A Detailed Guide to Concrete Mix Ratios: Getting it Right from the Start

Whether you’re building a patio, laying a foundation, or setting fence posts, getting your concrete mix ratio right is key. Too much cement and you’re wasting money; too little, and you risk weak, crumbly results. While there’s no “one size fits all” answer, understanding the basics and when to adapt will give your project the strength and durability it needs.

What Is a Concrete Mix Ratio?

In simple terms, a concrete mix ratio refers to the proportion of the main ingredients that make up concrete: cement, sand, and aggregate (stone or gravel). Sometimes water is added to the ratio, but it’s generally adjusted depending on the moisture content of the materials.

The standard way of writing a mix ratio looks like this: 1:2:4
This means 1 part cement, 2 parts sand, and 4 parts aggregate (measured by volume, not weight).

Why Mix Ratios Matter

• Strength: Some jobs need more strength than others. The right ratio ensures it holds up over time.

• Durability: In places with freeze-thaw cycles (like most of the UK), a weak mix will break down quicker.

• Ease of use: Workability changes with the mix. A smoother mix is easier to place and finish.

• Cost control: Cement costs more than aggregate, so using too much unnecessarily hits your wallet.

Common Concrete Mix Ratios and Their Uses

Here’s a breakdown of typical concrete mix ratios used in the UK, and what they’re best suited for from simple garden jobs to commercial-grade concrete.

General Purpose Mix – 1:2:4

Use for: Garden paths, steps, small walls, domestic slabs
This is a strong, versatile mix suitable for most light- to medium-duty tasks around the home.

Strong Mix – 1:1.5:3

Use for: Driveways, shed bases, small foundations
With more cement, this mix is stronger and great for areas that bear more load or traffic.

Lean Mix – 1:3:6

Use for: Non-structural concrete fill, mass foundations
A cheaper mix with lower cement content. Good for filling deep holes or bases under low-stress areas.

Fence Post Mix – 1:2.5:3.5

Use for: Setting wood or metal fence posts
This one sets relatively quickly and provides just enough strength to hold a post in place securely.

C30 Mix – 1:1:2.5 (approximate)

Use for: Reinforced slabs, load-bearing walls, light industrial floors
C30 concrete offers higher strength than most domestic mixes. You’d use this where structural integrity matters more like garage bases or commercial paving.

C40 Mix – 1:1.5:2.5 (approximate)

Use for: Structural beams, columns, heavy-duty floors, road slabs
This is a high-strength mix used in civil engineering and heavy-duty commercial applications. It’s not something you’d typically use at home unless you’re working on a large reinforced concrete structure.

Water-Cement Ratio

Water activates the cement, allowing it to bind the other ingredients together. But too much water weakens the final product. For high-strength mixes like C30 or C40, the water-cement ratio is kept low—often around 0.4 to 0.5. That means about 10–12.5 litres of water per 25kg bag of cement.

Always start with less water than you think you need. It’s easier to add a bit more than to try to fix an overly sloppy mix.

Should You Mix by Hand or Use a Mixer?

• For small jobs (like fixing a fence post or laying a step), hand mixing on a board or in a wheelbarrow is fine.

• For anything more than a few barrow-loads, a concrete mixer saves your back and helps ensure a consistent mix.

• For large-scale or structural work especially with mixes like C30 or C40 it’s better to order ready-mix concrete from a supplier. They’ll batch it properly and deliver it freshly mixed.

Quick Reference Table