Concrete Mix Ratios Explained: A Guide for DIYers and Self-Builders

Concrete is one of those materials that looks simple until you actually need to use it, and then the questions start. How much cement? What sand? Do I need ballast? What about a bagged mix versus mixing from scratch? This guide cuts through the confusion. We'll explain what concrete mix ratios mean, which mix you need for common jobs, and how to get it right first time.

What Is a Concrete Mix Ratio?

A concrete mix ratio tells you the proportions of cement, sand and aggregate (gravel or ballast) to combine. The ratio is always written as cement : sand : aggregate. So a 1:2:3 mix means 1 part cement, 2 parts sand, 3 parts aggregate. The more cement in the mix, the stronger, and more expensive, the concrete. Different jobs need different strengths.

The Most Common Concrete Mixes

1:2:3 — General Purpose / Structural

The workhorse mix. Strong and durable, suitable for most structural applications. Use it for strip footings and foundations, ground beams, garage floors and large slabs, and structural repairs.

1:2:4 — Moderate Strength General Use

Slightly leaner than 1:2:3, fine for less heavily loaded slabs and non-structural work: outbuilding floors, paths and walkways, drive edgings, base layers under paving.

1:3:6 — Blinding / Lean Mix

A weak mix used where you need a firm, even surface rather than structural strength. Common uses: blinding under foundations, sub-base levelling, non-load-bearing infill.

1:4 (Cement : Sharp Sand) — Post Mix

No coarse aggregate — just cement and sharp sand. Used for setting fence posts, gate posts, and anchoring garden structures. Many people use rapid-set post mix bags for this, available from MacBlair in handy 20kg bags.

1:5 or 1:6 (Cement : Sand) — Mortar

Strictly mortar, not concrete, but the same ratio logic applies. Use for bedding kerbs and edgings, bricklaying, and pointing paving slabs.

Mixing From Scratch vs Bagged Concrete

• Mixing from scratch: For large pours, mixing from bulk bags of sand and ballast is more economical. All-in ballast (combined aggregate) is the simplest approach, mix 1 part cement to 5 parts all-in ballast for a general-purpose mix.
• Bagged ready-mix: For smaller jobs such as setting a post or patching a step, a pre-blended bagged mix is far more convenient. Available in a range of sizes from MacBlair branches.
• Ready-mix delivery: For large structural pours such as house foundations and large floor slabs, ordering ready-mixed concrete delivered by truck is the professional choice.

Tips for Getting Your Mix Right

• Measure by volume, not weight — use a bucket as your measure for consistency
• Water is critical — the mix should hold its shape when squeezed, not run. Add water gradually
• Mix thoroughly — dry mix all ingredients first, then add water gradually over at least two minutes
• Work quickly — once water is added you have roughly 30–45 minutes before the mix goes off
• NI weather note: don't pour concrete when frost is expected within 24 hours

Frequently Asked Questions