Fence Post Concrete Calculator

How to use this calculator

1. Enter the hole diameter. Post hole diggers and augers are sold by diameter, so this is usually 8, 10 or 12 inches.
2. Enter the hole depth, measured from grade to the bottom of the hole.
3. Enter the number of posts to get the whole fence in one go.
4. Enter the post size if you want the post's own volume subtracted. A 4x4 is 3.5 inches actual, a 6x6 is 5.5 inches.
5. Leave 10 percent waste on. Holes are never as neat as the number you typed.

Enter the post size in the square field for timber posts and the round field for pipe or round posts. Leave both blank and you get the full hole volume, which is the safer way to order.

How deep and how wide should a post hole be?

The short version is below. The long answer is its own page, because it turns out there is no building code requirement for fence post depth at all and the four published versions of the one-third rule disagree with each other by 18 inches on the same fence. How deep should a fence post be works through what actually governs it: your local ordinance, your frost line, your soil, and the one published standard in fencing.

Two rules of thumb cover most residential fences, and both come straight from the bag manufacturers.

Depth: about one third of the post's height above ground. So a 6 foot fence on an 8 foot post wants roughly 2 feet in the ground. Diameter: about three times the post width. A 4x4 post takes a hole around 10 to 12 inches across. Sakrete's guide to setting posts and poles gives both rules, along with a 4 to 6 inch gravel layer in the bottom of the hole for drainage.

Frost depth overrides all of this. If your local frost line is deeper than one third of the post height, dig below the frost line instead, or the post will heave. Published frost lines run from 0 inches in low-elevation Arizona to 60 inches in Minnesota, and the model code leaves the figure blank for each jurisdiction to fill in, so it is a local lookup rather than something you can derive. Frost depth and fence posts has the values and the adfreeze mechanism that actually lifts a post.

If you want a single defensible number instead of a ratio, the only published depth standard in fencing is ASTM F567, reproduced by chain link manufacturers as a "Minimum footing depth, 24 in. plus an additional 3 in. for each 1 ft. increase in the fence height over 4 ft." That puts a 6 foot fence at 30 inches, which is the depth the worked examples on this page use.

The 50 lb bag counts are Sakrete's own figures from the Fast-Setting Concrete data sheet, which prints exactly this table for a 10 inch hole. Sakrete's project guide puts a standard 6 foot fence at about two bags of Fast-Setting per hole, which matches the 24 inch row.

The formula

A post hole is a cylinder, so the volume is the area of the circle times the depth. Subtract the post and you get the concrete that actually goes in:

Cubic feet per hole = π × (Diameter ft ÷ 2)² × Depth ft − Post volume

Worked example. A 10 inch hole is 0.833 ft across, so 2 feet deep it holds π × (0.833 ÷ 2)² × 2 = 1.09 cubic feet. A 4x4 post is 3.5 inches actual, so it displaces 3.5 in × 3.5 in × 2 ft = 0.17 cubic feet, leaving 0.92 cubic feet of concrete. Across 12 posts that is 11 cubic feet, about 0.41 cubic yards, which is 21 × 80 lb bags once you add 10 percent waste.

Which concrete for fence posts

Three products get used for this and they are not interchangeable. The difference that matters is not strength, it is whether you have to mix it, because a post has to be held plumb until the concrete grabs.

Fast-setting is the standard answer for posts and it is the one product in the list you do not mix. You pour the dry powder into the hole around the plumbed post, add water on top, and it reaches final set in 20 to 40 minutes. Quikrete's data sheet publishes 0.375 cubic feet per 50 lb bag and a worked example of two bags for a 3 inch square post in a 9 inch hole 30 inches deep, which is the figure the table above is built from.

Standard mix is cheaper per bag and gets to the same 28 day strength, but you mix it, and it stays workable for hours, so you will be babysitting braces most of the day. On a long fence line set in one pass, the time is worth more than the price difference.

Foam is worth a word because it is marketed as a concrete replacement. Sika publishes yield, set time and temperature limits for PostFix and publishes no compressive strength, no density and no pullout figure, with no test method named anywhere in either its US or Canadian data sheet. Its own limitations state the material is not intended for setting deck posts and should not be used for structural posts or posts exposed to a lot of sideways load. Sideways load is what a privacy fence post does for a living. Concrete, gravel or foam goes through the published data on all three.

For the brand-specific detail and the dry-pour method step by step, the Quikrete fence post guide covers the red bag, and the Sakrete mix types guide covers the equivalent. Price the bags with the Quikrete calculator or the Sakrete calculator.

