How Many Bags of Concrete for a Fence Post?
A post hole 300 mm wide and 600 mm deep holds about 0.042 cubic metres of concrete. At 0.012 m³ per 25 kg bag that is 3.5 bags, so four.
Try your own numbers
Last checked 10/11/2026. Sources: LSBUD: Linesearch Before U Dig (UK).
The post takes up some of the hole
The hole is a cylinder: π × 0.15² × 0.6 = 0.0424 m³. A 100 × 100 mm post buried 600 mm takes up 0.006 m³, leaving 0.0364 m³ of concrete, which is 3.03 bags. That rounds up to four bags, so the answer does not change here, but a slightly narrower hole could save a bag.
The yield of a 25 kg bag is an editorial rule of thumb (0.012 m³): brands differ, and the yield is printed on the bag, so enter yours in the yield box. For a run of posts, set the number of holes: ten holes of this size need 0.42 m³, which a ready-mix delivery or a mixer will handle more easily than 40 bags.
Before you dig, check for buried pipes and cables, for example with the free LSBUD service. Hole size and depth depend on the fence and the soil, so follow the fence maker’s guidance.
Common questions
Can I use fast-setting post mix?
Yes, if you enter its yield from the bag. Fast-setting mixes often have a different yield from general concrete.
How many bags for 8 posts?
Set holes to 8: that is 0.34 m³, about 29 bags of 25 kg at 0.012 m³ each, before allowing for the posts.