Convert concrete and building material measurements between metric, imperial and trade units — including metres, millimetres, feet, inches, cubic metres, cubic yards, litres, tonnes, kilograms, bulk bags and 25kg bags.
Free UK Concrete Unit conversion guides for 2026
Construction work in the UK is mostly specified and sold in metric units, but real projects still involve a mix of imperial and metric measurements. A homeowner might measure an old shed base in feet, a drawing might specify slab depth in millimetres, a US guide might talk about cubic yards, a builder’s merchant might sell 25kg bags, and a ready-mix supplier will usually ask for cubic metres. This page brings those conversions together in one practical concrete and building-material unit conversion guide.
The resource is designed for everyday UK estimating: concrete slabs, footings, garage bases, patio sub-bases, paths, fence post holes, screed, ballast, sand, gravel and MOT Type 1. It does not replace a structural working, specification or Building Control approval. It simply makes the measurement side clearer so you can move from the units you have to the units suppliers actually use.
UK trade note — metric is the legal selling basis
UK guidance says packaged and loose goods are generally sold using metric measurements such as grams, kilograms, millilitres and litres. Imperial can still appear as a supplementary measurement in some circumstances, but for buying concrete, aggregates and bagged materials you should keep your final order in metric units. SeeGOV.UK weights and measures guidance.
Metric to Imperial
Convert mm, cm and metres into inches, feet and yards for older drawings, imported guides and renovation measurements.
m³, ft³ and yd³
Move between UK-ready cubic metres, cubic feet and cubic yards without confusing US concrete guides with UK supplier units.
Bags and Bulk Bags
Estimate how many 20kg, 25kg, 800kg, 850kg or tonne bags are needed from a volume and material density.
Core Concrete Conversion Formulas
Most concrete estimating problems start with a simple volume working. Once you have length, width and depth in the same unit, volume is straightforward. The common UK mistake is mixing metres and millimetres in the same working. Always convert depth first: 100mm becomes 0.1m, 150mm becomes 0.15m, and 200mm becomes 0.2m.
Concrete volume:Length (m) × Width (m) × Depth (m) = Volume (m³)
Material mass:Volume (m³) × Density (kg/m³) = Kilograms
Bag count:Kilograms ÷ Bag size (kg) = Number of bags
Why millimetres matter
UK drawings and building notes usually express slab depth and insulation thickness in millimetres because it is more precise and avoids decimals. Concrete suppliers, however, price ready-mix by cubic metre. A 4m × 3m slab at 100mm deep is not 1,200m³; it is 4 × 3 × 0.1 = 1.2m³. That single unit step is the difference between a sensible order and an impossible one.
For small domestic pours, it is usually worth adding a sensible waste allowance. Concrete can be lost in uneven excavation, formwork gaps, barrow residue, pump lines or small level changes. Aggregates can compact, settle or spread beyond the measured edge. The conversion guide can show a 10% allowance for material yield mode, but important pours should follow the project specification and supplier advice.
Quick rule
If your project is measured in millimetres, divide by 1,000 before using the m³ formula. If your project is measured in inches or feet, convert to metres first for UK ordering.
Metric and Imperial Concrete Conversion Tables
Use these quick reference figures when checking a quote, a merchant product page, an old plan or a US-based construction guide. The guide above uses more precise values, while the tables below are rounded for practical site use.
