Concrete Calculator UK: How Much Concrete Do I Need?
Concrete Slab Calculator UK
Add prices to compare costs (optional)
Pick a shape and enter your measurements to see exactly how much concrete you need. This concrete calculator shows cubic metres, weight and the number of 20 kg or 25 kg bags. It also lists cement and ballast for mixing yourself, plus the cost of each option.
More free UK building calculators
Concrete is usually the first job, not the last. These tools cover the walls, sub-base and finishes that follow.
MOT Type 1, ballast, sand and gravel in tonnes and bulk bags.
Concrete, breeze and aircrete blocks, with mortar and cost.
Bricks per m², mortar and cost for any wall.
Slabs, block paving, sub-base, bedding mortar and patio fall.
MOT Type 1 tonnes, bulk bags, layers and dig-out depth.
Concrete, paving, grid or timber shed bases, with dig depth and cost.
Sharp, building and kiln-dried sand, plus sand and cement mixes.
Tonnage, coverage and driveway cost per m².
Why getting your concrete volume right matters
It happens more often than you would think. The ready-mix truck has emptied its drum, and your shed base still has a bare strip along one edge. The next delivery slot is tomorrow afternoon.
Fresh concrete will not bond properly to yesterday’s pour. That creates a cold joint, which is a weak line that can crack later. A second delivery also brings another delivery charge and usually a short-load surcharge.
Over-ordering costs money too, because nobody wants half a cubic metre of leftover concrete going off on the drive. So how much should you actually order? The answer takes two minutes with the calculator above, plus one habit we explain below.
Our habit: measure the depth in at least five places, use the deepest reading, then add 5–10%. Uneven ground swallows far more concrete than people expect.
How much concrete per m²?
A 100 mm slab needs 0.1 m³ of concrete for every m². That means one cubic metre covers 10 m². A 150 mm driveway slab needs 0.15 m³ per m², so one cubic metre covers just 6.7 m².
How many bags of concrete per m³?
A typical 20 kg bag of ready-to-use concrete yields about 0.010 m³. So you need roughly 100 bags for one cubic metre. That is two tonnes of bags to carry and mix.
How to use the concrete volume calculator
Our concrete calculator handles eight shapes and three measurement systems. Follow these steps for an accurate order.
- Pick the shape. Choose slab for bases, patios and paths. Use round column for post holes, and strip foundation for footings.
- Choose your units. Metres is the default, and each box accepts millimetres, centimetres or metres. Feet and yards also work.
- Enter the dimensions. Measure the excavation itself, not the finished paving. Use the deepest depth you find.
- Add a waste allowance. Use 5% for neat timber formwork. Allow 10% for hand-dug ground or uneven sub-base.
- Check density and bag yield. Normal concrete weighs about 2,400 kg/m³. Copy the yield printed on your bag if it differs from 0.010 m³.
- Mixing it yourself? Choose a ratio to see cement bags, ballast or sand and gravel, plus water.
- Add your prices. Enter quotes for ready-mix, bags, cement and ballast. The tool then highlights the cheapest option.
Our advice: never trust a single depth measurement. A sub-base dipping just 20 mm across a 10 m² base adds 0.2 m³. That equals twenty additional bags.
Concrete calculator formula: how to calculate cubic metres
Every concrete calculator converts your measurements to metres (or meters) and multiplies them. The answer comes out in cubic metres, written m³ or m3. Builders often call one cubic metre “a cube” of concrete, and suppliers price it that way.
Concrete per m² by slab thickness
| Thickness | m³ per m² | Area 1 m³ covers | 20 kg bags per m² | Typical use |
|---|---|---|---|---|
| 50 mm | 0.05 | 20 m² | 5 | Toppings over a sound base |
| 75 mm | 0.075 | 13.3 m² | 7.5 | Light paths and small pads |
| 100 mm | 0.10 | 10 m² | 10 | Shed bases, patios, garden paths |
| 150 mm | 0.15 | 6.7 m² | 15 | Driveways and garage floors |
| 200 mm | 0.20 | 5 m² | 20 | Heavy-duty bases |
Bags based on 0.010 m³ per 20 kg bag, before waste.
