Post Hole Concrete Calculator

Last updated: June 2026

Number of holes

holes

Hole diameter

Post diameter

Hole depth

Waste

Concrete volume

0.249 m3

25kg bags

21 bags

40kg bags

14 bags

Formula

concrete per hole = pi x (hole radius squared - post radius squared) x depth. Total volume = per-hole volume x holes x waste allowance.

About this calculator

The Post Hole Concrete Calculator estimates concrete needed for round holes around fence posts, gate posts, deck posts, pergolas, or garden structures.

Post Hole Concrete Calculator method

The calculator uses metric project dimensions, applies the material formula shown below, and then adds any waste or allowance entered. Quantities are rounded up where materials are normally purchased as whole boards, sheets, blocks, rolls, bags, or packs.

hole volume = pi x radius squared x depth
post displacement = pi x post radius squared x depth where used
concrete per hole = hole volume - post displacement
total concrete = concrete per hole x number of holes x (1 + waste percentage)

How to use the Post Hole Concrete Calculator

Measure or choose the hole diameter and depth.
Enter the number of post holes.
Enter post diameter if you want to subtract the post volume.
Add waste for irregular holes, spillage, and levelling.
Review total volume and bag estimate before buying post concrete.

Worked examples

Fence posts

Input: 8 holes, 250mm diameter, 600mm deep, 75mm posts

Calculation: Circular hole volume less post displacement, multiplied by 8.

Result: The result estimates total concrete volume for the fence run.

Gate posts

Input: 2 larger holes, 350mm diameter, 800mm deep, 10% waste

Calculation: Bigger diameter and depth increase volume quickly.

Result: The estimate helps decide between bagged concrete and ready mix.

When this calculator is useful

The Post Hole Concrete Calculator estimates concrete needed for round holes around fence posts, gate posts, deck posts, pergolas, or garden structures.

Use it before ordering materials, comparing supplier pack sizes, or checking whether a first estimate is realistic. Keep the result with your measured dimensions so you can update the calculation if the project size changes.

Inputs that affect the result

Hole shape: Hand-dug holes are rarely perfect cylinders, so include waste.
Post size: Large posts displace concrete but may also need larger holes for strength.
Ground and exposure: Wind load, gates, soft ground, and fence height can need deeper or wider holes.

Common mistakes to avoid

Check 1: Gate posts may need more concrete than ordinary fence posts.
Check 2: Do not assume shallow holes are adequate for exposed fences.
Check 3: Follow product instructions for water, curing, and setting time.

Material planning notes

Planning point	Why it matters
Measure finished area	Use the actual finished dimensions after allowing for openings, edges, posts, slopes, or returns.
Allow for waste	Cuts, breakages, joins, pattern matching, awkward shapes, and site handling usually mean ordering more than the exact calculated amount.
Round purchase quantities up	Most materials are bought as full boards, sheets, bags, blocks, rolls, packs, or bulk deliveries.
Check supplier specifications	Coverage, density, sheet size, block size, and pack size vary by product and supplier.

Limitations and safety

This calculator is for general project planning only. It does not replace a structural design, building regulations advice, manufacturer instructions, supplier specifications, or a competent tradesperson. Projects involving structure, roofs, retaining walls, foundations, drainage, electrics, gas, or safety-critical work should be checked properly before buying materials or starting work.

Frequently asked questions

Should I order exactly the calculated quantity?

Usually no. Order in the nearest practical pack, board, sheet, bag, roll, or delivery quantity, and keep a sensible waste allowance for cuts and mistakes.

What waste allowance should I use?

Simple rectangular jobs may only need 5-10%. Patterned, angled, fragile, or irregular projects may need more. The right allowance depends on the material and layout.

Why might my supplier quantity differ?

Suppliers use specific product sizes, densities, coverage rates, and pack quantities. Use the calculator as a planning estimate, then compare it with the product label or datasheet.

Can I use this for professional building work?

It can help with a rough take-off, but professional work should be checked against drawings, specifications, site conditions, and relevant UK rules or standards.

What measurements should I double-check?

Check length, width, depth, height, spacing, openings, pitch, and units. A small unit mistake can change a material order by a large amount.

Related calculators

fence-panel-calculator
concrete-calculator
concrete-footing-calculator

What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.