Reviewed, sourced guidance

Concrete mix grades: what the letters and numbers mean

Published 23 Sept 2026Reviewed 24 Sept 20262 min read

Direct answer

A full concrete strength class, such as C20/25, states characteristic compressive strength for cylinder and cube specimens. It does not state the consistence required for fresh concrete or how it will be placed. Strength, durability and fresh-concrete requirements come from the project specification; this calculator only estimates quantity.

Strength class describes hardened concrete. The full specification also sets durability and fresh-concrete requirements for the job.

Key takeaways

Strength classes use paired notation such as C20/25; quote both figures.
Strength class and fresh-concrete consistence are separate requirements.
The wider specification can also cover exposure and aggregate properties.
A quantity calculator cannot choose the correct specification.

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: Understanding what mix designations mean and why the choice belongs to the specification, not the calculator.
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: 2026-09-24
Commercial status: Information publisher only. No supplier is active, no lead capture runs, no enquiries are collected and no affiliate links are present.

This page does not select a class for a job. A full specification covers strength and durability as well as fresh-concrete requirements such as consistence and aggregate size.

How this guidance is produced · Correction submission status

Reading a strength class

A class such as C20/25 gives characteristic compressive strength for cylinder and cube specimens. Quote the full notation to avoid confusion.

The class describes hardened concrete. The consistence required for fresh concrete is specified separately for the placement method.

The class is a specification input, not a quantity input.
Strength class does not state fresh-concrete consistence.
Keep the full class separate from the volume calculation.

Sources for this section: The Concrete Centre, BS 8500 for building and civil structures (2024 guide to BS 8500:2023; observed 2026-09-25)The Concrete Society, Principles of mix design (published 2025-06-03; observed 2026-09-25)

What the class does not cover

Two mixes with the same strength class can have different requirements for fresh concrete, including consistence and maximum aggregate size.

Exposure conditions are also considered when setting durability requirements; strength class alone does not describe them.

Consistence is a separate requirement for fresh concrete.
Maximum aggregate size may also form part of the specification.
Exposure conditions inform durability requirements as well as strength.

Why this site does not pick one for you

Choosing a class means weighing loading, ground, exposure and any warranty or approval that applies. A quantity tool has none of that information.

The honest position is that the class belongs to the person specifying the job. This site calculates how much concrete a shape needs, and stops there.

Bring the specified class to the supplier rather than asking them to choose.
If no class has been specified, settle it before ordering.
Record what was ordered and delivered, so a future repair starts from facts.

Sources for this section: The Concrete Centre, BS 8500 for building and civil structures (2024 guide to BS 8500:2023; observed 2026-09-25)

Designated mixes and the current standard

UK concrete is specified under BS 8500, which complements BS EN 206. The current editions are BS 8500-1:2023 and BS 8500-2:2023, and Part 1 sets out how a specifier compiles a specification.

The standard offers several specification approaches, including designated, designed, prescribed and proprietary concrete, and it covers exposure classes for durability, aggregate classes, intended working life and consistence.

Designated mixes are pre-defined combinations intended for common uses.
Durability is handled through exposure classes rather than strength alone.
Guidance covers intended working life and consistence as well as strength.
The specifier chooses the approach; the supplier works to it.

Next steps

Confirm the full concrete specification before requesting a like-for-like price.
Keep the delivery note with the date and the class supplied.

Sources

Before you order