Bags are rounded up using the yield you provided.
Concrete per hole
Concrete total, incl. allowance
Total volume, incl. allowance
Gravel base volume
Yield per bag
Additional concrete allowance
How to calculate concrete for fence posts
Estimate concrete for one or more round or rectangular post holes. The calculator subtracts the gravel base and embedded post, applies an optional allowance and converts the required liters into bags using the actual yield of your mix. Supports square and round posts, or an empty hole.
Six 30 cm diameter × 70 cm deep holes, with a 10 cm gravel base and 9 cm square posts: about 37.55 L concrete per hole. With 10% allowance: about 247.84 L in total; at 12 L yield per bag, buy 21 bags.
Results are estimates and depend on the supplied values.
Formulas and buying decisions
1. Find the hole volume
For a round hole, calculate the cylinder; for a rectangular hole, calculate the box. Keep all lengths in matching units.
Round: π × (diameter / 2)² × depth · Rectangular: length × width × depth.
2. Subtract gravel and post
The gravel base fills the bottom of the hole. The post occupies part of the remaining concrete section. A square post has area side² and a round one has area π × (diameter / 2)².
Concrete/hole = (hole area − post area) × (hole depth − gravel depth).
3. Buy the right number of bags
Multiply the concrete volume per hole by the number of posts, add your allowance and divide by the actual fresh concrete yield per bag. Round up.
Bags = round up[(total liters × (1 + allowance/100)) / bag yield in liters].
Geometric estimate only. Hole dimensions, strength, anchoring and drainage depend on soil, wind loads, local requirements and the product instructions. This calculator does not specify structurally adequate foundations.
Frequently asked questions
How many 25 kg bags are needed per post?
It depends on the hole, post and actual mixed concrete yield. Two 25 kg bags do not necessarily produce the same volume; check the product data sheet.
Why subtract the post volume?
The buried part of the post takes space that cannot hold concrete. The calculation subtracts its cross section across the concrete-filled depth.
Can I include a gravel base?
Yes. Enter the gravel layer thickness and the calculator uses the remaining depth for concrete. It shows the geometric gravel volume, not the mass.
Can I use this for a gate or load-bearing post?
It estimates quantities only. Gates, wind and soil conditions may require a structural design that this calculator does not check.