Reviewed, sourced guidance

Concrete calculator questions answered

Published 29 Sept 2026Reviewed 29 Sept 20263 min read

Direct answer

One cubic metre needs about 100 bags of Blue Circle Multi-Purpose 20 kg or 125 bags of Heidelberg 40N MaxiPack20 at their cited yields; volume is length times width times depth; order allowances are job-specific and entered, not assumed.

Short answers to the volume, bag-count and ordering questions behind most concrete calculations, each grounded in the sources this site already cites.

Key takeaways

Bag counts depend on the product yield printed on the pack.
Volume is footprint times depth, component by component.
Order allowances belong to the job and supplier terms.
Slab depth follows the specification, not a rule of thumb.

Evidence note

Source-led homeowner information with its scope, limits and commercial status recorded.

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Produced by: My Concrete Calculator
Jurisdiction: Applies to work in the United Kingdom; specification requirements vary across England, Wales, Scotland and Northern Ireland
Page scope: Frequently asked questions about concrete quantity estimation, answered within the site’s published scope and cited sources.
Default subject: Concrete quantity estimation for slabs, footings and similar straightforward pours, including waste allowances and optional product-specific bag estimates.
Excludes: Structural, engineering or specification advice; Mix design or strength class selection for any specific job; Prices, quotations or supplier comparisons; Supplier recommendation, ranking or referral; Concrete supply, delivery, placing or finishing
Published: 2026-09-29
Reviewed: 2026-09-29
Commercial status: Information publisher only. No supplier is active, no lead capture runs, no enquiries are collected and no affiliate links are present.

Answers restate cited sources and tested arithmetic for planning. They are not a mix design, structural specification or supplier quotation for any specific job.

How this guidance is produced · Correction submission status

How many 20kg bags make one cubic metre of concrete?

At the cited yields, one cubic metre takes about 100 bags of Blue Circle Multi-Purpose Concrete 20 kg, whose data sheet gives 0.01 cubic metre per bag.

The same volume takes about 125 bags of Heidelberg Materials 40N MaxiPack20, cited at 0.008 cubic metre per bag.

Yields differ by product, pack size and revision, so check the packaging for the bag you would actually buy.

Sources for this section: Tarmac, Blue Circle Multi-Purpose Concrete product data sheet (December 2023; observed 2026-09-25)Heidelberg Materials, 40N Concrete technical data sheet (July 2025; observed 2026-09-25)

How do I work out how much concrete I need?

For a rectangular slab, multiply length by width by depth: 4 m by 3 m at 100 mm gives 1.2 cubic metres.

Break irregular plans into rectangles, right-angled triangles, circles or circle sectors, work out each volume, then add them up.

Areas must not overlap; the calculator adds components but does not detect overlap.

Sources for this section: The Concrete Society, Estimation of concrete volume requirement (published 2025-06-03; observed 2026-09-25)

How much extra concrete should I order?

There is no universal percentage. Suppliers and guidance differ, and the Concrete Society’s volume guidance discusses allowance examples rather than a fixed rule.

Treat the allowance as an entry you control: the calculator shows net volume and the allowance-adjusted planning volume side by side.

Settle the figure against your forming, access and placement conditions and the supplier’s terms for the job.

Sources for this section: The Concrete Society, Estimation of concrete volume requirement (published 2025-06-03; observed 2026-09-25)

What depth should my concrete slab be?

Depth follows the specification for the element, its loading and the ground, not footprint alone.

The same 20 square metre area needs 1.5 cubic metres at 75 mm but 3 cubic metres at 150 mm, which is why the specification matters before you order.

Structural context sits with the project specifier; Approved Document A and BS 8500 guidance describe the framework rather than a one-size depth.

Sources for this section: GOV.UK, Structure: Approved Document A, Building Regulations 2010 (observed 2026-09-25)The Concrete Centre, Standards for concrete: BS 8500 (observed 2026-09-25)

Is ready-mix cheaper than mixing bags myself?

There is no universal break-even volume; it depends on local prices, access, load terms and how much placing time you can manage.

BRMCA’s practical guide frames the comparison around system type, access and placement rather than price alone.

Price the same volume both ways using current local figures, then decide; this site does not carry prices.

Sources for this section: BRMCA, Ready-Mixed Concrete practical guide for site personnel (July 2020; observed 2026-09-25)

What does a grade like C25/30 mean?

C25/30 is a strength class: 25 is the characteristic cylinder strength and 30 the equivalent cube strength, in newtons per square millimetre.

The Concrete Centre’s BS 8500 overview explains strength classes alongside consistence and exposure, which is why a class is specified rather than chosen by a calculator.

A concrete quantity calculator does not select a mix class; that decision belongs to the specification for the job.

Sources for this section: The Concrete Centre, Standards for concrete: BS 8500 (observed 2026-09-25)

Next steps

Open the calculator and enter your measured areas, depth and allowance.
Check the chosen product packaging before converting a volume into a bag order.

Sources

Open the calculator