3 times the post's width, so a nominal 4x4 wood post (which measures about 3.5 inches square) wants a 10 to 12 inch hole. A hole the same width as the post leaves no concrete collar to resist sideways push.

The second is the gravel base. Six inches of crushed stone under the concrete gives water somewhere to go so the post end does not sit in a permanently wet cup. That 6 inches is why every concrete figure on this page subtracts it from the dig depth first. The third is the crown: trowel the concrete top so it slopes away from the post on all sides, which keeps rain from pooling against the wood.

Read the table as a shopping list for one post, then multiply by the number of posts and add 10 percent for waste. A 150 foot run of 6 foot privacy fence on 8 foot centers is about 19 posts, so the middle row means roughly 76 bags, which is a little over two pallets of the 50 pound size.

Local rules and the mistakes that cost a redo

Because the model code leaves fence post depth to local jurisdictions, the last word is whatever your city or county building department publishes, with local minimums in practice running from about 24 to 42 inches. Many places also want utility locates before you dig, so call 811 first; striking a buried line turns a weekend project into an insurance claim. University extension services such as University of Minnesota Extension publish frost depth guidance for their regions if you want a second source for the number your building department gives you.

The repeat offenders, in order of how often they show up:

• Buying the post too short. The 6 foot fence that needs a 9 foot post in frost country is the big one; measure before you load the truck.
• Skipping the frost check. A post set above the frost line heaves, and heaved fence lines never sit straight again.
• Holes too narrow. A 6 inch hole around a 4x4 has almost no concrete collar, so the post rocks no matter how deep it sits.
• No gravel, flat concrete top. Water sits against the post end and the collar, and rot starts at exactly that line.
• Backfilling with loose dirt instead of concrete. Tamped soil works for temporary fence, not for a line you want straight in five years.

None of these are hard to avoid on dig day, and all of them are miserable to fix after the concrete cures. A tape measure, the local frost depth, and one extra bag of mix are the whole checklist.