Fence Post Concrete Calculator
Work out how much concrete your fence-post holes need — and how many bags to buy — accounting for post displacement and a gravel base.
By Berjin B V
Published Updated
- Calculated posts
- —
- Concrete depth (per hole)
- —
- Hole volume (per hole)
- —
- Post displacement (per hole)
- —
- Net concrete (per hole)
- —
- Total concrete
- —
- Equivalent bags
- —
- Approx. bags per hole
- —
- Recommended purchase
- —
- Total bag weight
- —
Request a Free Fencing Quote
Tell us about your fencing project and location. We'll prepare a quote and send it to you when it's ready — Buildmator prepares these manually, and there's no obligation.
Request a Free Fencing QuoteNot a structural specification
Work out exactly how much post mix your fence-post holes need — as a precise volume in litres, an “equivalent bags” figure, and a whole-bag shopping list that won’t leave you short. This is a material quantity calculator: it doesn’t price the mix, size your holes, or check that a fence design is structurally sound.
How to use this calculator
- Measurement units — metres / millimetres or feet / inches. Switch any time; your numbers convert automatically.
- Post count — enter it directly, or choose Calculate from fence length and spacing and give the run length and target spacing; the post count updates live.
- Hole shape and size — round (diameter) or square (width), plus total hole depth and an optional gravel/hardcore base depth.
- Post shape and size — round (diameter) or square (width). The tool checks the post fits the hole, including the diagonal of a square post in a round hole.
- Bag size — defaults to a 20 kg post-mix bag; pick 25 kg to refill the yield, or choose Custom and enter your product’s stated figure.
Press Calculate for the full breakdown.
The formula
For each hole, the mix you need is the hole volume minus the buried post’s volume minus any gravel base:
- Ahole
- cross-sectional area of the hole
- Apost
- cross-sectional area of the buried post
- dfill
- depth actually filled with mix
- dhole
- total hole depth
- dgravel
- gravel/hardcore base depth (0 if none)
Cross-sectional area depends on shape:
- w
- diameter (round) or side width (square)
The whole run is the per-hole figure times the number of posts, and the exact bag count is that total over one bag’s yield:
The calculator’s Recommended purchase uses the first method. If you’re pouring a bag at a time per hole with no shared mixing, use the second:
- Mixed and poured together:
⌈ bagsequivalent⌉— round the run total up once. Leftover mix from one hole covers the next, so this is the smaller, realistic number. - Whole bags per hole:
⌈ Vmix÷ Ybag⌉ × nposts— round each hole up first. Safer with no shared mixing, at the cost of some spare material.
Fence run to post count
Rounding up keeps the last panel gap no wider than your target spacing; the + 1 puts a post at each end.
Worked example — round hole, 75 mm post
Ten 1.83 m fence panels make an 18.3 m run, so 11 posts at 1.83 m spacing. Holes are 250 mm diameter, 600 mm deep, with a 50 mm gravel base; posts are 75 mm square. Using 20 kg post-mix bags at ~8.5 litres:
| Step | Value |
|---|---|
Posts: ceil(18.3 / 1.83) + 1 |
11 |
Fill depth: 600 − 50 |
550 mm = 0.55 m |
Hole area: π × (0.125)² |
0.0491 m² |
Post area: (0.075)² |
0.0056 m² |
Mix per hole: (0.0491 − 0.0056) × 0.55 |
≈ 0.024 m³ = 24 L |
Run total: 24 × 11 |
≈ 263 L |
Equivalent bags: 263 / 8.5 |
≈ 30.9 |
Buy (mixed together): ceil(30.9) |
31 bags |
Buy (per hole): ceil(24 / 8.5) × 11 = 3 × 11 |
33 bags |
| Total dry mix (31 × 20 kg) | 620 kg |
Worked example — square hole, round post
A 12 m run, posts every 2 m, 200 mm square holes 700 mm deep, no gravel, 50 mm diameter round steel posts. Using 25 kg bags at ~12 litres:
| Step | Value |
|---|---|
Posts: ceil(12 / 2) + 1 |
7 |
Hole area: (0.2)² |
0.04 m² |
Post area: π × (0.025)² |
0.00196 m² |
Mix per hole: (0.04 − 0.00196) × 0.7 |
≈ 0.0266 m³ = 26.6 L |
Run total: 26.6 × 7 |
≈ 186 L |
Equivalent bags: 186 / 12 |
≈ 15.5 |
| Buy (mixed together) | 16 bags |
Common UK bag sizes and yields
Yields are approximate. Tarmac Blue Circle’s Postcrete product information, checked September 1, 2026, illustrates why post-setting products need their own coverage figure. Manufacturers vary, so check the yield printed on the bag you buy.