Length and depth conversions
- Imperial Unit: 1 inch; Metric Equivalent: 25.4mm; Practical UK Use: Small thickness/depth checks; Site Rounding Note: Do not round where precision matters
- Imperial Unit: 4 inches; Metric Equivalent: 101.6mm; Practical UK Use: Light slab examples; Site Rounding Note: Often discussed as 100mm
- Imperial Unit: 6 inches; Metric Equivalent: 152.4mm; Practical UK Use: Heavier slab examples; Site Rounding Note: Often discussed as 150mm
- Imperial Unit: 1 foot; Metric Equivalent: 0.3048m; Practical UK Use: Older plans and room sizes; Site Rounding Note: Use metres for final volume
- Imperial Unit: 1 yard; Metric Equivalent: 0.9144m; Practical UK Use: US guides and cubic yards; Site Rounding Note: Use metres for UK orders
Volume conversions
- Volume Unit: 1m³; Equivalent: 1,000 litres / 35.315 ft³ / 1.30795 yd³; Common Use: UK ready-mix ordering; Warning: Main unit for concrete supply
- Volume Unit: 1 cubic yard; Equivalent: 0.76455m³ / 764.6 litres; Common Use: US guides and guides; Warning: Do not order UK concrete in yd³
- Volume Unit: 1 cubic foot; Equivalent: 0.02832m³ / 28.32 litres; Common Use: Older imperial working; Warning: Small error becomes large over areas
- Volume Unit: 1 UK gallon; Equivalent: 4.54609 litres; Common Use: Legacy liquid volume; Warning: Different from US gallon
- Volume Unit: 1 US gallon; Equivalent: 3.78541 litres; Common Use: Imported product data; Warning: About 17% smaller than UK gallon
Concrete, Sand, Ballast and Aggregate Density Guide
Length and volume conversions are exact, but bag and tonne estimates depend on density. Density changes with moisture, grading, compaction, voids and supplier specification. That is why the guide uses typical estimating densities, not laboratory-certified design values. For purchasing, always check the merchant’s product label, delivery note or data sheet.
Density is not one fixed number
A cubic metre of dry loose topsoil is not the same weight as a cubic metre of compacted MOT Type 1 or cured concrete. Use density conversions for estimating, then confirm the exact product coverage with your supplier.
- Material: Normal concrete; Typical Density Used: 2,400 kg/m³; 1m³ Approx. Weight: 2.4 tonnes; Useful For: Mass checks and structural load awareness
- Material: Ballast; Typical Density Used: 1,750 kg/m³; 1m³ Approx. Weight: 1.75 tonnes; Useful For: Site-mix concrete estimating
- Material: Sharp sand; Typical Density Used: 1,600 kg/m³; 1m³ Approx. Weight: 1.6 tonnes; Useful For: Screed and bedding estimates
- Material: Building sand; Typical Density Used: 1,500 kg/m³; 1m³ Approx. Weight: 1.5 tonnes; Useful For: Mortar estimates
- Material: Gravel / shingle; Typical Density Used: 1,650 kg/m³; 1m³ Approx. Weight: 1.65 tonnes; Useful For: Drainage and decorative stone
- Material: MOT Type 1 compacted; Typical Density Used: 2,000 kg/m³; 1m³ Approx. Weight: 2.0 tonnes; Useful For: Sub-base working
Bag working are best treated as buying estimates. For example, 1m³ of ballast at 1,750kg/m³ is roughly 70 × 25kg bags before any waste allowance. But if the material is damp, loosely filled, compacted differently or sold as a bulk bag with a supplier-specific nominal weight, the real coverage can vary.
How UK Builders Actually Use Metric and Imperial Units
In day-to-day UK construction, the unit depends on the job stage. Drawings often use millimetres for dimensions because they reduce ambiguity. Materials are commonly ordered in cubic metres, tonnes, kilograms, linear metres, square metres or board counts. Homeowners, however, often think in feet and inches for old rooms, sheds, garden areas and renovation spaces.
This page is especially useful when you are joining those worlds together. You may measure a driveway in feet, convert the area to square metres, convert the sub-base depth from inches to millimetres, work out a volume in cubic metres, then convert that volume into tonnes or bulk bags. That is a normal UK estimating workflow.
Ready-mix concrete
Normally ordered in cubic metres. Suppliers may have minimum loads, part-load charges, waiting time and delivery-zone pricing.
Bagged concrete
Usually sold by kilograms per bag. 20kg and 25kg bags are common, with yield shown on the product label or data sheet.
Aggregates
Often sold in bulk bags, loose tonnes or 25kg bags. Coverage depends heavily on depth, compaction and moisture content.
