Group 1
Advanced options
Holes go 6 in below this. Ask your building department.
Drainage layer under the post. 4–6 in typical.
Sandy soil adds 20% to depth.
Use 15% for holes bored in loose sand.
This post hole calculator does two jobs: it tells you how deep and wide the holes should be for your fence height, post size and frost depth — then works out the concrete and gravel to fill them. It subtracts the post volume and the gravel base, which most calculators don’t.
The short answer
Bury one third of the total post length, with a 24 inch minimum, and go at least 6 inches below the frost line if your ground freezes. For a 6 ft fence that’s a 36 inch deep hole using a 9 ft post.
Hole diameter is three times the post width — a 4×4 post takes a 12 inch hole. That works out to about 3 bags of 80 lb concrete per post once you allow for a 6 inch gravel base.
On this page
How to use this post hole calculator
Pick what you’re setting, enter three things, and the post hole calculator recommends the hole size and returns the concrete needed. Four modes:
- Fence posts — enter fence height, post size and count; depth and diameter are calculated for you
- Gate / corner — the same, with the deeper spec these posts need
- Deck footings — you specify diameter and depth from your plan
- Custom hole — any hole you’ve already sized
Most fence jobs have both line posts and heavier gate or corner posts. Use Add another post group to enter each set separately, and the calculator totals the concrete into one order.
Two settings under Advanced options change the answer significantly: your local frost depth, and the gravel base depth. Set the frost figure if your ground freezes — it usually overrides everything else.
How deep should a post hole be?
Three rules apply at once, and the deepest one wins.
1. One third of the total post length
2. 24 inches minimum
3. Frost line depth + 6 inches
The one-third rule is the baseline. Note that it applies to the total post length, not the fence height — which means the buried depth works out to half the above-ground height. A 6 ft fence needs 36 inches in the ground, giving a 9 ft post with 36 of 108 inches buried. That’s exactly one third.
Worked example — 6 ft privacy fence, 4×4 posts
Dig 6 inches deeper than the post depth so the gravel base doesn’t reduce how far the post actually sits.
| Fence height | Buried depth | Post length | Dig depth with gravel |
|---|---|---|---|
| 3 ft | 24 in (minimum) | 5 ft | 30 in |
| 4 ft | 24 in | 6 ft | 30 in |
| 5 ft | 30 in | 7 ft 6 in | 36 in |
| 6 ft | 36 in | 9 ft | 42 in |
| 8 ft | 48 in | 12 ft | 54 in |
At 4 ft and below the one-third rule gives less than 24 inches, so the minimum takes over. Above 6 ft, check local code — many jurisdictions require a permit for tall fences and may specify their own depth.
Frost line depth overrides everything
If your ground freezes, this is the rule that matters. Water in soil expands as it freezes, and if the bottom of your post sits within that freezing zone, the ground lifts it. Thaw, refreeze, repeat — the post ratchets upward a little each winter until the fence is visibly crooked. That’s frost heave, and no amount of concrete prevents it if the footing is too shallow.
The fix is to place the bottom of the hole below the depth at which soil freezes, plus about 6 inches of margin. Frost depth varies enormously — effectively zero across the southern US, roughly 12–24 inches through the mid-Atlantic and lower Midwest, 36–48 inches across the northern tier, and deeper again in parts of Canada and mountain regions.
Frost depth is set locally and published by your municipal building department or code office. It is not something to estimate from a map or a national average — two neighbouring counties can differ. It’s a free phone call, it takes five minutes, and it’s the difference between a fence that stands for twenty years and one that leans after two winters.
How wide should a post hole be?
Hole diameter = 3 × post width
4×4 post (3½ in actual) → 10½ in, dig 12 in
6×6 post (5½ in actual) → 16½ in, dig 18 in
2⅜ in round steel → 8 in
Three times is the common standard. Some fence systems specify four times for extra holding power, and some sources allow two times where the post is set in compacted gravel rather than concrete. Wider gives the concrete more mass to resist side loads; narrower saves concrete but reduces anchorage.
Note that nominal lumber sizes aren’t actual sizes. A “4×4” measures 3½ × 3½ inches and a “6×6” measures 5½ × 5½. The calculator uses actual dimensions, which is why it recommends a 12 inch hole rather than 16.
How much concrete per post?
Concrete volume is the hole volume minus two things most calculators ignore: the gravel base at the bottom, and the post itself.
