Concrete Calculator

Concrete Mixing Guide

What's the key thing to understand about Bag Coverage?

Knowing how much area a bag of premix covers prevents the dispiriting trip back to the merchants mid-pour. A standard 25 kg bag of general-purpose concrete mix produces approximately 0.012 m³ of concrete (12 litres). A 40 kg bag produces about 0.019 m³ (19 litres). A worked example: a 1 m × 1 m × 100 mm slab is 0.1 m³, needing 0.1 ÷ 0.012 ≈ 9 bags of 25 kg or 6 bags of 40 kg. A typical fence post hole (300 mm diameter, 600 mm deep) is about 0.042 m³, needing 4 × 25 kg bags or 3 × 40 kg bags. Always add 10% to your calculated volume for waste, over-fill, and shape irregularities, which most calculators (including this one) do automatically. For larger pours, work out the volume first (length × width × depth, in metres, gives cubic metres), then convert to bags. Different concrete uses need different volumes: a typical garden path or patio base is 75–100 mm thick, a driveway 100–150 mm, structural foundations 150 mm or more depending on the load and soil. Don't underestimate post-hole concrete — fence posts especially need good volume to stay rigid in wind, and rushed undersize concrete causes posts to lean within a year. Bags are heavy and awkward; calculate the number you need before going to the merchants, and consider whether bagged mix is the right choice in the first place (see below).

What's the difference between Ready-Mixed and Bagged?

The decision between bagged concrete and ready-mix delivery is one of the most important practical choices for any concrete job, and getting it right saves money and back-breaking work. For volumes over about 0.5 m³ — equivalent to roughly 40 bags of 25 kg — ready-mixed concrete (delivered in a mixer truck) is usually more cost-effective and gives much more consistent results. The truck pours into wheelbarrows or directly into the formwork via a chute, and the concrete is mixed precisely to the right strength with the right water content. A typical driveway, garage base, or larger foundation is well over the 0.5 m³ threshold and is much easier with ready-mix. For small projects under about 0.3 m³ (around 24 bags), bagged premix is more practical: you can buy exactly what you need, mix only what you'll use in the next hour, and don't pay for a delivery you don't fully need. Between 0.3 and 0.5 m³ is the awkward middle ground — bagged is hard work but ready-mix can be wasteful if there's a minimum delivery charge (typically £80–150 plus the concrete cost). Another option for this range is a concrete trailer (some merchants rent these): you mix on-site with their machine, more concrete than bags but more flexible than a delivery. The biggest mistake with ready-mix is being under-prepared on delivery day: have the formwork ready, the access clear, the screed and trowels to hand, and enough labour, because once the truck arrives the concrete starts curing and there's no pausing it. For bags, the mistake is mixing in too-big batches and having concrete go off before it's used.

What should I know about Concrete Grades?

Different concrete grades are designed for different jobs, and choosing the right strength matters both for performance and for cost — buying high-strength mix for a job that doesn't need it is wasted money, while under-specifying for a structural application creates real risks. The grade number (C15, C20, C25, etc.) refers to the compressive strength in N/mm² (megapascals) at 28 days. C15 is a basic mix (1:2:4 cement:sand:aggregate by volume) for general non-structural work — garden paths, simple bedding, kerb support. C20 is the standard mix for domestic slabs, garden patios, and shed bases — strong enough for foot traffic and light loads. C25 is the right choice for driveways that take car weights, since vehicles concentrate loads on small tyre contact areas. C30 (1:1.5:3) is used for foundations, structural slabs, and reinforced work where strength matters. Higher grades (C35, C40+) are for heavily loaded structural work, usually with reinforcement and beyond domestic DIY. For post setting, a lean mix (1 part cement to 3 parts ballast, no aggregate separation needed) works fine for fence posts; for gateposts and structural posts, use C20 or postcrete (rapid-set concrete) for stronger anchoring. Postcrete (fast-setting, no aggregate-separation, just add water) is ideal for fence posts because it goes off in 5–10 minutes — you can plumb the post and have it set rigid by the time you've finished one row. For exposed external concrete (driveways, garden paths), a higher grade is more frost-resistant; air-entrained concrete is also available for areas exposed to freeze-thaw cycles.

