Concrete Post Calculator: Nail Your Fence Posts First Time—No More Running Out of Mix

Ever got halfway through setting fence posts, only to realise you’re two bags short and the merchant’s about to close? That’s a rite of passage—right up there with putting a post in upside down and pretending it was on purpose. Let’s save you the hassle (and the sheepish second trip) by walking through how to work out exactly how much concrete you need for your fence posts—whether you’re working in metres or feet, kilograms or pounds.

Why You Need a Concrete Post Calculator (And Why Guessing Is Like Pouring Money Down the Drain)

Here’s the bit most people get wrong: they just guesstimate the number of bags, chuck a few extra in for luck, and end up with half a pallet gathering dust—or worse, make do and have wobbly posts. Ordering concrete for fence posts without measuring is like doing your weekly shop with no list and a blindfold. Spoiler: you never get it right.

A proper concrete volume calculator for post holes stops you wasting cash, time, and effort. Plus, you’ll know you’ve got enough to make every post straight and solid—no more fence panels leaning like they’re sunbathing.

What Do You Actually Need to Calculate?

You’re looking to fill a series of holes, each shaped like a cylinder (or sometimes a square if you’re using a spade with attitude). You’ll need:

• The number of posts (holes)
• The diameter (width) and depth of each hole
• The size of the post itself (so you don’t count concrete where the timber/lumber sits)

And yes, every measurement comes in both metric and imperial—no need to play conversion roulette.

Standard Fence Post Hole Sizes (UK, US and Beyond)

Most fencing pros go by the old rule: bury at least one third of your post length in the ground. So, for a 1.8m (6ft) fence, you’re probably looking at a 2.4m (8ft) post, with 0.6m (2ft) of it underground.

For diameter/width:

• UK/metric: Typical post holes are 200–250mm (8–10in) wide
• US/imperial: 8–10in (200–250mm) diameter holes are standard

But check your local frost line (if you have one)—nobody wants fence posts doing the hokey-cokey every winter.

Concrete Volume Formula—The Simple Version

Here’s the quick-and-dirty formula for a single cylindrical hole:

Volume = π × radius² × depth

Where:

• Radius = half the diameter of your hole
• Depth = how much post you’re burying

And don’t forget to subtract the post itself (unless you like buying concrete for no reason).

Worked Example: Metric and Imperial

Let’s say you’re putting in 8 posts for a standard garden fence. Each post is 100mm x 100mm (4in x 4in), and you’re burying 600mm (2ft) of each post in the ground. You want a hole 250mm (10in) wide and 600mm (2ft) deep.

Step-by-Step Calculation

1. Work out the radius of the hole:
   • 250mm (10in) diameter → 125mm (5in) radius
   • 125mm = 0.125m (5in = 0.417ft)
2. Work out the volume for one hole:
   • Metric: π × (0.125m)² × 0.6m = 0.0295m³ per hole
   • Imperial: π × (0.417ft)² × 2ft = 1.09ft³ per hole
3. Subtract the post (optional for precision):
   • Post radius: 50mm (2in) = 0.05m (0.167ft)
   • Metric: π × (0.05m)² × 0.6m = 0.0047m³ (for the post)
   • Net concrete per hole: 0.0295m³ - 0.0047m³ = 0.0248m³
   • Imperial: π × (0.167ft)² × 2ft = 0.175ft³
   • Net concrete per hole: 1.09ft³ - 0.175ft³ = 0.915ft³
4. Multiply by number of posts:
   • Metric: 0.0248m³ × 8 = 0.198m³
   • Imperial: 0.915ft³ × 8 = 7.32ft³

How Many Bags Do You Need?

• UK/metric: Most postcrete/rapid set comes in 20–25kg (44–55lb) bags, yielding 0.011–0.013m³ (0.39–0.46ft³) per bag.
• US/imperial: Typical 50lb (22.7kg) bags yield about 0.375ft³ (0.01m³)

So:

• Metric: 0.198m³ / 0.011m³ per bag ≈ 18 bags
• Imperial: 7.32ft³ / 0.375ft³ per bag ≈ 20 bags

Chuck in a 10% wastage allowance—the spare teabags of the building world—so you’re never caught short.

