Fence Post Concrete Calculator
Find how many bags of post mix or concrete each fence post needs.
Tell us about your fence
Your answer
That is about 1.2 bags per post, for 10 posts.
This is a guide. For gates or posts that carry weight, check your local building rules or ask a builder or engineer.
What it costs
With VAT, Oct 2026. Post mix: £7.50, from only 2 stores. Concrete: £11, the middle of 4 prices from 3 stores.
Quick answers
- A 75 mm square post, 1.83 m (6 ft) fence
- About 1.2 bags of post mix per post. Dig 229 mm wide and 635 mm deep.
- 10 posts like that
- 13 bags of 20 kg post mix, about £98 with VAT.
- Hole size
- 3 times as wide as the post, says Blue Circle. Jacksons Fencing digs 600 mm deep for a standard fence.
- What one bag fills
- About 17.6 litres for 20 kg post mix, from Blue Circle's own table. A 20 kg bag of concrete makes 10 litres.
How to work it out
- Dig the hole 3 times as wide as the post.
- Make it as deep as ⅓ of the fence height, plus about 25 mm for gravel.
- Work out the hole area, and take away the area of the post.
- Multiply by the depth of concrete for the volume of each hole.
- Divide by what one bag fills: about 17.6 litres for post mix.
Hole: 3.142 × 11.45 × 11.45 cm = 412 cm²
Minus the post: 412 − 56 = 356 cm²
× 61 cm deep ÷ 1,000 = 21.7 litres
21.7 ÷ 17.6 = 1.2 bags per post
How much Postcrete per fence post: bags per hole
Blue Circle's data sheet says how many 20 kg bags of Postcrete fill each hole. Find your hole width and depth, then your post.
| Hole (wide × deep) | 50 mm round | 75 mm square | 100 mm square |
|---|---|---|---|
| 15 × 30 cm | ¼ | – | – |
| 15 × 45 cm | ⅓ | ⅓ | – |
| 15 × 60 cm | ½ | ½ | – |
| 15 × 75 cm | – | ¾ | – |
| 20 × 30 cm | ½ | – | – |
| 20 × 45 cm | ¾ | ¾ | ⅔ |
| 20 × 60 cm | 1 | ¾ | ¾ |
| 20 × 75 cm | – | 1 | 1 |
| 25 × 45 cm | – | 1⅓ | 1¼ |
| 25 × 60 cm | – | 1½ | 1⅓ |
| 25 × 75 cm | – | 1⅔ | 1⅔ |
| 30 × 45 cm | – | 2 | 1¾ |
| 30 × 60 cm | – | 2 | 2 |
| 30 × 75 cm | – | 2¾ | 2½ |
A dash means the sheet gives no figure for that post.
Blue Circle gives no volume for a bag. From its table, one bag fills 12 to 21 litres of hole. We use the middle figure, about 17.6 litres.
How deep and wide to dig a fence post hole
- Jacksons Fencing: 600 mm deep as standard. The hole is about 3 times the post's width: 230 mm for a 3 in post.
- Wickes: 450–600 mm deep and 300 mm wide.
- B&Q: 450 mm is usually enough. Dig deeper in soft ground.
- Blue Circle: as deep as ⅓ of the fence height above ground.
Buy posts at least 600 mm longer than the fence is high, say Jacksons and B&Q.
| Fence | Hole (⅓ rule) | Post (+600 mm) |
|---|---|---|
| 4 ft (1.22 m) | 406 mm | 1,819 mm (6 ft) |
| 5 ft (1.52 m) | 508 mm | 2,124 mm (7 ft) |
| 6 ft (1.83 m) | 610 mm | 2,429 mm (8 ft) |
Add about 25 mm to the hole for gravel. Feet worked out in metres.
Post mix or concrete for fence posts?
Post mix needs no mixing. Blue Circle says fill the hole ⅓ with water, then pour in the dry mix. It sets in 5–10 minutes.
Blue Circle says Postcrete is not for general concreting. For pads or bases, use ordinary concrete.
| Bag | Fills | Price | Bags | Per post |
|---|---|---|---|---|
| Post mix, 20 kg | 17.6 L | £7.50 (2 stores) | 1.2 | £9.24 |
| Concrete, 20 kg | 10 L | £11 (3 stores) | 2.2 | £23.86 |
Prices with VAT, from very few stores. Treat them as a rough guide.
By Blue Circle's table, post mix fills more per bag here, and each bag costs less.
We found no UK maker's advice on setting metal posts. For gravel under the post, see our gravel calculator. Prices for precast concrete posts are in the questions below.
Questions people ask
How much concrete for a fence post?
For a 75 mm square post on a 1.83 m (6 ft) fence, about 22 litres. That is 1.2 bags of 20 kg post mix, or 2.2 bags of 20 kg concrete.
How many bags of Postcrete per fence post?
Blue Circle's table goes from ¼ of a bag in a small hole to 2¾ bags in a big one. Hanson says one bag of its Postfix does one post.
How deep do fence posts in concrete need to be?
Jacksons Fencing digs 600 mm for a standard fence. Wickes says 450–600 mm, and B&Q says 450 mm is usually enough. Go deeper in soft ground.
Do fence posts need concrete?
Jacksons Fencing says timber posts must be set in concrete or post mix. B&Q says metal post spikes are quicker, but may not hold as well. We found no UK advice for decking posts.
How much are concrete fence posts?
An 8 ft concrete post was £30–£34.85 at only 2 sellers. A 9 ft post was £44.60 and a 6 ft post £27, at one seller each. Travis Perkins's prices include VAT; B&Q's page shows no VAT line.
How long does post mix take to set?
Blue Circle says Postcrete sets in 5–10 minutes. You can fix panels after 10 minutes. Hanson's fast Postfix sets in under 3 minutes.
Do you need gravel under fence posts?
Wickes puts about 25 mm of gravel or MOT Type 1 in the bottom, to help water drain. Jacksons uses at least 50 mm.
Where our numbers come from
- Post holes: Blue Circle Postcrete data sheet (bags per hole)
- Hanson packed products guide (2021): bags per pallet
- Post mix price: 2 UK stores (Oct 2026)
- Bag size: Blue Circle Multi-Purpose Concrete data sheet (20 kg makes about 0.01 m³)
- Bag price: the middle of 4 prices from 3 UK stores for 20 kg concrete (Oct 2026)
- Post holes: Jacksons Fencing guide (600 mm deep, 3× post width)
- Post holes: Wickes fence guide (450–600 mm deep, 300 mm wide)
- Post holes: B&Q fence guide (450 mm usually enough)
- 8 ft concrete post price: Travis Perkins (£34.85 inc VAT)
- 9 ft concrete post price: Travis Perkins (£44.60 inc VAT)
- 8 ft concrete post price: B&Q (£30)
- 6 ft concrete post price: B&Q (£27)
- Unit sizes: NIST Handbook 44, Appendix C (1 ft = 0.3048 m; 1 US gallon = 231 cubic inches)
Made by BuildersCount. Last checked 4 Oct 2026.
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