Ballast And Cement Calculator For Shed Base

It depends on your base's area, slab thickness and chosen concrete mix ratio. As a rough guide, a typical 8x6ft shed base (100mm thick, standard C20 mix, 10% waste) needs about 2 bulk bags of ballast and 7 bags of cement - but a larger, thicker, or heavier-duty base needs proportionally more. Enter your own dimensions above for an exact figure.

C20 (1 part cement to 5 parts ballast by volume) is the widely used UK trade default for a standard garden shed base - it comfortably handles foot traffic and the static weight of a timber shed. C15 (1:6) is a lighter, cheaper general-purpose mix sometimes used for very light structures; C25 (1:4) is a stronger mix worth stepping up to for a workshop, a larger base, or anything that will carry heavier equipment or vehicles.

Most standard garden sheds are fine on a 75-100mm concrete slab, with 100mm being the most common recommendation. Go thicker - 125-150mm - for a larger footprint, a heavier structure (workshop, log cabin, brick-built outbuilding), or if the base will need to bear occasional vehicle or machinery weight.

Most UK trade guidance recommends a compacted hardcore sub-base (typically MOT Type 1, 75-100mm compacted) beneath the concrete slab. It provides drainage, a stable, well-compacted platform, and helps prevent the slab cracking or settling unevenly over time - especially on softer or clay ground. You can untick the sub-base option above if you are pouring directly onto an already firm, well-drained base, but most sheds benefit from including it.

A DPM (a heavy-duty polythene sheet laid over the compacted hardcore, before the concrete is poured) is commonly recommended to stop ground moisture rising into the slab and, in turn, into a timber shed floor. It's inexpensive relative to the rest of the base and widely recommended in UK shed-base guides, even though it isn't calculated by this tool (it doesn't change your ballast/cement/hardcore quantities).

A standard bulk ("jumbo" or "builder's") bag of ballast holds around 0.85 tonnes - roughly the same as about 40-45 individual 25kg bags, though buying loose bulk bags is almost always cheaper per tonne than buying many small bags for a job this size. Bag weights vary slightly by supplier, so check the specific weight quoted by whoever you're buying from.

Yes - "all-in ballast" (a pre-blended mix of sharp sand and gravel/stone) is the standard, most convenient material for a domestic concrete shed base, which is what this calculator assumes. Some tradespeople prefer buying sharp sand and gravel separately for tighter control over the mix, but for a garden shed or outbuilding base, all-in ballast plus cement is the normal, widely recommended approach.

As a starting point, this calculator estimates roughly 140-190 litres of water per cubic metre of concrete depending on your chosen mix ratio, but treat this as a guide rather than a fixed recipe - add water gradually while mixing until you reach a firm, workable consistency (it should hold its shape on a shovel, not be sloppy or pourable like a thin batter). Weather, ballast moisture content and mixer type all affect the exact amount needed on the day.

The number refers to the concrete's compressive strength in newtons per square millimetre (N/mm²) once cured - so C25 is stronger than C20, which is stronger than C15. In UK trade terms this usually corresponds to progressively richer cement-to-ballast ratios (roughly 1:6, 1:5 and 1:4 by volume). Stronger mixes cost a little more (more cement per m³) but handle heavier and more concentrated loads.

Usually not for a typical garden shed base on its own, but planning rules depend on your shed's size, height, position relative to boundaries, and whether it counts within your property's permitted development allowance - this varies by local planning authority and isn't something a materials calculator can determine for you. Check gov.uk's planning permission guidance or your local council's planning department before building anything larger than a standard garden shed.

Concrete typically reaches enough strength to walk on and start light building work after about 3-7 days, but continues gaining strength for around 28 days. As a rule of thumb, avoid putting significant weight (building materials, a full shed frame) on the base for at least a few days, and check the specific guidance from your cement supplier if you're working to a tight schedule or in cold weather, which slows curing.

This calculator assumes a simple rectangular base (length x width). For an L-shaped, octagonal, or otherwise irregular base, split it into two or more rectangles, calculate each separately, and add the resulting bag counts together - or use the largest bounding rectangle as a conservative overestimate if the shape is only slightly irregular.

For a typical garden shed base, bagged cement plus bulk-bag ballast (mixed by hand or with a small mixer) is usually the most practical and cost-effective option this calculator is sized for. Ready-mixed concrete delivered by lorry becomes more cost-effective for larger volumes (broadly above 1-2m³, i.e. many workshop-sized or larger bases) since you avoid hours of manual mixing - but minimum order quantities and delivery access for a lorry are worth checking first.

10% is a commonly used default safety margin for a reasonably level, straightforward base, covering typical spillage and minor over-dig. Use a higher margin (15-20%) for rougher, sloped, or uneven ground where more material is likely to be lost preparing the excavation, or where you want a comfortable buffer rather than needing to make an emergency extra trip to the builders' merchant mid-pour.

No. It estimates material quantities from widely used UK trade mix ratios and sizing guidance for a standard domestic garden shed or outbuilding base. It is not a substitute for a structural engineer's assessment for anything unusual or heavier-duty, and it does not cover planning permission or building regulations, which depend on your specific project and local authority.