Fence Post Concrete Calculator
Each fence post sits in a hole about three times its width and at least 60 cm deep, with concrete packed around it. This calculator works out the concrete per hole after deducting the post itself, converts it to bags of post mix or general concrete, and rounds up per post rather than for the whole fence, because each hole is filled in one go.
Your measurements
Check your order
Your fence and post order
- Confirm the fence run, corners, gates and post spacing from the project specification.
- Check hole dimensions and the chosen concrete product’s yield before ordering bags.
- Plan bracing and alignment checks; a quantity estimate does not specify safe footing dimensions.
Tools for this job
Check hole dimensions for the concrete estimate, then brace each post upright and recheck it while placing the concrete.
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- Keep posts upright while fixing
Post level
Johnson 175-O post level
For checking that a post is upright while positioning and bracing it. Recheck alignment after fixing or placing concrete, before it sets.
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Short tape measure
STANLEY 25-foot tape measure
For checking lengths, widths and short runs before ordering. This is a 25-foot tape; use matching units in the calculator and check the scale on the selected model.
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How much concrete do I need per fence post?
A post hole is a cylinder with a post in the middle, so the concrete is the hole volume minus the post volume. The usual proportions are a hole three times the post width and a depth of about a third of the height above ground, never less than 60 cm (2 ft): a 100 mm (4 in) post goes in a 30 cm (12 in) hole, and a 1.8 m (6 ft) fence wants 75 cm (30 in). Doubling the hole diameter roughly quadruples the concrete.
Bags are rounded up per post, not for the whole fence: each hole is filled in one go around a post that is already plumb and braced, and a part bag of fast-setting mix does not keep. The calculator also shows the whole-fence rounding so you can see the margin.
The formula
Concrete and bags per post hole
The bag states its own yield or the hole size it fills; enter that as a custom yield if it differs from the 10 L and 11.7 L used here.
| Post | Hole diameter × depth | Concrete per post | 20 kg post mix (10 L) | 25 kg concrete (11.7 L) |
|---|---|---|---|---|
| 75 mm (3 in) | 23 × 60 cm (9 × 24 in) | 21.6 L | 3 bags | 3 bags |
| 75 mm (3 in) | 25 × 75 cm (10 × 30 in) | 32.6 L | 4 bags | 4 bags |
| 100 mm (4 in) | 30 × 60 cm (12 × 24 in) | 36.4 L | 5 bags | 4 bags |
| 100 mm (4 in) | 30 × 75 cm (12 × 30 in) | 45.5 L | 6 bags | 5 bags |
| 125 mm (5 in) | 38 × 75 cm (15 × 30 in) | 73.3 L | 9 bags | 7 bags |
| 150 mm (6 in) | 45 × 90 cm (18 × 36 in) | 122.9 L | 14 bags | 12 bags |
Worked example
Ten 100 mm (4 in) posts in 30 cm (12 in) holes, 60 cm (24 in) deep, set in 20 kg post mix. Hole volume = π × 0.15² × 0.6 = 0.0424 m³, or 42.4 L. The post takes up 0.1 × 0.1 × 0.6 = 0.006 m³ of that, so the concrete per post is 0.0364 m³, or 36.4 L. Add 10% = 40.1 L. At 10 L per bag that is 4.005, rounded up to 5 bags per post, so 50 bags for the fence, and 0.364 m³ of concrete in total before waste.
The 10% allowance tips this one just over four bags. With no allowance it is 36.4 ÷ 10 = 3.64, so 4 bags per post and 40 in total; rounding the whole fence at once would give 41. An auger gives exact holes, a spade in soft ground does not, so choose the allowance accordingly.
How deep and how wide should the hole be?
Go deeper rather than wider for a stiffer post: depth resists the lever action of the wind, width mostly adds weight. Use the top of each range in clay, on a windy site, or where frost goes deep.
| Fence height | Post length | Hole depth | Hole diameter, 100 mm post |
|---|---|---|---|
| 0.9 m (3 ft) | 1.5 m (5 ft) | 45 to 60 cm | 30 cm (12 in) |
| 1.2 m (4 ft) | 1.8 m (6 ft) | 60 cm (2 ft) | 30 cm (12 in) |
| 1.8 m (6 ft), no gravel board | 2.4 m (8 ft) | 60 cm (2 ft) | 30 cm (12 in) |
| 1.8 m plus gravel board | 2.7 m (9 ft) | 75 cm (30 in) | 30 cm (12 in) |
| 2.0 m, or an exposed site | 2.7 to 3.0 m | 75 to 90 cm | 30 to 35 cm |
Post mix or ordinary concrete?
Post mix is a dry blend of cement and aggregate, poured into the hole around the post and then watered; it stiffens in minutes, so panels can go up the same day, but it usually costs more per litre. Ordinary concrete, from 25 kg bags or mixed at 1:2:4 with the concrete calculator, is cheaper for a long run, but every post must stay braced until it has set, usually overnight.
Either way, a 5 cm (2 in) gravel layer under the post helps the end grain drain; the gravel calculator covers larger quantities and the fence calculator counts posts, panels and rails.
Mistakes that loosen a fence
- Too shallow. 45 cm looks fine on the day and leans in the first winter gale; 60 cm is the minimum, 75 cm for a 1.8 m fence.
- Too narrow, leaving a thin collar of concrete that cracks. Aim for three times the post width, never less than twice.
- Concrete stopped below ground level, making a cup that holds water against the post. Bring it above ground and slope it away.
- Buying the exact bag count. A root or a soft patch takes half a bag more, and one bag short means a second trip.
Sources and assumptions
Concrete volume is the circular hole volume minus the embedded post volume, with a digging allowance. Bags are rounded up for each hole, so the total can exceed rounding once for the whole fence.
- Tarmac Blue Circle: Postcrete: The manufacturer identifies the 20 kg product and directs users to its own no-mixing installation method, with water added to the hole before the product.
Before using this estimate
- The 20 kg and 25 kg yield presets are generic estimates. Enter the actual bag yield when it is available; different fast-setting products use different installation sequences.
- The result does not select a safe hole diameter or embedment depth for soil, frost, wind or gate loads. Confirm those dimensions before calculating bags.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.