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Concrete Mix Calculator

Work out cement, sand, aggregate and bag quantities for a pour.

How it works

Concrete is sold and mixed by volume, not weight, so the first step is always converting your project's dimensions into cubic metres — for a simple slab or path, that's length × width × depth, all measured in metres. Once you know the volume, a mix ratio (cement to sand to aggregate) determines how that volume splits into the weights of each raw material, which then convert into bag counts based on the bag sizes you're buying.

For a cylindrical shape like a fence post hole, the formula changes to account for the circular cross-section (using the radius, not a straight width) — using the wrong formula for the shape is a common source of ordering errors.

UK context

A common general-purpose mix ratio is 1:2:3 (one part cement, two parts sand, three parts aggregate), while foundations and other load-bearing work often use a stronger ratio such as 1:1.5:2.5 or 1:2:4. Stronger mixes use proportionally more cement — also the most expensive ingredient — so getting the ratio right matters for both durability and cost. See our full guide to working out how much concrete you need for a walk-through with worked examples.

If you're using pre-mixed bagged concrete rather than combining raw materials yourself, the bag's packaging usually states a yield (how much volume one bag produces) — in that case you can skip the ratio maths entirely and just divide your total volume by the stated yield.

Tips

Add a 5–10% buffer to your calculated volume — ground is rarely perfectly level and running short mid-pour is far more disruptive than having a little extra material.
For structural or load-bearing work (foundations, retaining walls), always check the manufacturer's guidance or consult a builder rather than relying on a general-purpose ratio.
Concrete needs time to cure properly before it takes full load — rushing this is a common cause of cracking and long-term weakness.

Frequently asked questions

How much extra concrete should I order over the calculated amount?

A buffer of roughly 5–10% is standard practice to account for an uneven sub-base, minor formwork gaps, and wastage during mixing or pouring.

What concrete mix ratio should I use for a garden path or slab?

A general-purpose ratio like 1:2:3 (cement:sand:aggregate) is commonly used for non-structural paths and slabs, but always check product-specific guidance, since ratios vary by manufacturer and intended use.

Do I need a different formula for a circular post hole?

Yes — a cylindrical shape like a post hole uses a volume formula based on the radius (not width), since it's a different geometric shape from a rectangular slab. Using a rectangular formula for a round hole will give the wrong answer.

Should I mix my own concrete or buy pre-mixed bags?

For small jobs, pre-mixed bagged concrete is usually simpler — just divide your total volume by the bag's stated yield. For larger pours, buying raw materials (cement, sand, aggregate) separately in bulk, or ordering ready-mix delivery, is often more cost-effective.

A quick note

Mix ratios, coverage rates and product yields vary by manufacturer and application — this page explains how the calculation works, not a substitute for product-specific technical guidance. Always check the manufacturer's data sheet, or consult a builder for structural or load-bearing work. See our full disclaimer.