2026 construction cost context
Unit conversions do not set prices, but they help compare prices correctly. In 2026, UK material estimates should allow for changing material markets, delivery costs, regional labour variation and taxation on primary aggregates. GOV.UK’s May 2026 construction materials commentary reported that the “All Work” material price index increased by 3.2% in April 2026 compared with April 2025. GOV.UK also lists the Aggregates Levy rate from 1 April 2026 as £2.16 per tonne for taxable aggregate in England, Wales and Northern Ireland, with Scotland moving to devolved arrangements. SeeGOV.UK construction materials commentaryandGOV.UK Aggregates Levy rates.
Worked Examples for Concrete Unit Conversion
Example 1 — feet and inches to m³ for a shed base
A homeowner measures a shed base as 10ft × 8ft and wants it about 4 inches thick. Convert each dimension first: 10ft = 3.048m, 8ft = 2.438m and 4 inches = 101.6mm = 0.1016m. The volume is 3.048 × 2.438 × 0.1016 = about 0.755m³. With a small allowance, the practical order may be around 0.85m³ depending on supplier minimums and site conditions.
Example 2 — cubic yards to cubic metres
A US article recommends 2 cubic yards of concrete for a small slab. In UK units, 2 yd³ × 0.76455 = 1.529m³. You would normally discuss this as about 1.53m³ before adding a project-specific waste allowance.
Example 3 — m³ to 25kg bags of ballast
If you need 0.5m³ of ballast and use a typical density of 1,750kg/m³, the estimated mass is 875kg. Dividing by 25kg gives 35 bags. With 10% extra, that becomes around 39 bags. A bulk bag may be cheaper and easier if access allows.
Example 4 — square metres and depth to litres
A 12m² area with a 50mm bedding layer is 12 × 0.05 = 0.6m³. Since 1m³ equals 1,000 litres, that is 600 litres of material before wastage or compaction adjustment.
Common Concrete Unit Conversion Mistakes
Using mm as metres
100mm must be entered as 0.1m in a volume working. Using 100 as metres creates an enormous overestimate.
Mixing UK and US gallons
UK imperial gallons and US liquid gallons are different. This matters for liquids, admixtures and imported product data.
Ignoring density
m³ to tonnes is material-specific. Concrete, sand, ballast, MOT Type 1 and topsoil all have different typical densities.
Ordering exactly the determined volume
Small wastage, uneven excavation and compaction can leave you short. Add a sensible allowance where appropriate.
Copying US concrete bag yields
US bag sizes and cubic-yard workflows do not always match UK merchant products. Check local bag labels.
Rounding too early
Convert accurately first, then round at the end. Early rounding can become significant on larger projects.
Which Concrete Unit conversion guide Should You Use?
Different parts of a concrete project use different units. The same job can move through millimetres, metres, square metres, cubic metres, kilograms and tonnes before anything is delivered. Choosing the right conversion guide saves time and prevents costly ordering mistakes.
Use the length/depth conversion guide when checking thickness
Depth is where many concrete mistakes start. A slab depth might be described as 4 inches by an older builder, 100mm on a UK guide, or 0.1m in the final volume formula. These are close but not always identical. For ordinary estimating, 4 inches is commonly treated as approximately 100mm, but 4 inches is actually 101.6mm. For structural work, levels, thresholds, insulation build-ups and drainage falls, use the exact specified value rather than a casual rounded value.
Use the area conversion guide for slabs, paths and driveways
Area conversion is useful when a homeowner measures a garden in feet but a contractor quotes per square metre. A patio that is 20ft by 12ft is 240ft². Converted to metric, that is about 22.3m². If a sub-base is then required at 150mm deep, the volume is 22.3 × 0.15 = 3.35m³ before compaction and wastage. Area alone does not tell you volume; it must be combined with depth.
