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Reviewed, sourced guidance

How deep should a concrete slab be?

Published Reviewed 4 min read

Direct answer

A slab depth depends on its load and ground support, not footprint alone. Paths, floors and vehicle-bearing surfaces may have different specifications. A 20 m² area uses 1.5 m³ at 75 mm or 3 m³ at 150 mm. Confirm the required depth with the project specifier.

Depth scales volume in direct proportion when the footprint stays the same: doubling it doubles the volume. Confirm the job specification before measuring for an order.

Key takeaways

  • Volume scales directly with depth, so doubling depth doubles the concrete needed.
  • For a specified structural element, design accounts for loading and support conditions; quantity alone does not establish depth.
  • Each entered component uses one uniform depth. Add separately measured components with their own depths where needed; depth changes within one component are not modelled.
  • Correct depth matters because required volume changes directly; confirm the specification before turning measurements into an order.

Why depth changes everything

Two jobs can cover the same area and need very different amounts of concrete. A 20 square metre slab at 75 mm takes 1.5 cubic metres. The same slab at 150 mm takes three.

That extra 1.5 cubic metres can materially change the order. Any bag comparison needs the yield from the chosen product.

  • Use the specified depth in the calculation; the calculator does not choose one for the job.
  • Measure the footprint accurately, but treat the depth as a given.
  • If the depth is not already specified, that is a conversation to have before ordering.

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

What actually sets depth

What the surface will carry is the first factor. A slab that only takes foot traffic is not built like one that takes a vehicle.

Support conditions matter too. Site investigation and ground suitability can affect how loads are transferred and what construction is appropriate.

  • Loading: what will be on the slab, and how concentrated is it.
  • Sub-base: what the concrete is being placed on, and how well it is compacted.
  • Exposure: conditions that may affect the concrete specification and durability.
  • Any specification, warranty or approval that applies to the work.

Sources for this section: GOV.UK, Structure: Approved Document A, Building Regulations 2010 (observed 2026-09-25)NHBC Standards, 2026 edition and applicability (observed 2026-09-24)The Concrete Centre, Standards for concrete: BS 8500 (observed 2026-09-25)

The component depth assumption

Each entered component uses one depth from edge to edge. Separate components can have different depths, but real ground may have high and low spots within an area, and a slab over excavation can use more concrete than the geometry alone suggests.

The Concrete Society notes that 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 general planning guides, not a specification for a particular pour.
  • The estimator cannot model changing depth within one component, deep hollows, reinforcement or irregular formwork.
  • Use the job specification and measured conditions when choosing an allowance.

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

Check the guidance for the element and jurisdiction

NHBC Standards are guidance for NHBC-registered home builders, not a general depth chart. The 2026 edition applies to qualifying homes whose foundations began on or after 1 January 2026.

A building floor, structural foundation and external path or driveway are different elements. Do not transfer floor or foundation guidance into an outdoor slab design.

  • Identify the element, location and applicable jurisdiction before interpreting a building standard.
  • Confirm external slab depth against the job specification, ground and expected loads.
  • A quantity calculator cannot select a safe structural depth.

Sources for this section: NHBC Standards, 2026 edition and applicability (observed 2026-09-24)

Foundations are different again

A strip foundation is not a floor slab and is sized differently. Approved Document A includes plain concrete strip foundation guidance for certain traditional residential buildings in England.

In that guidance, strip foundations generally extend at least 450 mm below ground except on rock, and deeper where frost or bearing-ground conditions require. Concrete thickness is at least 150 mm or the projection beyond the wall, whichever is greater.

For shrinkable clays with a Modified Plasticity Index of 10% or more, it lists underside minimums of 750 mm for low-, 900 mm for medium- and 1,000 mm for high-shrinkage clay. Greater depth may be needed for suitable bearing ground or nearby trees.

  • Underside at least 450 mm deep, unless founded on rock.
  • Shrinkable-clay examples: 750 mm low-, 900 mm medium- and 1,000 mm high-shrinkage clay.
  • Ground conditions, loads or nearby trees can require greater foundation depth.
  • Minimum thickness of 150 mm, or the projection, whichever is greater.
  • These foundation figures are not patio, path or driveway slab depths.
  • The Building Regulations 2010 set the legal requirement; the Approved Document gives guidance on one way to meet it.

Sources for this section: GOV.UK, Structure: Approved Document A, Building Regulations 2010 (observed 2026-09-25)legislation.gov.uk, Building Regulations 2010, Schedule 1 Part A Structure (observed 2026-09-25)

Next steps

  1. Get the specified depth in writing before you order.
  2. Enter the real measurements into the calculator rather than the drawing ones.

Sources

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