Concrete Mix Ratio UK: Grades, Ratios and What to Use for the Job

A mix ratio is the proportion of the three ingredients by volume: cement, then fine aggregate (sharp sand), then coarse aggregate (20 mm gravel). A ratio written as 1:2:4 means one part cement to two parts sharp sand to four parts gravel. You can also buy the sand and gravel pre-blended as all-in ballast, in which case the ratio collapses to two numbers, such as 1:5 cement to ballast. The cement is what binds everything; the sand fills the gaps between the stones; the stone gives the mix its bulk and compressive strength. Change the proportions and you change how strong the finished concrete is.

Takeaway: a mix ratio is cement to sharp sand to gravel by volume; with all-in ballast it becomes cement to ballast.

These are the working volume ratios UK trades use, with the approximate strength grade each one lands near.

Use sharp sand, not building (soft) sand, in concrete; soft sand is for mortar and makes weak, sticky concrete. And for anything outdoors that takes vehicles, air-entrained mixes (PAV1 or PAV2 as ready-mix) resist frost and de-icing salt far better than a plain site mix, which is why driveways are usually ordered rather than mixed by hand.

Takeaway: match the ratio to the load; 1:2:4 is a general-purpose slab mix, not a universal answer, and driveways want a stronger 1:1.5:2.5 or a specified paving grade.

When you order ready-mix, nobody talks in ratios. They talk in grades under BS 8500, the British standard that governs concrete specification. The two systems most homeowners meet are the designated GEN mixes for general work and the C strength classes for anything more demanding.

GEN0 (C6/8): kerb bedding, haunching, backing to walls. GEN1 (C8/10): mass concrete foundations, trench fill, drainage, blinding, unreinforced shed and garage bases. GEN2 and GEN3 (C12/15, C16/20): general slabs, paths, and unreinforced floors. PAV1 (C25/30, air-entrained): external paving and driveways for cars. PAV2 (C28/35, air-entrained): heavier paving and light HGV areas. C25, C30 and up, or RC classes: reinforced and structural concrete, specified to an engineer's design.

The number in a class like C25/30 is the characteristic strength in newtons per square millimetre: the first figure from a cylinder test, the second from a cube. Higher number, stronger concrete.

Takeaway: site mixes are set by ratio, ready-mix is ordered by grade; GEN1 covers most unreinforced domestic bases and C25 or a PAV grade covers driveways.

Say you are mixing on site for a 3 m by 4 m shed base in Sheffield, 100 mm thick, unreinforced. That is a GEN1 job, so a 1:2.5:3.5 mix, or 1:6 with all-in ballast.

Volume of the slab: 3 x 4 x 0.10 = 1.2 m3. Add 10% for waste and slump: 1.2 x 1.10 = 1.32 m3 to mix. At a 1:6 cement-to-ballast mix, a rough materials estimate is about 7 to 8 bulk 25 kg cement bags per cubic metre, so roughly 10 to 11 bags of cement plus around 2 bulk bags of all-in ballast for the 1.32 m3. In 2026, budget roughly £55 to £65 in cement and £90 to £120 in ballast from a merchant like Travis Perkins or Wickes, so about £150 to £185 in materials, before mixer hire.

For a base this size you are at the upper limit of what is sensible to mix by hand or in a small mixer in a day. Much bigger, and ready-mix delivered as GEN1 usually works out cheaper per cubic metre and far less exhausting.

Takeaway: work out the volume, add 10%, then apply the ratio; a 12 m2 shed base at 100 mm is about 1.3 m3 of GEN1, roughly £150 to £185 in site-mixed materials.

Everyone obsesses over the cement-to-aggregate ratio and then ruins the mix with the one number nobody writes down: water. Concrete strength is governed more by the water-to-cement ratio than by the sand-to-stone split, and the instinct to "add a bit more water so it spreads easier" is the single most common way DIY slabs end up weak and dusty. Wetter concrete is easier to place and noticeably weaker once cured. Aim for a mix that holds its shape when you form a ridge in it with the shovel, not a soup. If it is too stiff to work, the fix is a plasticiser or more mixing, not more water. And curing matters as much as mixing: keep a fresh slab damp and covered for the first few days rather than letting it dry out fast in the sun or wind, which is when surface cracks and dusting set in.

Takeaway: keep the water down and the curing slow; a slightly stiff, well-cured mix beats a wet, easy-to-spread one every time.

For small jobs, a shed base, a few post holes, a short path, site mixing from bagged cement and ballast is cheaper and more flexible. Once you are past roughly 1.5 to 2 cubic metres, or the pour needs to go in continuously, ready-mix delivered to grade starts to win on both cost and quality, because it is batched accurately and you are not fighting the clock with a small mixer. Driveways almost always want ready-mix in an air-entrained PAV grade, because you cannot reliably air-entrain a hand mix, and frost resistance is exactly what a driveway needs.

Takeaway: site-mix small unreinforced jobs, order ready-mix by grade for anything over about 1.5 m3 or anything structural or frost-exposed.

On site, groundworkers rarely mix by eye. They gauge with a bucket so every batch is the same ratio, because inconsistent batches give a slab with soft patches. They keep the water tight and reach for a plasticiser when they need workability rather than slackening the mix. And for anything that carries a vehicle or a structure, they specify a grade and order it in, rather than trusting a hand mix to hit a strength it was never batched for. The rule of thumb they work to is simple: if it holds a load or faces the frost, order it; if it is light and unreinforced, mixing on site is fine.

Takeaway: gauge every batch the same, control the water, and specify a grade for anything load-bearing rather than mixing by eye.

Once you have picked the mix, size the pour with the Concrete Volume Calculator, or use the Concrete Slab Bag Calculator to turn a slab into a bag count. For the layer underneath, the Gravel Under Concrete Slab Calculator sizes the sub-base, and for mortar or screed on the same job the Mortar Calculator and Screed Calculator cover those mixes.

Disclaimer: This guide is for planning and general work only. Mix ratios given are common volume proportions for unreinforced domestic concrete and are approximate. Anything structural, reinforced, or safety-critical must be specified by grade to BS 8500 and Approved Document A, and confirmed with a qualified engineer or your ready-mix supplier.