Concrete Mixing Ratios Explained

A concrete ratio is parts of cement : sand : coarse aggregate, measured by volume. For most garden and landscaping jobs you only need three: 1:2:4 (general purpose — pads, post footings, shed bases), 1:3:6 (mass fill and blinding), and 1:1.5:3 (stronger structural duty). The other half of the recipe is water discipline: add as little as gives you a workable mix, because excess water is the single biggest strength-killer in site-mixed concrete.

What each ingredient does

• Cement — the glue. More cement = more strength and cost, up to a point; it does nothing until water triggers the cure (hydration).
• Sand (fine aggregate) — fills the gaps between stones. Use sharp/grit sand for concrete, not soft building sand.
• Coarse aggregate — 10–20mm stone; the skeleton that carries load. "Ballast" is pre-blended sand+stone, convenient for small jobs (a common all-in recipe is 1 cement : 5 ballast for general purpose work).
• Water — enough to hydrate the cement and make the mix placeable, and no more. A mix that slumps like porridge will cure weak and dusty.

The common UK ratios and when to use them

• 1:3:6 — mass fill / blinding. Low strength, low cost: filling over-dig, blinding over hardcore, haunching where strength isn't critical.
• 1:2:4 — the general-purpose standard. Shed and greenhouse bases, fence-post footings, small pads, washing-line spurs — the default answer for domestic concrete.
• 1:1.5:3 — medium-high strength. Slabs taking heavier loads, garage floors, footings for masonry structures like an outdoor kitchen counter.
• 1:1:2 — high strength. Rarely needed domestically; thin structural sections and repairs where maximum strength is wanted.

For load-bearing or regulated work (house foundations, structural slabs), specifications call up designed concrete grades — order ready-mix to the engineer's spec rather than translating to buckets.

Concrete vs mortar — don't swap them

Mortar is cement + sand only (often with plasticiser). It bonds masonry and beds paving; it is not a structural pour. The two you'll actually use around paving: bedding mortar at about 1:4 cement:sharp sand mixed semi-dry (see the laying guide), and pointing mortar at about 1:3 for joints (jointing methods). Concrete with its coarse stone would make both jobs impossible.

Measuring ratios accurately

Use a bucket, not a shovel: shovelfuls of damp sand and dry cement weigh wildly differently. Gauge every ingredient with the same bucket, level-filled — 1:2:4 is one bucket cement, two sand, four stone. Batch consistently and successive mixes will colour-match and cure alike, which matters anywhere concrete stays visible.

Mixing: by hand or mixer

1. Blend the dry ingredients first — turned three times on a board, or 2–3 minutes in a drum mixer.
2. Form a crater, add roughly half the water, and turn/mix.
3. Add remaining water a little at a time until the mix just moves and holds a ridge — then stop.
4. Use within about 45 minutes to an hour; never "refresh" a stiffening mix with extra water (retempering kills strength).

Weather rules

• Cold: don't pour below about 3°C and falling — hydration stalls and frost can wreck an uncured pour. Cover fresh work with insulation or hessian in marginal weather.
• Hot/windy: the mix loses water too fast — dampen the sub-base first, work in shade if you can, and cover or mist-cure the surface so it doesn't dry before it cures.
• Curing: keep concrete damp for the first days rather than drying it out; it reaches most of its working strength at 28 days but takes foot traffic much sooner.

Safety

Wet cement is caustic: it burns skin on prolonged contact and the dust harms lungs and eyes. Waterproof gloves, eye protection and a dust mask when handling dry cement are non-negotiable, and any cement in the eye is an immediate rinse-and-A&E job.

Frequently asked questions