Concrete slab calculator: worked examples for common UK jobs
We ran each example through the concrete calculator. Use them as sensible checks against your own figures.
Case study 1: a 3 × 2.4 m shed slab
A 3 × 2.4 m shed base, 100 mm deep, on a compacted sub-base
Lay 75–100 mm of compacted MOT Type 1 or crushed concrete first. Our shed base calculator adds the hardcore, DPM and formwork, and our MOT Type 1 guide covers compaction.
Case study 2: driveway concrete calculator
A single car driveway, 6 × 3 m, 150 mm deep
Planning a front driveway or patio? Check our guide to permits for concrete patios before you dig.
Case study 3: concrete footing calculator for a garden wall
A 10 m strip footing, 600 mm wide and 250 mm deep
Footing sizes depend on the wall and the ground. Our garden wall footing guide explains typical depths and widths.
Case study 4: fence post holes
Ten round holes, 300 mm across and 600 mm deep
Concrete bag calculator: how many 20 kg and 25 kg bags?
Divide the volume by the yield printed on the bag, then round up. Most 20 kg ready-to-use bags, such as Blue Circle multipurpose concrete, yield about 0.010 m³. Typical 25 kg bags yield about 0.011 m³.
| Volume | 20 kg bags | 25 kg bags | Weight to carry |
|---|---|---|---|
| 0.10 m³ | 10 | 10 | about 200 kg |
| 0.25 m³ | 25 | 23 | about 500 kg |
| 0.50 m³ | 50 | 46 | about 1 tonne |
| 1.00 m³ | 100 | 91 | about 2 tonnes |
Here is a figure that surprises most people. Bagged concrete costs roughly six to eight times more per cubic metre than ready-mix. Bags still make sense for post holes and small repairs, where convenience matters more than price.
Concrete mix calculator: cement, sand and ballast per m³
Dry sand and aggregate pack together once you add water. So you need about 1.54 m³ of loose dry material for every cubic metre of finished concrete. The calculator splits that by your chosen ratio.
| Mix by volume | Typical use | Cement | Aggregate | Water |
|---|---|---|---|---|
| 1 : 1.5 : 3 (≈ C25) | Driveways, garage floors | 403 kg (17 bags) | 672 kg sand + 1,260 kg gravel | 200 L |
| 1 : 2 : 4 (≈ C20) | Patios, paths, bases | 317 kg (13 bags) | 704 kg sand + 1,320 kg gravel | 160 L |
| 1 : 3 : 6 (≈ C10) | Blinding, mass fill | 222 kg (9 bags) | 739 kg sand + 1,386 kg gravel | 110 L |
| 1 : 5 ballast | Shed bases, general use | 370 kg (15 bags) | 2.18 t all-in ballast | 185 L |
| 1 : 6 ballast | Foundations, footings | 317 kg (13 bags) | 2.24 t all-in ballast | 160 L |
Per cubic metre. Bags are 25 kg. Add water gradually, because damp sand already carries some.
All-in ballast is sharp sand and gravel already blended. It is the simplest way to mix concrete for DIY jobs. Bulk bags work out cheaper per tonne than small bags once you need more than a few. You can also price the ballast in tonnes with our aggregate calculator.
Cement is alkaline and can cause serious skin burns. HSE publishes clear guidance on working safely with cement. Wear gloves, long sleeves and eye protection whenever you mix.
Concrete cost per m³ in 2026: ready-mix, bags or mix yourself?
We checked UK merchant and supplier prices in October 2026. Ready-mix, often called liquid concrete, typically costs £100–£135 per m³ for C20 or C25 before VAT. Delivery comes on top. A 25 kg bag of cement costs about £7–£8. An 850 kg bulk bag of ballast typically costs £60–£77.
| Volume | 20 kg bags | Mix yourself (1:5) | Ready-mix delivered | Best value |
|---|---|---|---|---|
| 0.25 m³ | ≈ £225 | ≈ £100 | ≈ £300 (minimum charges) | Mix yourself |
| 0.6 m³ | ≈ £540 | ≈ £210 | ≈ £300 | Mix yourself |
| 1.0 m³ | ≈ £900 | ≈ £320 | ≈ £300 | Ready-mix |
| 2.5 m³ | ≈ £2,250 | ≈ £770 | ≈ £520 | Ready-mix |
Materials only, including VAT. Assumes £9 per concrete bag, £7.50 per cement bag and £70 per bulk bag. Ready-mix assumes £120 per m³ plus delivery and a short-load charge. Mixer hire and labour are extra.
