Reviewed, sourced guidance

Concrete calculator: volume, bags and waste allowance

Published 23 Sept 2026Reviewed 27 Sept 20263 min read

Direct answer

Enter plan dimensions, a count and depth in metric units. For each row, multiply its supported plan area by depth and count, then add the non-overlapping row volumes. A 4 m by 3 m slab at 100 mm is 1.2 cubic metres before waste.

Add rectangles, right-angled triangles, circles or circle sectors. Set a count for repeated identical areas, then choose metres, centimetres or millimetres and a shared or per-area depth.

Concrete quantity estimate

Measure each pour area and depth; set a count for repeated identical areas. Product yields are optional for bag counts.

Depth overrides are optional. Leave an area blank to use the default depth below.

Default depth Depth unitRequired only when one or more area rows have a blank depth override.

Waste allowance (%) Enter the supplier or job-specific allowance, or enter 0 for the base quantity. No default is assumed.

Bagged product presetExamples are not brand recommendations. Verify current packaging before ordering.

Product yield (m³ per bag) Optional. Without a yield, the tool returns concrete volume and no bag count.

Density (kg per m³) Optional. Enter the density from your mix specification or supplier to estimate weight. No typical density is assumed for you.

Method and limitations

Model version: 2026-09-30-v1

Each plan area uses its own depth when entered, or the shared default. Areas must not overlap; the calculator cannot detect overlap or confirm a site drawing.
Rectangles, right-angled triangles, full circles and circular sectors are supported. Other shapes need to be split into non-overlapping supported components.
The entered waste allowance is a planning value, not a project specification. The Concrete Society general guide gives different overage ranges for formed and unformed elements.
Reinforcement, formwork and deep hollows are not modelled.
Bag yield is product-specific. No generic bag count is assumed.
Bag counts round up to whole packs. This estimate does not calculate a price or select a structural depth or mix.
Weight appears only when you enter a density; the figure is that density applied to the calculated volume, not a materials guarantee.

Product yields and sources

Tarmac, Blue Circle Multi-Purpose Concrete product data sheet (December 2023; observed 2026-09-25): about 0.01 m³ per bag.
Heidelberg Materials, 40N Concrete technical data sheet (July 2025; observed 2026-09-25): about 0.008 m³ per bag.

Key takeaways

Set the count for repeated identical areas; each row uses its own depth or the shared default.
Enter a supplier or job-specific waste allowance, or enter 0 for the base quantity; no default is assumed.
Bag estimates use one of two cited product yields or a yield entered by the user.
Bag counts are rounded up, because buying a part bag is not possible.
The calculator never returns a price.

Evidence note

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

View the full evidence note

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: Non-overlapping rectangles, right-angled triangles, circles and circle sectors; each row represents 1 to 1,000 identical areas at one uniform depth.
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: 2026-09-23
Reviewed: 2026-09-27
Commercial status: Information publisher only. No supplier is active, no lead capture runs, no enquiries are collected and no affiliate links are present.

A count repeats the same dimensions and depth; use a separate row when either differs. Areas must not overlap, and overlap is not detected. Without a product yield, bag counts are omitted; published presets are examples, not recommendations.

How this guidance is produced · Correction submission status

How the calculation works

Choose metres, centimetres or millimetres for plan dimensions and depth. The calculator converts plan dimensions to metres and normalizes entered depths to millimetres; 100 mm becomes 0.1 m for the volume formula.

For rectangles use length x width; for a right triangle use base x perpendicular height / 2; for a circle use pi x radius squared. A circle sector uses the same circle area multiplied by its central angle / 360. Multiply each area by its depth and row count, then add the volumes. A row count repeats identical dimensions and depth; repeated and separate areas must still be non-overlapping.

The calculator accepts metric units only. To convert an imperial measurement before entering it, multiply feet by 0.3048 for metres and inches by 25.4 for millimetres. A 30 foot by 40 foot slab 4 inches thick becomes 9.144 m by 12.192 m at 101.6 mm, which is about 11.33 cubic metres before waste. That conversion is arithmetic; it does not add imperial input fields.

4 m x 3 m x 0.1 m gives 1.2 cubic metres.
Four circular footings with a 150 mm radius and 600 mm depth total about 0.17 cubic metres before waste.
A 90-degree sector with a 2 m radius has an area of about 3.14 m². Each entered component must be separate and non-overlapping; overlap is not detected.
A 5 per cent allowance on 1.2 cubic metres adds 0.06, giving 1.26.
At 1.26 cubic metres, Blue Circle Multi-Purpose Concrete yields an estimated 126 bags; Heidelberg Materials 40N MaxiPack20 yields 158.

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)

Why there is a waste allowance

The Concrete Society notes that concrete overage may account for spills, uneven ground, excavation variation and waste.

Its general guide is 5 per cent for formed elements and 10 to 15 per cent for unformed elements. These are not project specifications; consider reinforcement and site conditions too.

No allowance is preselected; enter a supplier or job-specific figure, or enter 0 for the base quantity.
The calculator cannot measure hollows or account for reinforcement and formwork.
Use the specified dimensions and confirm project-specific quantities with whoever specified the pour.

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

Cited product yield reference

Bag counts are only as good as the yield behind them, and yields differ by product, pack and revision.

These are the two published presets the calculator offers, restated so you can check them against the pack you would actually buy.

Published yields for the two cited products
Product	Pack	Yield per bag	Bags per cubic metre
Blue Circle Multi-Purpose Concrete	20 kg	0.010 cubic metres	about 100
Heidelberg Materials 40N Concrete MaxiPack20	20 kg	0.008 cubic metres	about 125

Reading the result

The net volume reflects the entered geometry. The allowance-adjusted figure is a planning estimate, not an order instruction; confirm the final quantity with the supplier.

A bag count appears only after you select or enter a product yield. For larger pours, compare the volume with current supplier delivery terms.

Compare the delivered quote with bag cost and the time needed to mix and place the concrete.
Supplier minimum loads, part-load charges and delivery arrangements vary; confirm them before comparing.
Product yields can vary by pack size and revision; check the selected product packaging before ordering.

Sources for this section: The Concrete Society, Estimation of concrete volume requirement (published 2025-06-03; observed 2026-09-25)BRMCA, Ready-Mixed Concrete practical guide for site personnel (July 2020; observed 2026-09-25)

Next steps

Confirm the depth for your job before you order anything.
Check the volume against any minimum load your supplier applies.

Sources

Before you order