Concrete Calculator Guide - Volume & Mix Ratios UK

Concrete is the foundation of most UK garden and construction projects—from patio slabs and shed bases to driveways and fence posts. Getting the quantity right is crucial: order too little and you'll have an unfinished job; order too much and you're paying for waste you can't easily dispose of.

This guide covers the calculations for UK concrete projects, mix ratios for different applications, 2025 costs for ready-mix versus hand-mixing, and practical advice for pouring in British weather conditions.

Basic Volume Formula

Common Project Depths

Concrete Mix Ratios

Bags of Cement Needed

For hand-mixing with a 1:2:4 ratio:

Ready-Mix vs Hand-Mix

Ready-Mix Concrete UK 2025

Prices vary by region. Pumping adds £150-300. Saturday delivery typically 50% extra.

UK Concrete Strength Classes

Fence Post Calculation

Curing Times UK

UK Concrete Mix Ratios: C20, C25, and C30 Explained

In the United Kingdom, concrete is classified by its compressive strength, measured in Newtons per square millimetre (N/mm²) after 28 days of curing. The designation number refers to this strength rating, so C20 concrete achieves 20 N/mm² compressive strength. Understanding which grade you need is essential for both structural integrity and compliance with UK Building Regulations.

C20 (GEN 3): A general-purpose mix suitable for domestic foundations, floor blinding, and non-structural work. The typical ratio by volume is 1 part cement to 2 parts sand to 4 parts coarse aggregate (1:2:4). This is the most commonly used grade for garden projects, shed bases, and paths.

C25 (ST 2): A standardised structural mix used for domestic foundations, including those for house extensions and conservatories. The ratio is approximately 1 part cement to 2 parts sand to 3 parts coarse aggregate (1:2:3). Most UK local authorities require a minimum of C25 for strip foundations under Building Regulations Part A.

C30 (ST 3/PAV 1): A higher-strength mix used for reinforced foundations, driveways, and areas subject to vehicle loading. The ratio is roughly 1 part cement to 1.5 parts sand to 3 parts coarse aggregate (1:1.5:3). This grade is also specified for pavements and hard standings where durability under traffic is required.

Curing Time and Weather Considerations

Concrete does not simply dry out; it undergoes a chemical reaction called hydration that requires moisture and appropriate temperatures. Proper curing is critical for achieving the rated strength.

Under normal UK conditions (approximately 15 to 20 degrees Celsius), concrete reaches about 50% of its design strength within 3 days, 75% within 7 days, and its full rated strength at 28 days. However, UK weather frequently presents challenges that affect these timelines.

Cold weather: When temperatures drop below 5 degrees Celsius, the hydration process slows significantly. Below freezing, it can stop entirely, and if fresh concrete freezes before gaining sufficient strength, it can be permanently damaged. During winter months, it is advisable to use fast-setting cement or to cover freshly poured concrete with insulating blankets. Many UK builders avoid pouring concrete between December and February unless precautions are taken.

Hot weather: While less common in the UK, temperatures above 25 degrees Celsius can cause concrete to cure too quickly on the surface, leading to cracking. Keep the surface moist by covering it with damp hessian or polythene sheeting for at least the first 7 days.

Rain: Light rain on concrete that has already started to set (usually after 2 to 4 hours) is generally harmless. However, heavy rain on fresh concrete can wash cement out of the surface, weakening it. Always have tarpaulins or polythene sheets ready to cover freshly poured concrete if rain is forecast.

Coverage and Ordering from UK Suppliers

One cubic metre of mixed concrete covers the following approximate areas depending on thickness: 10 square metres at 100 mm thick, 6.7 square metres at 150 mm thick, and 5 square metres at 200 mm thick. Always order 5-10% more than your calculated requirement to account for uneven ground, spillage, and slight variations in the sub-base level.

Ready-mixed concrete delivery: For larger projects (typically 1 cubic metre or more), ordering ready-mixed concrete from a local batching plant is more practical and consistent than mixing by hand. Most UK suppliers deliver in standard mixer trucks carrying 6 cubic metres. Smaller loads can be ordered but may incur a surcharge for part-loads (often £15-£30 per cubic metre below the minimum). Companies such as Hanson, Cemex, and Tarmac operate across most of the UK.

Bagged concrete: For smaller projects, pre-mixed bags from B&Q, Wickes, or Screwfix are convenient. A standard 20 kg bag of post-mix yields approximately 0.009 cubic metres, so you would need about 110 bags per cubic metre. At typical retail prices of £4-£6 per bag, this works out considerably more expensive than ready-mixed delivery for anything beyond very small jobs.

Reinforcement for Concrete Projects

Many UK concrete projects require steel reinforcement to prevent cracking and increase tensile strength. For domestic driveways and garage floors, A142 or A193 steel mesh is commonly specified. This mesh comes in standard sheets of 4.8 m x 2.4 m and is laid on spacers within the concrete pour to sit approximately one-third of the way up from the bottom of the slab.