Our view is simple. Below about 0.75 m³, mixing your own usually wins on price. Above one cubic metre, ready-mix wins on both price and quality. Expect prices in London and the South East to run roughly 15–25% higher. Always ask suppliers about part-load charges, waiting time and pump hire before you book. Request every additional charge in writing before booking.
UK concrete grades: which mix do you need?
UK concrete follows BS EN 206 and BS 8500. Your drawings or Building Control set the grade for structural work. For everyday domestic jobs, these designations cover most needs.
| Designation | Approx. strength | Typical domestic use |
|---|---|---|
| GEN0 / ST1 | 8–10 N/mm² | Blinding and kerb bedding |
| GEN1 / ST2 | 10–15 N/mm² | Unreinforced strip and trench-fill foundations |
| GEN2 / ST3 | 15–20 N/mm² | House floor slabs, shed and conservatory bases |
| GEN3 / ST4 | 20–25 N/mm² | Garage floors and concrete flooring under heavier loads |
| PAV1 / PAV2 | C25/30–C28/35, air-entrained | Driveways exposed to frost and de-icing salt |
| C25/30 and above | 25–30 N/mm² cylinder | Reinforced foundations, beams, retaining walls |
Here is our contrarian take. Many people order a stronger mix than they need and skip the sub-base. A well-compacted sub-base protects a slab far better than an extra strength class. For foundation guidance, read Approved Document A. The Concrete Centre also publishes free technical guidance.
Concrete weight calculator: how heavy is a cubic metre?
Weight equals volume multiplied by density. Normal concrete weighs about 2,300–2,400 kg per m³, so one cubic metre weighs roughly 2.4 tonnes. Lightweight concrete can drop to 1,400–2,000 kg/m³.
in one cubic metre of normal concrete
in one m² of 100 mm slab
in one cubic yard
These figures matter when you hire a skip for broken concrete, or plan van and trailer loads for bags.
Concrete ordering mistakes that cost money
Our thinking changed considerably over time. We once concentrated on mix strength, but most unsuccessful pours come from poor planning, not chemistry.
- Measuring depth once. Excavations are never flat. Measure five or more spots and use the deepest.
- Forgetting the sub-base. Concrete laid on soft ground cracks. Compact MOT Type 1 in layers first, and size it with our sub-base calculator.
- Ignoring truck access. A full mixer can weigh over 25 tonnes. Check the width, overhead cables and how far the chute reaches.
- Booking too little help. Concrete starts stiffening within an hour or two. Line up enough barrows and people before the truck arrives.
- Skipping curing. Keep new concrete damp and protected for several days. Fast drying in sun or wind weakens the surface.
- Pouring in a frost. Frost below about 5 °C slows curing and can permanently damage fresh concrete. Check the forecast and cover the slab.
Curing deserves its own explanation. Read our article on how long concrete takes to cure before you walk or drive on it.
Concrete calculator FAQs
How the concrete calculator gets its numbers
Each shape uses its standard geometric formula after converting every input to metres. The calculator then adds your waste allowance. Weight equals volume multiplied by density. Bag counts divide the volume by the stated yield and round up.
Mix quantities use the dry-volume method with a factor of 1.54. Loose densities are 1,440 kg/m³ for cement, 1,600 for sand, 1,500 for gravel and 1,700 for ballast. Prices reflect UK merchant listings checked in October 2026.
Remember that ready-mix truck with an empty drum and a bare strip of shed base? Measuring the depth in five places would have prevented it. Measure carefully, add sensible waste and compare costs before you book. Reviewed in October 2026. Treat results as estimates, and follow your engineer or Building Control for structural work.
What are you pouring next? Tell us about your project or ask a question through our contact page. We read every message, and good questions often become new guides.