Bags per post, by hole size

This is the table most people want before they get to the store. It is 50 lb fast-setting bags at 0.375 cubic feet each, for a 4x4 post at its actual 3.5 inches, with the post volume subtracted and no waste allowance added. Counts are rounded up to whole bags, because you cannot buy two thirds of one.

Read the pattern rather than just your row, because it decides where to spend your digging. Depth adds bags in a straight line. Diameter adds them with the square. Going from an 8 inch hole to a 12 inch hole at the same depth more than doubles the concrete, while going from 24 to 36 inches deep adds half again. And the resistance you get runs the other way: by the code embedment equation, doubling the hole diameter cuts the required depth by only about 29 percent. Deep and narrow beats wide and shallow, for the same holding power and fewer bags.

For a 6x6 post at 5.5 inches actual, the post itself eats about 2.5 times as much of the hole, so in a 12 inch hole the bag counts drop by roughly one. Enter the post size in the calculator and it does that subtraction for you.

These are higher than the counts on the Quikrete fence post table, and the difference is the assumption rather than the arithmetic. That table allows 6 inches of gravel in the bottom and brings the concrete to 3 inches below grade, so it fills 9 inches less of the hole. Both are correct for what they describe. Pick the one that matches how you are actually setting the post.

Working out the whole fence

One hole is arithmetic. A fence is a post count, and getting the post count wrong is the commonest way people end up short.

How many posts

Spacing is set by your rail length, not by engineering, and USDA NRCS Maryland states the causation outright for a board fence: "Posts shall be spaced a maximum of 8 feet on center to accommodate rail lengths of a maximum of 16 feet." Rails come in 16 foot lengths, so posts go at 8 feet. No building code specifies fence post spacing at all.

Published maximums for other fence types, from the same family of specifications: conventional woven wire 10 feet, barbed wire 16 feet, high tensile smooth wire 16 feet or 30 feet with battens. For chain link, Michigan State's construction standard requires line posts "equally not exceeding 10-foot on center." The 6 foot spacing often recommended for tall privacy fences has no primary source; it appears in trade content as the conservative option.

To count posts for a straight run: divide the length by the spacing, round up, then add one for the far end. A 96 foot run at 8 feet is 12 bays and 13 posts. Then add one post at every corner and one on each side of every gate, because a gate needs a post on both sides and neither of them is shared with a line run.

A worked example

A 6 foot privacy fence, 96 feet in a straight line plus one corner and a single 4 foot gate, on 4x4 posts in 10 inch holes 30 inches deep.

• Line posts: 96 ÷ 8 = 12 bays, plus 1 = 13 posts
• Corner: 1 post
• Gate: 2 posts, one each side
• Total: 16 posts
• Volume per hole at 10 in by 30 in, post subtracted: π × (5 ÷ 12)² × 2.5 = 1.36 cu ft, minus the post at 3.5 in square over 2.5 ft = 0.21 cu ft, so 1.15 cu ft
• Across 16 posts: 18.4 cu ft, which is 0.68 cubic yards
• In 50 lb fast-setting bags at 0.375 cu ft: 18.4 ÷ 0.375 = 50 bags, or 54 with 10 percent waste

Gate and corner posts usually want to be bigger than that, which pushes the total up. USDA NRCS specifications consistently set them deeper: California puts line posts at 24 inches and gate and brace posts at 3 feet, Vermont sets line posts at 30 inches and corner and gate posts at 36. Run those holes through the calculator separately rather than averaging them in.

Buy a spare bag or two whatever the arithmetic says. A hole that caves wider than you dug it, or one bag that splits, will eat a 10 percent cushion on its own.

Bags or ready-mix

Posts are almost always a bagged job, and the reason is the delivery minimum rather than the volume. The worked example above needs 0.68 cubic yards for a whole fence, and a ready-mix truck is not interested in that: the short-load threshold and fee make a sub-yard delivery expensive per yard, and the concrete arrives all at once when you need it hole by hole over a day.

The crossover is somewhere around 2 to 3 cubic yards of post holes, which is 50 or more holes, and at that scale you are usually a contractor with a machine rather than a homeowner with a clamshell digger. How much concrete is in a truck covers the delivery economics, and the bag calculator converts any volume into bag counts at every size.

One practical note on buying. Fast-setting mix is sold in 50 lb bags, where almost everything else on a pallet is 60 or 80 lb. Check the weight on the bag before you divide, because a bag count worked out at 80 lb yields will leave you a third short.

Setting a mailbox post, a deck footing or a sign post? The geometry is identical, and for a tube form use the cylinder and tube calculator instead. A deck footing has a second question the volume does not answer, which is how wide it has to be for the load: the deck footing size calculator handles that off the IRC table. For the gravel in the bottom of each hole, if your soil is one where gravel helps, that calculator does the tonnage.