Concrete Post Mix Calculator
Estimate post mix bags for fence posts, gate posts and garden posts from hole diameter, depth, post size, bag yield and waste allowance.
Post Hole Details
Post Mix Result
Bag counts are rounded up. A spare bag is often cheaper than stopping a fence job with one hole unfinished.
What The Estimate Includes
This estimator works out the wet concrete volume needed around posts. It treats the hole as a cylinder and subtracts a simple square post displacement, then multiplies the result by the number of posts. The allowance covers rough digging, loose soil, stones, small overfills and the reality that garden holes rarely match a neat drawing.
The result is most useful for bagged post mix, quick-setting fence concrete and similar products where the label gives a wet yield in litres. It is not a structural design check. Tall fences, heavy gates, retaining structures, exposed sites, poor ground or boundary work may need a builder, fencing contractor or engineer. Check underground services before digging and use safe manual handling for heavy bags.
Formula And Method
Hole volume = pi x radius x radius x depthPost displacement = post width x post width x depthConcrete per hole = max(0, hole volume - post displacement)Order litres = concrete litres x posts x (1 + waste percentage)Bags = order litres / wet yield per bag, rounded upAll measurements are converted from millimetres into metres before volume is calculated. The final bag count is rounded up because part bags do not help if the last hole is short. The post displacement is approximate; round posts, slotted posts and H-section concrete posts may need their own judgement.
Hole Size Reference
| Post type | Typical hole idea | Check before digging |
|---|---|---|
| Light garden stake | 150 to 200 mm diameter | May not need bagged concrete at all. |
| 75 mm timber fence post | 200 to 250 mm diameter | Depth depends on fence height and ground. |
| 100 mm timber post | 250 to 300 mm diameter | Common for domestic fence runs. |
| Gate post | 300 mm or more | Gate weight and wind load matter. |
| Concrete slotted post | Follow supplier advice | Post shape changes displacement. |
| Poor or loose ground | Larger or deeper hole may be needed | Get site advice for weak ground. |
Worked Examples
Eight Fence Posts
Eight 250 mm holes at 600 mm depth with 100 mm posts need roughly 186 L before allowance. With 15% extra and 11 L bags, the order rounds to 20 bags.
Two Gate Posts
Two wider 350 mm holes at 750 mm depth can need a surprising number of bags. Gate loads justify bigger foundations, so do not shrink the hole only to save material.
Small Garden Screen
Light posts in sheltered areas may use shallower holes, but the estimate should still include waste because small bag shortfalls are common.
Practical Mixing And Ordering Notes
Post mix products vary. Some are poured dry into a wet hole, others are mixed or wetted according to the bag instructions. Follow the manufacturer’s label for water and setting time. Do not assume one brand’s wet yield matches another brand. If the label says a 20 kg bag yields 10 L, enter 10 L even if another product gives 12 L.
Digging accuracy matters. A powered auger can give cleaner holes than a spade in some soils, while stony ground can create wider, rougher sides. If holes stand open before posts are fitted, rain and collapse can change the volume. Recheck the first hole before buying the rest of the materials for a long run.
For long fence runs, set out the line, string, levels and post spacing before opening every bag. A small change in spacing can alter the number of holes, while one deeper corner post can change the order. Keep unopened surplus bags dry until the work is complete, because damp cement-based products can harden in storage and become unusable.
Safety And Boundary Checks
- Check for buried cables, pipes and drainage before digging.
- Wear gloves and eye protection when handling cement-based products.
- Keep concrete away from skin and wash splashes quickly.
- Brace posts while the mix sets; do not rely on wet concrete to hold a post plumb.
- Check boundary position before setting permanent posts.
FAQs
How many bags of post mix per post?
It depends on hole diameter, depth, post size and bag yield. A common domestic fence hole may use two to three 20 kg bags, but wide or deep holes can need more.
Should I subtract the post from the hole volume?
Yes, the post occupies space in the hole. The calculator subtracts a simple square post displacement so the estimate is closer than a plain cylinder volume.
What waste allowance should I use?
Ten to twenty percent suits many domestic fence jobs. Use more for rough, collapsing or overdug holes.
Can I use this for structural posts?
No. It is a material estimate for ordinary garden and fence posts. Structural work, large gates and retaining loads need proper design advice.
Why does bag yield matter?
Two bags with the same dry weight can have different wet volume. Use the litre yield printed on the bag whenever possible.
Sources
- Health and Safety Executive. (2026). Cement. HSE. https://www.hse.gov.uk/construction/healthrisks/hazardous-substances/cement.htm
- Health and Safety Executive. (2026). Avoiding danger from underground services. HSE. https://www.hse.gov.uk/pubns/books/hsg47.htm
- British Standards Institution. (2023). BS 1722: Fences. BSI. https://knowledge.bsigroup.com/
- National Institute of Standards and Technology. (2008). Guide for the Use of the International System of Units (SI), Special Publication 811. NIST. https://www.nist.gov/pml/special-publication-811