Mixing concrete is not complicated, but small mistakes early on are expensive later. Get the ratio wrong, add too much water, or skip the curing and you end up with a slab that cracks, dusts or crumbles within a season. Get it right and a hand-mixed base will outlast the building on top of it.

This guide covers the mix ratios used on UK sites, how to mix by hand and with a mixer, how much water to add, how to judge consistency, and how to cure the pour so it reaches full strength. When you need exact quantities of cement, sand and ballast for your slab, work them out with our concrete calculator.

Understand the mix ratio

A concrete mix ratio is written as three numbers, for example 1:2:4. The first number is cement, the second is sand, and the third is coarse aggregate (stone). The numbers are parts by volume, so 1:2:4 means one bucket of cement to two of sand to four of aggregate.

Many UK builders mix with all-in ballast instead of separate sand and stone. Ballast is sand and gravel pre-blended, so the ratio collapses to two numbers, cement to ballast. A 1:2:4 mix becomes roughly 1 part cement to 6 parts ballast, which is the most widely used domestic ratio.

The common UK mixes

Ratio (cement:sand:aggregate) Rough ballast equivalent Typical grade What it suits
1:3:6 1:9 GEN1 (C12/15) Trench fill, kerb bedding, mass fill where load is low
1:2:4 1:6 GEN3 (C20) The general all-rounder: shed bases, paths, strip foundations, ground slabs
1:1.5:3 1:4 C25 and up Driveways, reinforced slabs, anything carrying heavy load

The 1:2:4 general mix is the one most domestic jobs need. Step up to 1:1.5:3 for a driveway or a load-bearing slab, and drop down to 1:3:6 only for low-strength fill where cost matters and strength does not.

A note on cement and sand: use Ordinary Portland Cement (the grey 25kg bags) and sharp sand for concrete, not the fine soft sand used for bricklaying mortar. Soft sand makes a weak, dusty concrete.

These ratios sit within the UK designated mix system under BS 8500. For anything structural, confirm the grade your design or building inspector requires before you mix.

Mixing by hand or with a mixer

By hand

Hand mixing is fine for small jobs, roughly up to a few barrow loads. Beyond that the work and the time get punishing, and consistency suffers.

  1. Lay out a clean, hard mixing surface: a mixing board, an old sheet of ply, or a clean section of slab. Bare grass or soil contaminates the mix.
  2. Measure your dry materials by the bucketful so every gauge is the same. Tip the sand or ballast out first, then the cement on top.
  3. Turn the dry pile over with a shovel at least three times, until the colour is an even grey with no streaks of pale cement or dark ballast.
  4. Form a crater in the middle, add a little water, and fold the dry edges into the water. Keep adding water in small amounts and turning the whole pile until it is uniform and workable.

The single most common hand-mixing error is adding too much water too fast. You cannot take it back out.

With a mixer

A drum mixer is worth hiring or buying for anything bigger than a couple of barrows. It mixes more evenly and far faster.

  1. Start the drum, then add roughly half your water to wet the inside.
  2. Add about half the ballast (or sand and aggregate), then the cement, then the rest of the ballast. Layering stops the cement balling up.
  3. Add the remaining water in small splashes until the mix looks right. Let it turn for two to three minutes after the last addition so it is fully blended.
  4. Tip into the barrow, and rinse the drum out before the residue sets hard.

Never run a mixer bone dry with cement in it, and never lean a shovel into a turning drum.

Getting the water right

Water is where most pours are won or lost. Too little and the concrete is stiff, hard to place and full of voids. Too much and you dilute the cement paste, which weakens the finished concrete and leaves a dusty, crack-prone surface.

As a rough working guide the water to cement ratio sits around 0.5 to 0.55 by weight. For a mix using one 25kg bag of cement, that is in the region of 13 to 14 litres of water, but treat that as a starting point, not a target. Aggregate that is already damp from the yard needs less added water.

Always add water gradually and judge by consistency, not by the bucket count.

