How to use the fence post calculator
- Enter how many posts you're setting and whether you dig square holes (spade) or round holes (auger).
- Enter the hole width (or diameter) and depth, and the width of the post itself.
- Choose your bag. In the UK, 20 kg post-mix is common; in the US, choose a bag size or enter your product's yield.
The formula
concrete per post = (hole area − post area) × hole depth
bags = concrete per post × posts ÷ yield per bag (rounded up)
The hole area is width² for a square hole or π × (diameter ÷ 2)² for a round one. We subtract the post because the post itself fills part of the hole. If you put gravel in the bottom of the hole for drainage, reduce the depth by that amount.
Where our post-mix figure comes from
Manufacturers of 20 kg post-mix don't all publish a yield. We worked one out from a supplier's published table: for a 75 mm post, 3 bags fill a 300 × 600 mm hole and 2.25 bags fill a 300 × 450 mm hole. Both rows give the same answer — about 16.9 litres of filled hole per bag, which is what this calculator uses. If your bag states a different yield, choose “Other bag”.
Worked examples
UK: 10 posts of 75 mm in 300 × 300 mm holes, 600 mm deep → 3 bags per post → 30 bags of 20 kg post-mix.
US: 10 posts (3.5 in) in 10 in round holes, 24 in deep → 2.5 bags per post → 25 bags of 50 lb concrete mix.
Tips
- Dig one hole first and measure it — real holes are often wider than planned, which uses more concrete.
- Buy a spare bag or two. Running out with a post half-set is worse than returning an unopened bag.
- Gate posts carry more load; follow the gate manufacturer's guidance for hole size.
Common mistakes to avoid
- Holes wider than planned. Dug holes are often bigger than intended — measure one before buying.
- Forgetting drainage gravel. If you put gravel in the bottom, reduce the depth you enter.
- Using a US bag yield for a metric bag (or the other way round). Choose the bag that matches what you're buying.
Measuring an awkward shape? See how to measure an irregular area.
Frequently asked questions
How many bags of postcrete per fence post?
It depends on the hole. One supplier’s table for a 75 mm (3 in) post gives 3 bags of 20 kg post-mix for a 300 × 600 mm hole and 2.25 bags for a 300 × 450 mm hole. Fencing suppliers commonly suggest 2 bags per post in firm ground and up to 3 in soft ground or for taller fences. Enter your hole size above for a tailored answer.
How much concrete is needed for a 4×4 post?
A nominal 4×4 post is about 3.5 in square. In a 10 in round hole, 24 in deep, the concrete volume is about 0.92 cubic feet — about 2.5 bags of 50 lb concrete mix per post.
Can I use normal concrete instead of post-mix?
Yes — this calculator works with standard bagged concrete too. Choose the bag size, or pick “Other bag” and enter the yield printed on your product. Post-mix products are designed to set quickly; follow the instructions on the bag.
How deep should a fence post hole be?
Follow the fence or post manufacturer’s instructions and any local building rules — required depths vary with fence height, wind exposure, soil and (in cold climates) frost depth. Then enter that depth here.
Sources
Last updated 10 November 2026. Found a mistake? Tell us.