Concrete Calculator

Last updated: June 2026

Project type

Length

Width

Depth

Volume

0.6 m³

25kg bags

51 bags

40kg bags

32 bags

Formula

Slab volume = length x width x depth. Post hole volume = pi x (hole radius² - post radius²) x depth x number of holes. 25kg bags are estimated at 0.012m³ each.

About this calculator

Estimate concrete volume and bag quantities for slabs, paths, footings, and post holes. It is useful for deciding whether bagged concrete is practical or whether ready-mix delivery may be worth comparing.

Concrete volume method

The calculator uses metric dimensions and practical UK planning assumptions. Results are estimates, so check product labels, site conditions, and supplier guidance before ordering materials.

slab volume = length x width x depth
post hole volume = pi x (hole radius² - post radius²) x depth x holes
25kg bags = ceiling(volume / 0.012m³)

How to use the concrete calculator

Choose the project type or measurement mode where the calculator offers one.
Enter all dimensions in metric units.
Add allowances such as coats, waste, pattern repeat, roll width, or bag size where relevant.
Review the main highlighted result first.
Check secondary results such as packs, bags, rolls, posts, or tin combinations.
Round up to supplier pack sizes and confirm the label coverage before buying.

Worked examples

Small slab

Input: 3m x 2m slab at 100mm depth

Calculation: 3 x 2 x 0.1 = 0.6m³.

Result: About 50 x 25kg bags using 0.012m³ per bag.

Fence post holes

Input: 4 holes, 200mm diameter, 0.6m deep, 100mm post

Calculation: Subtract post volume from hole volume and multiply by 4.

Result: The calculator estimates concrete around the posts only.

Bagged concrete vs ready-mix

For small jobs, bagged concrete can be convenient. For larger volumes, especially around or above 1m³, ready-mix may be easier and more consistent. Delivery, access, time, and labour matter.

Depth matters

A small depth change can significantly alter volume. Always convert millimetres to metres before multiplying length and width.

Common mistakes

Mixing centimetres, millimetres, and metres without converting first.
Ordering exact quantities with no allowance for cuts, waste, damage, or uneven surfaces.
Forgetting that products are sold in tins, packs, bags, rolls, or boards rather than exact calculated quantities.
Using average deductions or coverage rates when the project has unusual shapes, textures, slopes, or access limits.
Not checking manufacturer instructions before final ordering.

Home improvement estimate disclaimer

This calculator is for general DIY planning only. It is not a substitute for a professional survey, structural design, building control advice, manufacturer instructions, or installer measurement.

Actual product coverage and pack sizes vary by supplier.
Site conditions can change material quantities.
Structural, roofing, electrical, gas, and regulated building work may need qualified advice.

Frequently asked questions

How much does a 25kg concrete bag cover?

The calculator uses about 0.012m³ per 25kg bag as a planning estimate. Check the specific bag label.

Does this calculate cement, sand, and aggregate separately?

No. It estimates concrete volume and bag quantities. Mix ratios are shown for context only.

Should I add waste?

For practical jobs, allow some margin for uneven excavation, spillage, and low spots.

Can I use this for structural work?

Use it for planning quantities only. Structural concrete should follow appropriate specifications and professional guidance.

Why subtract post diameter?

For post holes, the post occupies part of the hole, so the concrete fills the ring around it.

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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.