What's the key thing to understand about Concrete Mix Ratios and Strength?

Concrete strength is determined by the water-to-cement ratio and aggregate proportions. Common mixes: C15 (1:2:4 cement:sand:aggregate) — general purpose, garden paths, light-duty. C20 (1:2:3) — most domestic slabs, shed bases, fence posts. C25 (1:1.5:3) — driveways, heavier loads. The critical variable is water content: excess water reduces compressive strength. Mix only as much as can be placed and finished within 90 minutes. For structural concrete — retaining walls, beams, foundations — always use a properly designed mix rather than a rough DIY ratio, since the consequences of underspecified structural concrete are serious.

Worked Example: Calculating Concrete for a Garden Shed Base

Take a shed base measuring 3m × 2.4m at a standard 100mm (10cm) thickness.

This base needs roughly 0.76 cubic metres of concrete including a sensible waste allowance — small enough that most ready-mix suppliers would quote a minimum load charge rather than an exact-volume price, which is worth checking before ordering, since many suppliers have a minimum order of around 1 to 1.5 cubic metres below which delivery becomes uneconomical. For a job this size, bagged concrete mixed on site (around 61 bags of 25kg general-purpose concrete mix) is often more practical and cheaper overall than paying a ready-mix minimum charge for less than a full load, even though mixing 61 bags by hand or with a small mixer is considerably more labour-intensive than having a truck pour it directly.

Curing Time and Strength Development

Concrete doesn't reach its design strength the moment it sets hard enough to walk on — it continues gaining strength for weeks after pouring through an ongoing chemical reaction (hydration) between cement and water. A widely used rule of thumb is that concrete reaches roughly 50% of its 28-day design strength within the first 3-7 days, enough to walk on carefully or support light loads, but it isn't considered fully cured and ready for its full rated strength until 28 days have passed, which is why structural engineers and concrete specifications almost universally reference "28-day strength" as the standard benchmark figure rather than any earlier point. Loading a slab too early — driving a vehicle onto a new driveway, or erecting a heavy shed on a base poured only a week earlier — risks cracking or settling because the concrete hasn't yet developed the strength the design assumes. Curing conditions matter too: concrete that dries out too fast in hot weather cures weaker and is more prone to surface cracking, which is why covering fresh concrete with plastic sheeting or periodically dampening it in warm weather (sometimes called "wet curing") is standard practice for anything beyond a small DIY job, and why pouring concrete in freezing conditions needs special additives or heated covers since hydration essentially stops below around 5°C.

Reinforcement: When Concrete Needs Mesh or Rebar

Plain, unreinforced concrete is adequate for many small domestic jobs like a shed base or garden path, but larger slabs, driveways, and anything subject to vehicle loading typically need steel reinforcement to resist cracking from ground movement and tensile stress that concrete alone handles poorly. Reinforcing mesh (a welded steel grid, commonly A142 or A193 mesh for domestic use, denoting the cross-sectional area of steel per metre) is laid within the slab, ideally supported on spacers so it sits roughly in the middle third of the slab's depth rather than resting on the ground or sitting too close to the surface, where it does little to resist cracking and can even promote surface corrosion staining over time. A rule of thumb used by many contractors is that any slab wider than about 3-4m in any direction, or subject to vehicle weight, benefits from mesh reinforcement, while smaller, foot-traffic-only slabs like shed bases often get by without it provided the sub-base beneath is well compacted and the slab itself isn't unusually thin. Reinforcement doesn't prevent all cracking — concrete still needs properly placed movement joints on long runs like driveways — but it keeps any cracks that do form tightly closed rather than allowing them to open up and let water in, which is the main long-term durability benefit.