Pro tip: Or skip the maths and try the concrete post hole calculator instead. It’ll do the heavy lifting for you.

Hole Size	Depth	Posts	Concrete Needed (m³)	Bags (25kg/55lb)
250mm/10in dia	600mm/2ft	8	0.198	18–20

The Right Way to Use a Concrete Post Calculator

Bet the merchant never told you this: the secret is to measure everything before you buy, not after you’ve dug most of the holes (we’ve all been there). Use the post hole concrete calculator by entering:

• The number of posts
• The diameter and depth of each hole (don’t forget to convert to the right units)
• The size of your post

Let it spit out the total concrete required, and round up to the nearest full bag. Nobody has ever finished a job saying they wish they’d bought less.

Don’t Forget These Practical Tips

• Always check your soil type—soft ground may need wider or deeper holes
• Pack a bit of gravel/aggregate at the base for drainage
• Make sure posts are plumb before the mix sets (unless you like creative fencing)
• Don’t skimp on the concrete—wind, pets, and time are all out to get you

Questions People Are Actually Asking

How much concrete post hole?

The amount of concrete for each post hole depends on the hole’s size and depth. For a typical 100mm x 100mm (4in x 4in) post with a 250mm (10in) diameter by 600mm (2ft) deep hole, you’ll need about 0.025m³ (0.9ft³) of concrete per post. Use a dedicated post hole concrete calculator for your exact numbers.

How much concrete post in the Philippines?

Concrete posts in the Philippines are often used for perimeter fences. The amount of concrete per post depends on the post’s size and hole dimensions, just like anywhere else. For a 150mm x 150mm (6in x 6in) post with a 300mm (12in) diameter by 600mm (2ft) deep hole, you’ll need around 0.042m³ (1.5ft³) per post. Local bag sizes may vary, so check your supplier.

How much fence post should be in the ground?

General rule: at least one third of the post’s length should be buried. For a 2.4m (8ft) post, that’s about 0.8m (2.5ft) in the ground. Deeper is better in soft or frost-prone soils.

How much post concrete do I need?

Total concrete required is the volume per post multiplied by the number of posts. For 10 posts with standard holes (250mm/10in diameter, 600mm/2ft deep), you’ll need roughly 0.25m³ (9ft³), or about 20–23 bags of 25kg (55lb) mix. Or just plug your numbers into the post hole concrete calculator to get an exact answer.

How much fence post goes in the ground?

Aim for at least one third of the post’s total length below ground. So, for a 1.8m (6ft) fence, use a 2.4m (8ft) post and bury 600–800mm (2–2.5ft).

How much fence post below ground?

The same as above: at least one third of the total post length, or more if your ground is soft or you’re in a windy area. Deeper posts are sturdier posts.

How much concrete post?

If you’re asking about pre-cast concrete posts (not just the concrete needed for timber/lumber posts), you’ll need to check the specific product weight and size. For setting posts, follow the volume calculations above.

Conclusion

Ordering the right amount of concrete for your fence posts doesn’t have to be guesswork (or involve three trips to the merchant and a lot of muttering). Use the formulas above or, better yet, let the concrete post hole calculator do the legwork for you. Your back—and your fence—will thank you.

Free tools to put this into practice

Reading is one thing — working out your own numbers is what saves time and money. Here are the free HowMuchDoINeed calculators that pair best with this guide:

• Fence Post Spacing Calculator — work out exactly how much you need and what it will cost.
• Post Hole Calculator — get an instant materials estimate for your project.
• Concrete Steps Calculator — size up quantities and costs before you head to the merchant.
• Concrete Weight Calculator — plan your order so you avoid over- or under-buying.

Explore every free calculator on HowMuchDoINeed — no sign-up required.