How the Fence Post Concrete Calculator Works

Enter your post size, hole diameter, hole depth, and the number of posts. The calculator finds the volume of each hole, subtracts the volume of the post sitting inside it, multiplies the difference across all posts, and adds 10% for spillage and uneven holes.

The subtraction is the step naive estimates miss: the post itself occupies a real share of the hole. For a 100mm post in a 300mm round hole at 600mm deep, the hole holds 0.042 m³ but the post occupies 0.006 m³ of it, so each hole actually needs about 0.036 m³ of concrete. Across ten posts, that difference is a whole extra bag saved or wasted. If you are calculating the actual posts and panels needed for the boundary, use our dedicated Fence Panel Calculator.

Two Rules That Size Every Hole

Trade practice sizes post holes using two simple rules. First, the hole diameter should be three times the post width: a 100mm timber or concrete post takes a 300mm hole, giving the concrete enough mass around the post to resist wind loads. Second, the hole depth should be 600mm minimum for a standard 1.8m fence, deepening to 750mm on exposed or windy sites, following the broader principle of burying roughly a third of the post's height.

Gate posts and corner posts break this pattern deliberately: they carry lateral and swinging loads that line posts never see, so give them a wider hole, extra depth, and an extra bag. Before any digging, check for buried services along the fence line, since gas and water pipes have a habit of following boundaries.

Bags or Mixed Concrete

For most domestic fence jobs, the answer is postcrete: a fast-setting dry mix poured straight into the hole around water, setting in 5 to 10 minutes and taking the strain off bracing. A 20kg bag yields roughly 0.01 m³, so the 300mm hole above takes 3 to 4 bags per post. Budget 1 to 2 bags for smaller 250mm holes in firm ground, and 2 to 3 bags in soft ground or for taller fences.

Site-mixed concrete at a 1:2:4 or 1:6 ballast ratio costs less per litre and suits big runs, and our Post Mix Calculator UK converts hole volumes into exact bag counts for either route. The crossover point: once a fence line needs more than roughly a cubic metre (around 25 or more standard holes), mixing your own or pricing a mini-mix delivery beats hauling a hundred individual bags.

Setting Posts So They Stay Set

Concrete only performs if the hole and pour are done right. Drop 100mm of gravel in the base first for drainage: this matters most under timber posts where trapped water causes rot at ground level (the classic failure point). Brace the post plumb in two directions before pouring, checked with a level on adjacent faces, and pour to just above ground level rather than below it. Finish by doming the concrete surface away from the post so rainwater sheds instead of pooling around the timber.

Fast-set mixes take foot traffic loads in 4 hours, whereas standard mixes want 24 hours before panels go on. Resist the temptation to hang the fence the same afternoon on standard concrete, since levering a green post out of half-cured concrete ruins both.

Worked Example

A 12-post run of 1.8m fence on 100mm posts, 300mm holes at 600mm deep:

• Volume per hole: 0.042 m³ minus post 0.006 m³ = 0.036 m³
• Total: 12 × 0.036 = 0.43 m³, plus 10% = 0.48 m³
• As postcrete at 0.01 m³ per 20kg bag: 48 bags
• As site-mixed 1:6 ballast: roughly one bulk bag of ballast plus six 25kg cement bags

At 48 bags, the site-mixed route saves real money, which is exactly the comparison worth running before making a trip to the merchant.