How Much Concrete Do I Need for Fence Posts?
You have a run of fence to put up, the posts are stacked in the garden, and the question is simple: how much concrete do I need for fence posts? Buy too little and you are back at the builders’ merchant with one post half set; buy too much and bags harden in the shed. The answer is a short piece of geometry, shown below with two fully worked examples (metric and imperial) and a quick-reference table.
First, size the hole correctly
Concrete quantity depends almost entirely on the hole, so get the hole right before you do any maths. Three widely used rules of thumb cover most garden and boundary fences in the UK and US:
- Width: make the hole about three times the width of the post. A 100 mm (4 in) post wants a hole roughly 300 mm (12 in) across; a 75 mm (3 in) post about 225 mm (9 in).
- Depth in the ground: bury about one third of the height that will stand above ground. A 1.8 m (6 ft) fence needs around 600 mm (2 ft) of post below ground, so you would buy 2.4 m (8 ft) posts.
- Drainage layer: dig an extra 100 to 150 mm (4 to 6 in) and put gravel or hardcore in the bottom so water drains away from the end grain of timber posts. Concrete only fills the space above this layer.
In colder parts of the US, the bottom of the post should sit below the local frost line, which can be much deeper than 2 ft. Your local building department can tell you the figure for your area. Before digging anywhere, check for buried cables and pipes: in the US call 811, and in the UK use the free LinesearchbeforeUdig service or ask your utility providers.
How to calculate concrete for one fence post, step by step
The concrete fills the hole minus the space taken up by the post. That gives a five-step method:
- Measure the concrete depth. This is the depth of the hole minus the gravel layer, not the full hole depth.
- Work out the hole volume. Square hole: width x width x concrete depth. Round hole: 3.1416 x radius x radius x concrete depth.
- Work out the post volume over the same depth: post width x post width x concrete depth.
- Subtract the post volume from the hole volume. That is the concrete for one post.
- Multiply by the number of posts, add 10% for waste, and divide by the yield of your bag to get the number of bags.
Keep every measurement in the same unit. In metric, metres give cubic metres (multiply by 1,000 for litres). In imperial, inches give cubic inches, and you divide by 1,728 to get cubic feet.
How much does one bag make?
The yield printed on the bag is the number that matters most. In the US, standard concrete mix bags typically state 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb. That works out at roughly 2,140 kg of dry mix per cubic metre of finished concrete, so a 20 kg bag of UK post mix or concrete mix makes about 9 to 10 litres (0.009 to 0.010 m³). Rapid-set post mixes vary a little between brands, so always check the back of the bag you are actually buying.
Worked example 1: a UK garden fence in metric
You are putting up a 1.8 m panel fence with 10 posts, each 100 x 100 mm. You dig square holes 300 mm wide and 700 mm deep with a spade, and put 100 mm of gravel in the bottom.
- Concrete depth: 700 mm minus 100 mm gravel = 600 mm = 0.6 m
- Hole volume: 0.3 x 0.3 x 0.6 = 0.054 m³
- Post volume: 0.1 x 0.1 x 0.6 = 0.006 m³
- Concrete per post: 0.054 minus 0.006 = 0.048 m³ (48 litres)
- For 10 posts: 0.048 x 10 = 0.48 m³
- Add 10% waste: 0.48 x 1.1 = 0.528 m³ (528 litres)
- Bags at about 9.36 litres each: 528 / 9.36 = 56.4, so buy 57 bags of 20 kg
That is around five bags per post, far more than the “one bag per post” many people expect. The difference is the hole. A round 250 mm hole from a post-hole digger holds 3.1416 x 0.125 x 0.125 x 0.6 = 0.0295 m³, so concrete per post falls to about 23.5 litres, roughly 2.5 bags.
For cost, multiply bags by the price per bag: purely as an illustration, 57 bags at £7 (or €8) each is £399 (€456). Use the price on your own quote, and at over half a cubic metre it is worth pricing bulk or ready-mixed concrete too.
Worked example 2: a US privacy fence in imperial
You are setting 12 posts of nominal 4×4 lumber. A 4×4 actually measures 3.5 x 3.5 in, which is the size to use. Holes are dug with a 10 in auger to 30 in deep, with 6 in of gravel in the base.