Volume = π × (Diameter ÷ 2)² × (Depth − gravel) − post volume
12 in hole, 36 in deep, 6 in gravel, 4×4 post
Hole: 3.1416 × 0.5² × 2.5 = 1.96 ft³
Post: 0.29² × 2.5 = 0.21 ft³
= 1.75 ft³, about 2.9 × 80 lb bags
The post displaces about 11% of the concrete in a 4×4 in a 12 inch hole, and considerably more for a 6×6. On a twelve-post fence that’s roughly four bags you’d otherwise buy and not need.
| Hole size | Concrete depth | ft³ per hole | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 8 in ⌀ × 24 in | 18 in | 0.40 | 0.7 | 0.9 |
| 10 in ⌀ × 30 in | 24 in | 0.92 | 1.5 | 2.0 |
| 12 in ⌀ × 36 in | 30 in | 1.75 | 2.9 | 3.9 |
| 12 in ⌀ × 42 in | 36 in | 2.10 | 3.5 | 4.7 |
| 12 in ⌀ × 48 in | 42 in | 2.45 | 4.1 | 5.4 |
| 18 in ⌀ × 42 in | 36 in | 4.67 | 7.8 | 10.4 |
Figures assume a 6 inch gravel base and a 4×4 post (5½ in for the 18 inch hole). Round up when buying — you cannot pour most of a post.
Why the gravel base matters
Four to six inches of compacted gravel under the post does one job: it stops the post standing in water. Concrete around a post is not waterproof, and water that reaches the bottom of the hole has nowhere to go if it sits in a sealed concrete cup. Wood rots from the base up, and steel corrodes at the same spot.
Use angular crushed stone rather than pea gravel — the angular faces lock together and stay put. Compact it before setting the post, and dig your hole that much deeper so the gravel doesn’t eat into your post depth.
Two details that decide whether a wood post lasts. Slope the top of the concrete away from the post so rainwater sheds outward instead of pooling against the wood. And don’t cap the post base in concrete at the bottom — the gravel layer is what lets water drain out. A post entombed in concrete top and bottom rots from the inside of the hole.
Quick reference chart
| Application | Post | Hole ⌀ | Depth | 80 lb bags each |
|---|---|---|---|---|
| 4 ft picket fence | 4×4 | 12 in | 24 in | 1.8 |
| 6 ft privacy fence | 4×4 | 12 in | 36 in | 2.9 |
| 6 ft fence, gate post | 6×6 | 18 in | 44 in | 8.2 |
| Chain link line post | 2⅜ in | 8 in | 30 in | 1.1 |
| Chain link terminal post | 2⅞ in | 10 in | 36 in | 2.1 |
| Mailbox post | 4×4 | 10 in | 24 in | 1.2 |
| Deck footing | 6×6 | 12 in | Below frost | 3.5+ |
Adjusting for soil type
| Soil | Depth adjustment | What to watch |
|---|---|---|
| Clay | Standard | Excellent lateral support once set, but holds water against the post — use a deeper gravel base |
| Loam | Standard | The baseline case |
| Sandy | +20% | Poor lateral support and holes belly out below grade — allow 15% extra concrete |
| Rocky | Standard | Hard digging but excellent support once in |
Sandy soil is the difficult case. It offers the least resistance to a post being pushed sideways, and bored holes tend to collapse and widen below the surface where you can’t see it. Both the depth and the concrete estimate need padding.
Gate and corner posts need more
Line posts carry only wind load. Gate posts carry the cantilevered weight of a gate that swings, slams and gets leaned on, and corner posts take tension from two directions at once. Both need upgrading:
- Go deeper — commonly half the above-ground height rather than a third of the post length, so around 44 inches for a 6 ft fence.
- Go bigger — step up to a 6×6 where line posts are 4×4, or a 2⅞ inch terminal post in chain link.
- Always concrete — line posts can sometimes be set in compacted gravel; gate and corner posts should not be.
A sagging gate is the most common fence complaint, and it’s nearly always the post, not the hinges.
Setting the posts
- Call before you dig. In the US, dial 811 a few days ahead for free utility locating. Hitting a gas or fibre line is expensive and dangerous.
- Dig deeper than the post depth by the thickness of your gravel base.
- Add and compact 4–6 inches of angular gravel.
- Set the post and brace it plumb in both directions before any concrete goes in.
- Pour concrete to just below grade, then crown it so water sheds away from the post.
- Recheck plumb — concrete shifts posts as it’s placed, and you have a few minutes to correct it.
- Wait before hanging anything. Standard mix takes 24–48 hours before it will take load, and reaches full strength at 28 days. Fast-setting mix is usable in hours.
Products designed for posts are poured into the hole dry and then have water added on top — no mixing. Follow the bag, because the water quantity matters and the working time is short. Standard concrete mix must be mixed properly before it goes in; pouring standard mix dry into a hole gives you weak, patchy concrete.
Post hole mistakes to avoid
- Ignoring frost depth. The single most common cause of a leaning fence in cold regions.
