What Size Gravel Is in Ballast? (And Does It Matter?)

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What Gravel Size Is Used in Ballast?

  • ✔ Most common sizes: 10mm and 20mm

    Ballast usually comes with either 10mm or 20mm gravel mixed in. Both are blended with sharp sand to create a strong, workable base for concrete mixing.
  • ✔ 10mm ballast gives a smoother finish

    Smaller gravel makes it easier to screed, float and finish the surface — ideal for patios, paths, internal slabs or decorative finishes where looks matter.
  • ✔ 20mm ballast is better for strength

    The larger gravel creates a chunkier concrete mix that’s perfect for foundations, footings, shed bases and anywhere strength is more important than finish.
  • ✔ Available bagged or in bulk

    Whether you're buying small 25kg bags or loose loads by the tonne, most merchants offer both 10mm and 20mm ballast — just check before delivery.
  • ✔ Gravel size can affect ease of mixing

    10mm ballast is easier to mix by hand, especially for small DIY jobs. 20mm might need a cement mixer to get a consistent, even result.

Recommended Gravel Size by Project Type

Project Type Recommended Gravel Size Why It Works Best
Path or patio slab 10mm Gives a smoother finish and is easier to level
Shed base 20mm Stronger mix for heavier loads and deeper pours
Concrete footings 20mm Ideal for structural strength and durability
Fence post mix 10–20mm Either works — strength isn’t a major issue for post holes
Internal floor slab 10mm Easier to finish smooth for painting or polishing later

10mm Gravel in Ballast

  • Smoother finish — great for patios, paths, and internal slabs.
  • Easier to mix by hand or in small mixers.
  • More workable for surface screeding and detail work.

10mm Gravel in Ballast

  • Less structural strength than 20mm in deep pours.
  • May not be ideal for heavy-duty foundations or driveways.

20mm Gravel in Ballast

  • Stronger concrete for heavy loads or structural work.
  • Ideal for driveways, shed bases, footings and outdoor slabs.
  • Readily available at most builders’ merchants.

20mm Gravel in Ballast

  • Rougher finish — harder to smooth for decorative surfaces.
  • Harder to mix evenly without a mechanical mixer.
🔹 What size gravel is used in standard ballast?
Most ballast used in UK construction contains either 10mm or 20mm gravel, blended with sharp sand. The size you need depends on the project — 10mm is better for smooth finishes, while 20mm is ideal for structural strength and deeper pours.
🔹 Can I use 10mm ballast for foundations or footings?
You can, especially for smaller or lightly-loaded structures like garden walls or sheds. However, 20mm ballast is generally preferred for deeper or more structural footings because it creates a stronger, chunkier concrete mix.
🔹 Is 20mm gravel harder to work with?
Yes — 20mm ballast can be more difficult to mix by hand, especially for small batches. It may also give a rougher finish, so it’s better suited for driveways, deep bases, or where appearance isn’t a top priority.
🔹 Does gravel size affect concrete strength?
Yes — larger gravel (like 20mm) tends to make concrete stronger and more suitable for heavy loads. Smaller gravel (10mm) makes it more workable and easier to finish, but may be slightly less robust in load-bearing scenarios.

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