- Concrete depth: 30 in minus 6 in gravel = 24 in
- Hole volume: 3.1416 x 5 x 5 x 24 = 1,885.0 cu in
- Post volume: 3.5 x 3.5 x 24 = 294.0 cu in
- Concrete per post: 1,885.0 minus 294.0 = 1,591.0 cu in; divided by 1,728 = 0.921 cu ft
- For 12 posts: 0.921 x 12 = 11.05 cu ft
- Add 10% waste: 11.05 x 1.1 = 12.15 cu ft (or 0.45 cu yd, since 1 cu yd = 27 cu ft)
- 80 lb bags at 0.60 cu ft: 12.15 / 0.60 = 20.3, so buy 21 bags
Per post, that is about 1.5 bags of 80 lb, or 2.5 bags of 50 lb. If your bags cost $8 each (again purely illustrative), 21 bags comes to $168.
Quick-reference table: concrete per post
These figures use the steps above, with no waste allowance added. Bag counts assume 9.36 litres per 20 kg bag and 0.60 cu ft per 80 lb bag.
| Post size | Hole | Concrete depth | Concrete per post | Bags per post |
|---|---|---|---|---|
| 75 x 75 mm (3 in) | 225 mm square | 600 mm | 27 L (0.95 cu ft) | 2.9 x 20 kg |
| 100 x 100 mm (4 in) | 250 mm round | 600 mm | 23.5 L (0.83 cu ft) | 2.5 x 20 kg |
| 100 x 100 mm (4 in) | 300 mm square | 600 mm | 48 L (1.70 cu ft) | 5.1 x 20 kg |
| 125 x 125 mm (5 in) | 375 mm square | 600 mm | 75 L (2.65 cu ft) | 8.0 x 20 kg |
| 4×4 (3.5 in actual) | 10 in round | 24 in | 0.92 cu ft (26 L) | 1.5 x 80 lb |
| 4×4 (3.5 in actual) | 12 in round | 24 in | 1.40 cu ft (40 L) | 2.3 x 80 lb |
| 6×6 (5.5 in actual) | 16 in round | 36 in | 3.56 cu ft (101 L) | 5.9 x 80 lb |
Two patterns stand out. Hole width matters far more than post size, because volume grows with the square of the width: going from a 10 in to a 12 in auger adds about 50% more concrete. And square holes dug by hand use noticeably more than round ones of the same width.
Tips for a post that stays put
- Crown the top. Finish the concrete just above ground and slope it away from the post so rain runs off.
- Brace before it sets. Plumb the post on two adjacent faces and hold it with battens; rapid-set mixes firm up in minutes.
- Protect your skin. Wet cement is strongly alkaline and can burn, so wear gloves and eye protection.
- Consider gravel instead. Some fencers set timber posts in compacted gravel, which drains freely and makes a rotten post easier to replace. Our gravel calculator will size that for you.
Frequently asked questions
How many bags of postcrete do I need per fence post?
For a 100 mm (4 in) post in a 600 mm deep hole, expect about 2.5 bags of 20 kg in a 250 mm round hole, or about 5 bags in a 300 mm square hole. Work out the volume of your actual hole, then divide by the yield printed on the bag.
How deep should a fence post be in concrete?
A common guide is one third of the above-ground height, so about 600 mm (2 ft) for a 1.8 m (6 ft) fence, with an extra 100 to 150 mm of gravel below. In the US, go below your local frost line where that is deeper.
Should I subtract the post when calculating concrete?
Yes. The post fills part of the hole, and for a square post in a narrow hole that can be 10 to 20% of the volume. In the worked UK example the post took up 0.006 of the 0.054 m³ hole, about 11%.
Work it out for your own fence
The quickest route is to plug your own post size, hole shape and depth into the free Quoteen concrete calculator, which works in metric and imperial. If your merchant quotes in cubic yards and you worked in cubic metres, the cubic metre, foot and yard converter switches between them. Measure twice, add your 10%, and the last post goes in with concrete to spare.