Concrete Calculator UK

How to Calculate Concrete for a Slab

Working out concrete volume is the same for a slab, footing or any poured area - multiply length by width by depth. Here's a worked example for a 3m × 3m slab at 100mm deep:

1. Measure the area: 3m × 3m = 9 m².
2. Multiply by depth: convert depth to metres first - 100mm = 0.1m. 9 × 0.1 = 0.9 m³.
3. Add waste: 10% is standard for a straightforward pour. 0.9 × 1.10 = 0.99 m³.
4. Choose how to buy it: this calculator converts that volume into 25kg or 20kg bagged mix, ballast and cement to mix your own, or the m³ to order delivered.

Post holes work differently - the concrete only fills the gap between the hole and the post, not the whole hole volume, so this calculator uses the manufacturer's own published bags-per-hole-size table for postmix rather than a volume conversion (see below).

The Four Ways to Buy Concrete in the UK

These aren't interchangeable names for the same thing - each is a genuinely different product, and "ready-mix" in particular gets used loosely to mean more than one of them:

• Postmix / rapid-set bags: a fast-setting powder (Blue Circle Postcrete, Hanson Post Fix and similar) poured dry into a part-filled hole around a post. Sets in 5-10 minutes. For fixing posts only - not rated for general concreting.
• Bagged concrete mix: pre-blended aggregate and cement in 20kg or 25kg bags, sold by Wickes, B&Q and Travis Perkins - add water and pour. Convenient for small-to-medium jobs with no separate materials to measure out.
• Ballast + cement (mix your own): ballast (combined sand and gravel) and cement bought separately and mixed at a chosen ratio. Cheaper per m³ than bagged mix, but more measuring and mixing effort.
• Delivered concrete: ordered by the m³ and poured directly from a ready-mix lorry (fixed volume, minimum load applies) or a volumetric mixer truck (mixes on site, only charges for what's actually used - worth asking about for smaller jobs near the crossover point below, where a lorry's minimum load can be poor value).

Concrete Mix Ratios: 5:1 vs 6:1

For bagged mix and mixing your own, the ratio of ballast to cement (by volume) determines strength:

• 5:1 ballast:cement - roughly equivalent to a C20 mix, the standard specification wherever the concrete needs to bear real load: slabs that carry a structure, footings, and anything else load-bearing. Our shed base calculator uses this exact ratio for the same reason.
• 6:1 ballast:cement - a leaner, cheaper general-purpose mix. DIY Doctor's published mixing guide cites 6 parts ballast to 1 part cement as the standard ratio for garden paths and similar non-structural, non-load-bearing work. We don't recommend it for a shed base, footing or anything that has to carry weight without cracking - use the 5:1 mix for those.

When you mix dry ballast and cement with water, the finished concrete takes up noticeably less space than the loose dry materials did, since water fills the gaps between the aggregate particles. As a rule of thumb, you need about 1.5 times the loose dry volume for every cubic metre of finished, compacted concrete - this calculator applies that factor automatically.

Bags, Mixing Your Own, or Delivered: Where's the Crossover?

The right choice mostly comes down to volume. Small jobs favour bags because a delivery's minimum load and call-out charge outweigh any per-m³ saving; large jobs flip that the other way once the sheer physical effort of mixing by hand starts to dominate.

• Under roughly 1m³: bag it yourself. A single mixer or a few bought-in bags handle this comfortably in an afternoon.
• Roughly 1-2m³: genuinely borderline. Get a quote for a delivered load - including any minimum-load charge - and compare it against this calculator's bag cost before deciding. It can go either way depending on your local supplier and how much mixing time you're willing to put in.
• Over roughly 2m³: get it delivered. Mixing that much by hand or in a small mixer takes a very long time, and a delivered load usually ends up cheaper once your own labour is accounted for.
• Near the crossover specifically: ask whether a volumetric mixer truck is available in your area. Unlike a ready-mix lorry, it only charges for the concrete it actually discharges, which avoids paying for a lorry's minimum load when you're just over the 1m³ mark.

How Many Bags of Postmix for a Post Hole?

Fixing a garden fence post? Use our fence calculator - it gives bags per post directly by timber or concrete post type. For other posts (pergolas, rotary washing lines, gate posts, signs), this calculator uses Blue Circle's own published Postcrete Product Data Sheet (December 2023), which gives bags required by hole size and post size directly, since the concrete only fills the gap between the hole and the post - not a simple volume conversion. Laying block paving? Our block paving calculator counts the blocks, and this calculator covers the concrete for the edge restraints.

Full table (all hole sizes) is built into the calculator above - select your post size and hole size for an exact figure. Blue Circle's own guidance: Postcrete may be used for decking posts, rotary washing lines and small-to-medium gateposts, but not for general concreting or anywhere a specified compressive strength is required - for anything larger or load-bearing, use bagged concrete mix or ballast and cement at the structural 5:1 ratio instead.

Frequently Asked Questions

What's the difference between postmix, bagged concrete and ballast and cement?

Postmix (also called postcrete or post-fix) is a fast-setting powder for fixing fence, gate and other posts only - it sets in 5-10 minutes and isn't rated for general concreting. Bagged concrete mix is pre-blended aggregate and cement in 20kg or 25kg bags - add water and it's ready to pour, used for slabs and footings. Ballast and cement are bought separately and mixed at your own ratio - cheaper per m³ but more work. Delivered concrete (a ready-mix lorry or a volumetric mixer) is ordered by the m³ and poured directly, no bags or mixing at all.

How many bags of postcrete does a fence post need?

That depends on post type and hole size - see our fence calculator for the exact bags-per-post-type table (1.5 bags for a 75mm timber post, 2 for a 100mm timber post, 3 for a 100mm concrete post, based on a 250-300mm hole). This calculator's post-hole mode is for other posts - pergolas, washing lines, gate posts and signs - using the manufacturer's published hole-size table directly.

Is postmix strong enough for a pergola post?

Check the post size and load first. Blue Circle's own published guidance for Postcrete says it may be used for decking posts, rotary washing lines and small-to-medium gateposts (under 1m wide), but explicitly not for general concreting or anywhere a specified compressive strength is required. A light pergola post fits the first category; anything larger, load-bearing, or supporting a roof is safer built with bagged concrete mix or ballast and cement instead, mixed at the structural 5:1 ratio.

When does delivered concrete become cheaper than bags?

Under about 1m³, bagging it yourself (postmix, bagged mix, or ballast and cement) is almost always cheaper and simpler. Between 1 and 2m³, it's worth comparing the bag cost this calculator gives you against a local supplier's minimum-load charge. Above about 2m³, delivered concrete usually works out cheaper and is far less labour, since mixing that volume by hand takes a very long time.

What's the difference between a ready-mix lorry and a volumetric truck?

A ready-mix lorry pre-mixes concrete at a plant and delivers a fixed volume - you pay for the whole load even if you use less, and any surplus is wasted. A volumetric truck mixes on site from separate ingredients and only charges for what's actually used, which matters most for smaller jobs near the bagged-vs-delivered crossover point, where a fixed-volume lorry's minimum load can otherwise be poor value.