- Confusing fence height with post length. The one-third rule applies to total post length.
- Using nominal lumber sizes. A 4×4 is 3½ inches, so 3× gives 10½, not 12.
- Skipping the gravel base and leaving the post standing in water.
- Treating gate posts like line posts. They carry a different load entirely.
- Flat or dished concrete tops that pool water against the post.
- Not calling 811 before digging.
Frequently asked questions
How deep should a fence post be?
Bury one third of the total post length, with a 24 inch minimum, and go at least 6 inches below the local frost line if your ground freezes. For a 6 ft fence that’s 36 inches buried using a 9 ft post. Dig 6 inches deeper still to allow for a gravel base.
How wide should a post hole be?
Three times the post width. A 4×4 post is actually 3½ inches, so the hole should be about 10½ inches — dig 12. A 6×6 needs 18 inches. Some fence systems specify four times the post width for extra holding power.
How many bags of concrete per fence post?
About 3 bags of 80 lb mix for a standard 12 inch diameter hole at 36 inches deep with a 6 inch gravel base and a 4×4 post — that’s 1.75 cubic feet of concrete. Shallower 24 inch holes take under 2 bags; deep frost-line holes at 48 inches take 4 or more.
How deep should a post be for a 6 foot fence?
36 inches, using a 9 ft post so exactly one third is buried. Dig 42 inches to allow for a 6 inch gravel base underneath. If your frost line is deeper than 30 inches, go 6 inches below it instead.
Do fence posts need gravel at the bottom?
Yes — 4 to 6 inches of compacted angular crushed stone. Without it the post sits in a sealed concrete cup where water collects, and wood rots or steel corrodes from the base up. Use angular stone rather than pea gravel so it locks together and stays put.
What is frost heave and how deep do I need to go?
Water in soil expands when it freezes, lifting anything whose footing sits within the freezing zone. Repeated freeze-thaw cycles ratchet posts upward until the fence leans. Set the bottom of the hole at least 6 inches below your local frost line — that ranges from effectively zero in the southern US to 48 inches or more in northern regions. Your building department publishes the figure.
Should gate posts be deeper than line posts?
Yes. Gate posts carry the cantilevered weight of a swinging gate and corner posts take tension from two directions. Set them around half the above-ground height rather than a third of the post length, step up a post size, and always use concrete rather than gravel backfill.
Can I set fence posts without concrete?
Line posts can be set in compacted angular gravel, which drains better and makes future replacement far easier. It needs a wider hole and thorough compaction in lifts. Gate posts, corner posts and anything in sandy soil or high wind should be concreted.
How long before concrete post footings set?
Standard mix needs 24 to 48 hours before you attach rails or panels, and reaches full strength at 28 days. Fast-setting mixes designed for posts are usable within a few hours. Don’t hang a gate on green concrete.
Should concrete be above or below ground level?
Bring it to just below grade and crown the top so it slopes away from the post, shedding water outward. A flat or dished top holds water against the wood, which is exactly what causes rot at the base.
How far apart should fence posts be?
6 to 8 feet on center for most residential fences, though the correct spacing depends on the panel or rail system you’re using. Pre-built panels dictate spacing exactly. Closer spacing gives a stiffer fence in windy locations.
Can I use this post hole calculator in metric?
Yes. Switch the header toggle to m / mm and enter dimensions in metres and millimetres. Results show cubic feet, cubic yards and bag counts for both US and metric bag sizes.
Sources and method
- Depth and diameter rules compiled from published fence contractor and treated lumber manufacturer guidance; the one-third rule and 3× diameter rule are consistent across sources
- Frost depth requirements are set by local building codes and vary by jurisdiction — always confirm locally
- Bag yields from published concrete mix manufacturer data
- Utility locating: 811 in the United States, before any digging
Related calculators
How this post hole calculator works out its numbers
Recommended depth takes the largest of three values: one third of the total post length, a 24 inch minimum, and frost depth plus 6 inches. Because buried depth forms part of the total post length, the one-third rule resolves to half the above-ground height. Sandy soil adds 20%, and gate or corner posts use 60% of the above-ground height instead. Recommended diameter is three times the post’s actual width, rounded up to the nearest 2 inches — actual dimensions, so a 4×4 is treated as 3½ inches.
Concrete volume is the cylinder above the gravel base, minus the volume of the post occupying that space. Square posts are subtracted as squares, round posts as circles. Bag counts use published yields of 0.60 ft³ for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb.
This calculator produces quantities and general guidance, not a structural design. Frost depth, minimum footing dimensions and permit requirements are set by your local building code and always take precedence over any rule of thumb on this page. Deck footings carrying structural load should be sized to your local code or an engineer’s design, not to a fence rule.