Judging consistency (the site version of a slump test)

On big jobs, workability is measured with a formal slump test to BS EN 12350-2. For a hand or small mixer batch you can judge it by eye and feel:

  • Scrape the surface of the mix with a trowel. A good mix holds a small ridge or peak. If the ridges slump flat and shiny, it is too wet. If they crumble and crack, it is too dry.
  • It should look like thick, even porridge: cohesive, not soupy, not crumbly.
  • Squeezed in a gloved hand it holds its shape without water running out of it.

When in doubt, mix it slightly stiffer. You can always work it a little, but you cannot remove water.

Placing and finishing

Pour the concrete into your shuttering and work it into the corners with a shovel. Tamp it with a straight timber edge to compact it and bring a little paste to the surface, removing air pockets as you go. Level it off, then float or trowel to the finish you want. Do not overwork the surface or you will draw too much water and fines to the top, which weakens it.

Curing: do not skip this

Concrete does not dry, it cures. Curing is a chemical reaction called hydration where cement and water bond, and it needs moisture to keep going. Let the surface dry out too soon and that reaction stalls, leaving weak, dusty concrete.

According to Cemex UK’s curing guide, concrete reaches roughly three quarters of its strength after about a week and its practical 28-day strength after a full curing period, so keep it protected for as long as you can.

  • Keep the surface damp for at least the first seven days. Cover it with plastic sheeting, damp hessian, or spray it with water regularly to hold the moisture in.
  • Protect a fresh pour from frost. Avoid pouring when freezing temperatures are likely in the first 24 to 48 hours, and insulate the slab if cold weather catches you out.
  • In hot, dry or windy conditions, shade the pour and keep it damp so it does not dry too quickly and crack.

Stay off the slab for 24 to 48 hours, keep heavy load off it until it has cured properly, and reckon on 28 days for it to reach full strength.

Common mistakes to avoid

  • Too much water. The biggest cause of weak, dusty, cracking concrete. Add water slowly and stop sooner than you think.
  • The wrong sand. Soft building sand belongs in mortar. Use sharp sand or all-in ballast for concrete.
  • Inconsistent gauging. Eyeballing the parts gives you a different mix every batch. Use the same bucket for everything.
  • Mixing on dirt. Grass, soil and leaves contaminate the mix and weaken it. Always use a clean board or slab.
  • Skipping the cure. A pour left to dry in the sun or wind never reaches its strength. Cover it and keep it damp.
  • Pouring in frost. Concrete that freezes before it sets is permanently damaged.

FAQ

What is the strongest concrete mix ratio for a driveway?

A 1:1.5:3 mix, around grade C25, is the usual choice for driveways and other load-bearing slabs. A 1:2:4 general mix is the minimum many would accept for a light domestic drive, but step up for anything carrying vehicles regularly.

How much water do you add to a bag of cement?

For a 25kg bag of cement, somewhere around 13 to 14 litres is a reasonable starting point at a 0.5 to 0.55 water to cement ratio. Add it gradually and judge by consistency, because damp aggregate already carries water.

How long before you can walk on fresh concrete?

Usually 24 to 48 hours for light foot traffic, depending on the mix and the weather. Keep heavy loads off it and allow up to 28 days for it to reach full strength.

Can you mix concrete by hand?

Yes, for small jobs of up to a few barrow loads. For larger pours a drum mixer gives a more even mix and saves a lot of effort, and you can hire one cheaply by the day.

How much does it cost to mix your own concrete?

It varies by region and supplier, but as a rough guide a 25kg bag of cement runs about £5 to £9 and a 25kg bag of ballast a few pounds, with bulk bags of ballast (around 850kg to 1 tonne) typically falling somewhere around £45 to £70 plus VAT and delivery. Always get a current quote from your local merchant, as prices move with location and demand.

Work out your quantities first

Before you buy a single bag, work out exactly how much cement, sand and ballast your pour needs with our concrete calculator. Enter the length, width and depth of the slab and it gives you the volume and the materials for your chosen mix, so you order once and do not run short halfway through a pour.

Sources

Prices and figures here are general guidance for UK domestic work and vary by region, supplier and date. For structural work, follow your building inspector’s or engineer’s specified concrete grade.