Fence Post Depth Calculator
Work out how deep to set a fence post and how much concrete each hole needs. Enter the above-ground height, post size, depth rule, frost line, gravel base and hole diameter, and get the hole depth, embedment, total post length, concrete per hole and bags to buy, with an optional cost — plus by-height and by-diameter tables and a downloadable PDF.
| Height | Rule depth | Hole depth | Total post |
|---|---|---|---|
| Enter a post to compare | |||
| Diameter | Concrete / hole | Bags / hole | Total bags |
|---|---|---|---|
| Enter a post to compare | |||
How to use the fence post depth calculator
Four quick steps turn your fence height, frost line and hole size into hole depth, post length and the concrete each hole needs — with by-height and by-diameter tables, a chart and a downloadable report.
Enter height & posts
Give the above-ground post height in inches and how many posts you’re setting.
Pick post size & rule
Choose a 4×4, 6×6 or round post and bury 1/3 of the height, or 1/2 for tall or windy fences.
Set frost, gravel & hole
Enter your frost line depth, gravel base, hole diameter, bag size and an optional price per bag.
Read results & save a PDF
Get hole depth, post length, concrete and bags per hole and in total — then download a report.
How fence post depth is calculated
A post needs enough of its length in the ground to resist leaning, and its footing must sit below the frost line so winter heave can’t lift it. Here is the exact math the calculator runs.
Worked example — 6 ft (72″) fence, 4×4 post, 1/3 rule, 36″ frost line, 4″ gravel, 12″ hole, 50 lb bags
Rule depth: 72 ÷ 3 = 24" · Embedment: deeper of 24" and 36" frost = 36"
Hole depth: 36 + 4 = 40" · Post length: 72 + 36 = 108" = 9 ft
Concrete: (113.1 − 12.25) × 36 ÷ 1,728 ≈ 2.10 cu ft · Bags: 2.10 ÷ 0.375 = 5.6 → 6 bags
≈ dig 40″ deep × 12″ wide, set a 9 ft post, and use 6 bags (50 lb) of concrete per hole.
Fence post depth reference charts
Handy lookups for hole depth by fence height, concrete by hole diameter, and bags by bag size.
| Fence height | Rule depth | Hole depth — no frost | Hole depth — 36″ frost |
|---|---|---|---|
| 3 ft | 12″ | 16″ · 4.0 ft post | 40″ · 6.0 ft post |
| 4 ft | 16″ | 20″ · 5.3 ft post | 40″ · 7.0 ft post |
| 5 ft | 20″ | 24″ · 6.7 ft post | 40″ · 8.0 ft post |
| 6 ft | 24″ | 28″ · 8.0 ft post | 40″ · 9.0 ft post |
| 8 ft | 32″ | 36″ · 10.7 ft post | 40″ · 11.0 ft post |
In cold climates the frost line usually sets the depth; in warm climates the 1/3 rule does.
| Hole diameter | Concrete / hole | Bags / hole | Bags for 20 posts |
|---|---|---|---|
| 8″ | 0.79 ft³ | 3 | 60 |
| 10″ | 1.38 ft³ | 4 | 80 |
| 12″ | 2.10 ft³ | 6 | 120 |
| 14″ | 2.95 ft³ | 8 | 160 |
| 16″ | 3.93 ft³ | 11 | 220 |
About 3× the post width is a good hole size — 10–12″ for a 4×4. Wider holes resist leaning better but use much more concrete.
| Bag size | Yield per bag | Bags / hole | Bags for 20 posts |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 8 | 160 |
| 50 lb | 0.375 ft³ | 6 | 120 |
| 60 lb | 0.45 ft³ | 5 | 100 |
| 80 lb | 0.60 ft³ | 4 | 80 |
Bags per hole are rounded up. Bag yields vary by brand — check the label.
What the fence post depth calculator gives you
Fence height, frost line and hole size give you a complete dig-and-set plan for every post.
Post setting at a glance
Who uses a fence post depth calculator
Whether you’re setting one mailbox post or a whole fence line, the same calculator sizes each hole.
Setting posts for a privacy or picket fence and needs the hole depth, post length and concrete to buy.
- Enter fence height
- Check your frost line
- Total bags for all posts
Tired of posts heaving every winter and wants footings that sit below the frost line.
- Enter local frost depth
- Add a gravel base
- Buy longer posts
Quoting a fence job and wants fast concrete totals and post lengths for the materials order.
- Use ½ rule for gates
- Compare hole sizes
- Share the PDF
Tips for setting fence posts
A post set right stays straight for decades — these habits help.
Fence post depth calculator FAQ
Quick answers about how deep to set fence posts, frost lines, hole size, post length and concrete.
A common rule is to bury about 1/3 of the above-ground height, or 1/2 for a tall, heavy or windy fence. A 6 ft fence needs at least 24 inches in the ground.
In cold climates the post must also go below the frost line, so use whichever is deeper, then add a gravel base.
With a 36 inch frost line, the post goes 36 inches below grade — deeper than the 24 inch rule. Add 4 inches of gravel and the hole is 40 inches deep.
The post is 72 + 36 = 108 inches, or 9 ft long, so buy a 10 ft post and trim it.
When the ground freezes it expands and can push posts up — called frost heave. Footings set below the frost line stay put.
Frost depth is 0 in warm climates and often 36 to 48 inches in cold regions. Check your local building code.
A good rule is about 3 times the post width. For a 4×4 (3.5 inches actual), that is roughly 10 to 12 inches.
Wider holes hold more concrete and resist leaning better, but use more: a 16 inch hole needs nearly twice the concrete of a 12 inch hole.
Subtract the post area from the hole area and multiply by the embedment depth. A 12 inch hole with a 4×4 post set 36 inches deep needs about 2.10 cu ft.
That is 6 bags of 50 lb concrete per hole, or 4 bags of 80 lb.
Multiply the bags per hole by the number of posts. At 6 bags of 50 lb per hole, 20 posts need 120 bags.
With 80 lb bags it is 4 per hole, or 80 bags. At $6 per 50 lb bag, 120 bags cost about $720.
Add the above-ground height and the embedment depth. A 6 ft fence with 36 inches in the ground needs a 9 ft post.
In a warm climate with no frost line, the same fence needs 24 inches in the ground and an 8 ft post.
Yes. About 4 to 6 inches of gravel under the post lets water drain away from the end grain, which helps wood posts last longer.
The calculator adds the gravel to the hole depth but does not fill it with concrete.
Planning guide only — check your local code. This calculator works in US units (inches, feet) and buries about 1/3 of the above-ground height (1/2 for tall, heavy or windy fences), always below the frost line, plus a gravel base. Soil type, slope, gate and corner posts, and wind load may call for deeper or wider footings, and bag yields vary by brand. Always call 811 to locate utilities before digging, and follow your local building code and permit rules.