Work out how deep to bury fence posts, the right hole size and the concrete needed. Enter post size, soil type and frost depth for a full footing estimate.
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Fence posts fail at the base, not the top. A post that sits too shallow in sandy soil will lean within a season. This fence post depth calculator works out the burial depth your posts actually need, given the height above ground, the soil type, and the local frost line. It also sizes the hole, subtracts the post’s own volume, and returns the concrete or gravel needed to fill it.
Pick a post preset or enter your own dimensions. Choose square or round. The tool loads a footing plan with the governing rule shown in plain language, whether that’s the standard third rule, the frost line, or the absolute minimum. Then it prices the posts, the concrete, and the labour per post, so you can see cost per post rather than a lump sum that hides the details. Metric and imperial units convert on the fly.
Pick the unit system: metric (mm / m) or imperial (in / ft).
Choose a post size preset. Custom sizes are allowed.
Enter the height of the post above ground.
Set how many posts are in the run.
Pick the soil type. Sandy soil needs deeper burial than rock.
Enter the frost line depth for your area. Zero if there’s no frost.
Confirm the hole diameter, or leave it in auto mode.
Choose the footing material, then enter your unit costs.
The Summary tab shows burial depth first, with hole size and bags below. Detailed Breakdown lists every intermediate value.
Burial depth comes from three competing requirements. The calculator picks the deepest one, since that’s the rule that governs the job.
Formula: Base burial = Height above ground × 0.5
Formula: Soil-adjusted burial = Base × Soil factor
Formula: Frost requirement = Frost depth + 0.15 m
Formula: Absolute minimum = 0.45 m (rocky) or 0.60 m (other soils)
Formula: Burial depth = max(soil-adjusted, frost requirement, absolute minimum)
Formula: Total post length = Height above ground + Burial depth
That 0.5 multiplier isn’t arbitrary. It buries one third of the total post, which is the classic rule for fence posts. If above-ground height is two thirds of the post, then the buried third is half the above-ground figure. A 1.8 m post above ground buries 0.9 m.
Soil factors scale the base. Normal loam is 1.00. Clay is 1.10. Compacted fill is 1.25. Sandy or loose soil is 1.30, the largest multiplier because it holds the least. Rocky or hardpan is 0.75, which reflects how hard it is to dig and how little the surrounding material can move.
Frost is the rule that catches people by surprise. When frost depth is greater than zero, the calculator adds a 150 mm (about 6 in) margin and treats that as a minimum. In cold climates, this often drives the depth well past the third rule. A 900 mm frost line means the footing needs to sit at 1,050 mm minimum.
The hole diameter auto-sizes at three times the post width, floored at 150 mm. That gives enough room for the fill material to surround the post. You can override it if your auger only comes in specific sizes.
Net fill volume is hole volume minus post volume. The tool computes each separately with the right geometry, then subtracts. The result is the actual material you’d buy, not the total hole volume that includes the post itself.
Bags come from net fill divided by bag yield. Yields are entered in cubic metres (metric) or cubic feet (imperial), based on standard bag sizes. Round up to whole bags.
Say you’re setting ten 100 mm square posts, each 1.8 m above ground, in normal loam with a 600 mm frost line. Hole diameter is auto. Footing is 25 kg concrete bags. Costs: $4 per metre of post, $8 per bag, $5 per post labour.
Base burial: 1.8 × 0.5 = 0.900 m
Soil factor (normal): 1.00
Soil-adjusted burial: 0.900 × 1.00 = 0.900 m
Frost requirement: 0.600 + 0.150 = 0.750 m
Absolute minimum: 0.600 m
Burial depth: max(0.900, 0.750, 0.600) = 0.900 m (governed by the 1/3 rule)
Post length each: 1.8 + 0.9 = 2.700 m
Total post length: 27.00 m
Hole diameter (auto): 0.100 × 3 = 0.300 m, above the 150 mm floor
Hole volume each: π/4 × 0.300² × 0.900 = 0.0636 m³
Post volume in ground: 0.100 × 0.100 × 0.900 = 0.0090 m³
Net fill per hole: 0.0636 − 0.0090 = 0.0546 m³
Total fill: 0.0546 × 10 = 0.546 m³
Bags per hole: ⌈0.0546 ÷ 0.0117⌉ = 5 bags
Total bags: 50
Post cost: 27.00 × $4 = $108
Concrete cost: 50 × $8 = $400
Labour cost: 10 × $5 = $50
Total: $558
Cost per post: $55.80
Five bags per hole is on the high side for a post that size. That’s because 25 kg bags yield less than 80 lb ones, and the hole is 300 mm across. Drop the hole to 250 mm manually and the concrete drops to roughly four bags per hole.
Bury one third of the post’s total length. A post standing 1.8 m above ground needs 0.9 m in the ground. Frost line and soil type can push that deeper.
Below the frost line, plus a 150 mm safety margin. A 600 mm frost line needs a footing at 750 mm minimum. The calculator adds that margin automatically when frost depth is entered.
A 100 mm post in a 300 mm hole, 900 mm deep, needs about 0.055 m³, roughly 5 bags of 25 kg mix. Larger posts need less concrete because the post displaces more of the hole.
Three times the post width is the standard. A 100 mm post gets a 300 mm hole. A 4-inch post gets a 12-inch hole. Larger holes offer more lateral stability but use more concrete.
Yes. Sandy soil multiplies the base burial depth by 1.30, so a post that needs 0.9 m in loam needs about 1.17 m in sand. Loose soil holds less, and depth compensates.
Yes, in well-drained soil with light loads. Gravel backfill drains better and is easier to remove if the post needs replacing. Concrete provides more lateral stability and is standard for privacy fences.
Local building departments publish frost depth maps. So do many extension offices. Enter the figure in metres or inches and the calculator handles the margin.
0.60 m for normal soils and 0.45 m for rocky ground. The calculator uses those floors even when the third rule would produce a shallower figure.
Concrete mix is the standard for wood and metal fences carrying any load. Fast-set post concrete cures faster. Gravel works for short fences in draining soil. Native soil works only for temporary fences.
Bury them below the frost line with at least 150 mm of margin. The calculator applies that margin automatically when you enter a frost depth. Skipping it is the main reason posts lean after a few winters.
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