For strip foundations, reinforcement bars (rebar) may be required depending on ground conditions and the structural engineer's specification. In areas with clay soils prone to shrinkage, such as much of the South East of England, deeper foundations with additional reinforcement are often necessary.

Frequently Asked Questions

How much concrete do I need for a shed base?

A typical garden shed base of 2.4 m x 1.8 m with a concrete thickness of 100 mm requires approximately 0.43 cubic metres of concrete. Using C20 mix, this equates to roughly 48 bags of 20 kg post-mix or a very small ready-mix delivery. Adding 10% for wastage brings the total to approximately 0.47 cubic metres. For larger sheds or workshops, increase the thickness to 150 mm for added durability.

Can I pour concrete in the rain in the UK?

Light drizzle on concrete that has been setting for at least 2-4 hours is unlikely to cause problems. However, you should never pour concrete into standing water or during heavy rainfall, as this washes the cement paste from the mix and severely weakens the finished product. Check the Met Office forecast before scheduling a concrete pour, and always have covering materials on site. If rain begins during a pour, cover the surface immediately with polythene sheeting.

What is the minimum thickness for a concrete driveway in the UK?

For a domestic driveway supporting standard cars, the minimum recommended concrete thickness is 100 mm (4 inches) over a compacted sub-base of at least 100 mm of MOT Type 1 hardcore. If heavier vehicles such as delivery lorries or motorhomes will use the driveway, increase the concrete thickness to 150 mm and use C30 grade concrete with A193 steel mesh reinforcement. The sub-base must be thoroughly compacted using a plate compactor or whacker plate before pouring.

Concrete in the UK: Standards, Suppliers, and Sustainability

The United Kingdom consumes approximately 100 million tonnes of concrete each year, making it the most widely used construction material in the country. The UK concrete industry is worth over 5 billion pounds annually and supports approximately 70,000 jobs across manufacturing, delivery, and installation. Major UK ready-mix concrete suppliers include Hanson, Cemex, Tarmac, and Aggregate Industries, all of which operate batch plants throughout England, Scotland, Wales, and Northern Ireland.

Concrete in the UK is specified according to British Standards and European Norms, primarily BS 8500 and BS EN 206. For domestic projects, the most commonly ordered mixes are C20 (general-purpose foundations and footings), C25 (driveways and footpaths), C30 (heavier-duty applications and reinforced foundations), and C35 (commercial structural work). The "C" number refers to the characteristic compressive strength in megapascals (MPa) at 28 days. When ordering ready-mix concrete, you will need to specify the strength class, maximum aggregate size, consistency (slump), and any special requirements such as sulfate resistance for clay soils.

For smaller domestic projects, many UK homeowners opt for bagged concrete from builders' merchants such as Jewson, Travis Perkins, or Buildbase, or from DIY retailers like B&Q and Wickes. Standard bags come in 20 or 25 kilogram sizes and typically yield around 0.01 cubic metres per bag. For larger volumes, ready-mix concrete delivered by a truck-mounted mixer is more economical and consistent. Minimum order quantities for ready-mix delivery in the UK are typically 1 to 3 cubic metres, with prices ranging from 65 to 120 pounds per cubic metre depending on strength, additives, and location.

The UK construction industry is increasingly focused on reducing the carbon footprint of concrete, as cement production accounts for approximately 5 percent of global CO2 emissions. UK concrete manufacturers now offer low-carbon alternatives using ground granulated blast-furnace slag (GGBS), pulverised fuel ash (PFA), and recycite aggregates to reduce embodied carbon by up to 50 percent compared to traditional Portland cement mixes.

Practical Concrete Tips for UK DIY Projects

• Check whether you need planning permission: Most domestic concrete work such as garden paths, shed bases, and patios falls under permitted development in England and Wales. However, if the concrete area covers more than 5 square metres, you must use permeable materials or direct surface water to a lawn or border to comply with drainage regulations and avoid the need for planning permission.
• Allow for UK weather conditions: Do not pour concrete when temperatures are below 3 degrees Celsius or when heavy rain is forecast within 24 hours. In the UK, the ideal concreting season runs from April to October. If pouring in colder months, use frost protection blankets and consider a faster-setting mix.
• Calculate volume accurately to avoid waste: Ready-mix concrete is perishable and begins setting within 90 minutes of mixing. Over-ordering wastes money and creates disposal problems, as you cannot simply dump excess concrete. Under-ordering leaves an incomplete job and a visible cold joint where new concrete meets partially set concrete.
• Hire a mixer for medium-sized projects: For projects requiring 0.5 to 2 cubic metres, consider hiring a cement mixer from a tool hire company such as HSS or Speedy. A half-bag mixer costs approximately 30 to 50 pounds per day to hire and allows you to mix concrete at your own pace using bags of cement, sand, and aggregate from a builders' merchant.

Concrete Project Questions

James Mitchell, ACCA

Chartered Accountant & Former HMRC Advisor

James is a Chartered Certified Accountant (ACCA) specialising in UK personal taxation and financial planning. With over 12 years in practice and a background as a former HMRC compliance officer, he brings authoritative insight to complex tax topics.