Use the volume conversion guide for ready-mix ordering
Volume is the main result for concrete. UK ready-mix concrete suppliers usually quote in cubic metres, so this should be your final ordering unit. Cubic feet and cubic yards are useful when reading imported guides, but UK suppliers are unlikely to price a domestic delivery in those units. Convert the imported figure back to m³, add your allowance, and then check supplier minimum load and part-load charges.
Use the mass conversion guide for bags, aggregates and handling
Mass matters when buying bagged products, planning van loads, checking trailer limits or organising manual handling. A 25kg bag is manageable for some workers but still requires safe lifting practice. An 850kg bulk bag needs mechanical delivery and a suitable flat area for offloading. A tonne of aggregate is not a volume until you know the material density.
Use the density conversion guide for m³ to tonnes
Density links volume and mass. It is the reason 1m³ of concrete is much heavier than 1m³ of loose topsoil. The guide includes common estimating densities, but these are not a replacement for supplier data. Dense, wet or compacted materials can weigh more than expected. Lightweight insulation, voided blocks and foamed materials can weigh far less than ordinary aggregates.
Step-by-Step UK Concrete Conversion Workflow
For most home and small-site projects, the safest workflow is to convert dimensions first, work out volume second, apply wastage third, and only then compare ready-mix, bags or site-mix materials. Jumping straight from feet to bags or from inches to tonnes usually creates avoidable errors.
- Step 1 — Measure accurately:Measure length and width at several points if the area is not perfectly square. Use the average only if the shape is genuinely regular; otherwise split the area into rectangles.
- Step 2 — Convert all dimensions to metres:Convert feet, inches, centimetres or millimetres before working out m³. Keep enough decimal places until the end.
- Step 3 — Work out volume:Multiply length × width × depth. For irregular shapes, work out each section and add them together.
- Step 4 — Add allowance:Add 5–10% for many domestic slabs, more for uneven excavations, deep footings or awkward access. Do not add arbitrary waste to engineered pours without checking the specification.
- Step 5 — Choose supply method:Ready-mix suits larger and time-sensitive pours; bagged concrete suits very small repairs; site mixing may suit remote or awkward access but requires more labour and quality control.
- Step 6 — Confirm with supplier:Check minimum order, delivery access, discharge time, waiting charges, product grade, slump and any extra services such as pumps or barrow-in labour.
Best estimating habit
Keep a written trail: original measurements, converted metres, determined m³, allowance, final order and supplier confirmation. This prevents arguments and helps if the project changes.
Bagged Concrete and Aggregate Conversion Guide
Bagged products are convenient, but they are easy to underestimate. Small bags look cheap until the bag count rises, and multiple bags can create more handling, storage and mixing time than expected. The guide’s material-yield mode helps turn m³ into kilograms, 25kg bags and bulk bags, but the final buying decision should consider access, labour and waste.
20kg and 25kg bags
Small bags are useful for patch repairs, fence posts, small pads, doorstep repairs and awkward areas where a ready-mix truck cannot reach. For larger slabs, the number of bags rises quickly. For example, if a bag yields roughly 0.01m³ of mixed concrete, 1m³ can require around 100 bags. That is a lot of lifting, mixing water, clean-up and time. Bag yield varies by product, so always check the manufacturer’s label.
Bulk bags and tonne bags
Bulk bags are better for aggregates such as ballast, sand, gravel and MOT Type 1. A merchant may describe a bag as a bulk bag, jumbo bag, dumpy bag or tonne bag, but the actual filled weight may be 800kg, 850kg or 1,000kg depending on supplier and product. Never assume every bulk bag equals exactly one tonne. If your working is close to the boundary, check the stated bag weight before ordering.
Loose loads
For larger quantities, loose delivery by tipper or grab vehicle may be cheaper than buying many bags. Loose aggregate is usually priced by tonne or by load, and the conversion from m³ to tonnes depends on density. Access is important: the lorry needs a safe place to tip, and the material needs to be stored where it will not block drains, pavements or neighbours.