| Bag | Approx. yield | Bags per 100 L |
|---|---|---|
| 20 kg post mix | ~8.5 L (8–9 L) | ~12 |
| 25 kg concrete mix | ~12 L | ~8.5 |
Typical hole and post dimensions
Common starting points, not a structural specification — fence height, wind exposure, and soil all matter. Check manufacturer or trade guidance.
| Fence | Post | Hole diameter | Hole depth |
|---|---|---|---|
| 0.9 m picket | 75 mm | 200 mm | 450 mm |
| 1.8 m panel fence | 75–100 mm | 250 mm | 600 mm |
| 1.8 m exposed / windy | 100 mm | 300 mm | 600–750 mm |
| 2.4 m close-board | 100–125 mm | 300 mm | 750 mm+ |
Rule of thumb: hole diameter ≈ 3 × post width; hole depth ≈ a third of the post’s total length.
Unit conversions used
Why subtract the post?
A fence post needs the gap around it filled, not a solid plug. Ignoring the post’s own volume overstates the mix by the size of the post — for a 75 mm post in a 250 mm hole, about 6%; for a 125 mm post in a 300 mm hole, closer to 17%.
Why a gravel base helps
- Keeps the end grain of a timber post out of standing water, slowing rot from below.
- Gives water a path away from the post.
- Slightly reduces the mix volume, since gravel fills depth that would otherwise be concrete.
Optional — many installers skip it in free-draining ground. Set the field to 0 if you’re not using one.
Common mistakes this calculator avoids
- Pricing the full hole volume instead of the mix around the post.
- Assuming yield from bag weight rather than the product’s stated figure.
- Rounding equivalent bags down — 2.8 bags is not 2. The tool shows the true figure and a separate whole-bag purchase count.
- Forgetting the end posts —
posts = spans + 1.
Worked planning scenario
For a 20 m garden fence with posts no more than 1.8 m apart, ceil(20 ÷ 1.8) + 1 gives 13 posts, not 12. With 100 mm square posts in 250 mm round holes 600 mm deep and no gravel base, each hole needs about 23.5 L of mix. Using the calculator’s approximate 8.5 L yield for a 20 kg bag, the whole run is about 35.9 equivalent bags: 36 bags when mixed across the project or 39 bags when each hole is rounded to three bags. Check the yield on the post-mix you buy, since that figure can change the bag count.
Methodology
This tool computes material quantity only — the geometric volume of mix implied by your dimensions, as an exact figure and a whole-bag purchase count rounded up so rounding can’t leave you short. It does not assess structural adequacy, embedment depth, soil bearing, or Building Regulations compliance; those depend on your site. Choose or confirm your hole dimensions first, then use this calculator for materials.
Want the background rather than a specific calculation? Read our guide to how much concrete a fence post needs — the geometry, a bag-yield table, and worked examples for 1, 10, and 20 posts.
FAQs
Is a postcrete calculator the same as this fence post concrete calculator?
"Postcrete" is the common name for fast-setting post-mix concrete — originally a product name that has become a general term. Whether you call it a postcrete calculator, a fence post concrete calculator, or a post hole concrete calculator, the job is identical: work out the volume of mix in the gap around the post and turn it into a bag count.
How many bags of postcrete per fence post?
For a typical 250 mm diameter, 600 mm deep hole (50 mm gravel base) with a 75 mm post, one hole needs about 24 litres of mix — roughly 2.8 of a 20 kg bag at ~8.5 litres, so you buy 3 bags per hole if you pour holes separately. Across a full run the shared-mix total is lower, since a part-used bag from one hole covers the next.
How much postcrete for a concrete-in fence post?
One 20 kg bag of post mix sets roughly 8–9 litres of concrete. A standard garden-fence hole (250 mm × 600 mm around a 75 mm post) needs about two to three bags; a taller or exposed fence with a wider, deeper hole needs more. The calculator gives the exact figure for your dimensions.
Do I need hardcore or gravel under a fence post?
Many UK installers put 50 mm of gravel or hardcore at the bottom of the hole so the end of a timber post isn't standing in water. It's optional in the calculator — set it to 0 if you're not using one.
How deep should a fence post hole be in the UK?
A commonly cited minimum is around 600 mm for a standard 1.8 m garden fence, going deeper for taller or exposed fences. Roughly a third of the post's total length is a useful rule of thumb. This calculator doesn't set depth for you — check manufacturer guidance or a local tradesperson, then use the tool for materials.
Fast-setting post mix or standard concrete?
Fast-setting post mix is poured dry and soaked, letting you brace and move on the same day — quicker for a handful of posts. Standard concrete is mixed first and is more economical for a long run. Enter whichever product's stated yield you're using.