Manual handling note
Repeatedly lifting 20kg or 25kg bags can still cause injury. Plan storage, keep bags dry, avoid twisting lifts, and use mechanical help for bulk bags. CheckHSE manual handling guidancefor safe lifting principles.
UK Regulations, Selling Units and Trade Accuracy
ConcreteMath.co.uk unit conversion guides are built around practical UK trade use. They show imperial values because people still use feet, inches, cubic feet and cubic yards in conversation, old plans and imported content. But UK material supply is normally based on metric quantities. This is especially important for quotes, invoices, product labels and building-material comparisons.
GOV.UK guidance states that goods sold by weight or measure in Great Britain generally use metric measurements. It also explains that packaged goods must show their quantity clearly and that metric units are the primary legal basis in most trade settings. For construction customers, that means final supplier communication should use m³, m², linear metres, kilograms, tonnes or litres depending on the product.
When a supplier gives a quote, check whether VAT is included, whether delivery is included, whether the product is sold by exact weight or nominal bag weight, and whether the quoted coverage assumes a certain depth. A bag that covers 10m² at 10mm deep covers only 2m² at 50mm deep. A unit conversion cannot fix a misunderstood coverage depth.
Accuracy policy for this conversion guide
Exact unit conversions, such as inches to millimetres or cubic feet to cubic metres, use standard mathematical factors. Material yield working use typical estimating densities and therefore produce planning estimates, not guaranteed delivered volumes. The page is intended for quantity planning, quote comparison and homeowner understanding, not laboratory testing, structural engineering or dispute resolution.
Concrete Unit Glossary
Cubic metre
The main UK volume unit for ready-mix concrete. Equal to a cube 1m long, 1m wide and 1m deep.
Cubic foot
An imperial volume unit used in older and overseas material guides. 1ft³ is about 0.0283m³.
Cubic yard
A common US concrete unit. 1yd³ is about 0.7646m³, so it must be converted for UK ordering.
Kilogram
The everyday metric mass unit used for bagged concrete, cement, sand and many packaged products.
Tonne
1,000kg. Common for loose aggregates, sub-base, ballast and bulk deliveries.
Density
Mass per unit volume, normally kg/m³. Used to estimate tonnes or bags from cubic metres.
Before You Order: Unit Conversion Checklist
Before you send a quantity to a supplier, run through this short checklist. It is deliberately simple, because the most expensive mistakes are usually simple unit mistakes: a depth left in millimetres, a cubic-yard figure copied from an American page, a bulk bag assumed to be one tonne, or a determined volume ordered with no allowance for uneven ground.
- Have all dimensions been converted to metres?Length and width can be in metres, but depth is often still in millimetres. Convert it before working out volume.
- Is the shape regular?If the area is L-shaped, curved, tapered or stepped, split it into smaller sections and work out each part separately.
- Is the result a volume, a mass or a bag count?m³, tonnes and bags are not interchangeable. You need density to move from volume to mass.
- Have you checked supplier minimums?Ready-mix companies may charge part-load fees, while merchants may sell only full bags, pallets or bulk bags.
- Is the unit from a UK source?US cubic yards, US gallons and US bag yields are not the same as UK trade practice.
- Does the project need professional measurement?Foundations, retaining structures, load-bearing slabs and structural alterations should be specified by a competent designer or engineer where appropriate.
A good conversion guide gives you confidence, but a good ordering process prevents delays. Keep your working, quote and delivery notes together. If the supplier changes bag weight, product density or delivery method, update the working before confirming the order.
Use Conversions for Estimating — Not Structural Design
This unit conversion guide helps with quantities and supplier communication. Always follow the project specification for structural concrete grade, reinforcement, slab thickness, foundation depth, curing, drainage and Building Regulations requirements.
Concrete Unit conversion guide FAQs
Answers to common UK questions about metric, imperial, cubic metres, cubic feet, cubic yards, bags and concrete material conversions.
Related ConcreteMath.co.uk Resources
Use these guides after converting your units to plan quantities, costs and deliveries.