Concrete Calculator UK: Volume, Cost, Bags & Mix Ratio
Concrete Calculator UK
Free UK concrete estimator - volume (m³), weight, bags, cost per m³ and ready-mix truck estimates for slabs, footings, columns, curbs and stairs.
Slab / Wall - plan & section
SVG · liveMix & waste settings
These apply to every shape above and drive the weight, purchase and bag estimates.
Concrete Calculator Results: Volume, Weight, Bags and Purchase Quantity
Bag estimates vary by product. Check the manufacturer's stated yield before purchasing.
Concrete cost estimate
Optional - add your local pricing to estimate a total.
Prices shown in GBP (£) by default - this reflects the price you enter, not a live market rate. Confirm final pricing with your UK concrete supplier.
Ready-mix truck estimate
Optional - see how many truck loads your pour needs.
Truck capacities vary by supplier and local regulations - treat this as a rough planning estimate.
This free concrete calculator UK homeowners, tradespeople and concrete estimators use to plan a pour works as a volume calculator, weight calculator, bags calculator and cost estimator in one tool. Pick the shape that matches your job - slab, hole or column, circular slab, curb and gutter, stairs, or footing - using the tabs above, or start from a preset like Driveway or Patio. Enter dimensions in mm, cm, m, in or ft; the diagram and every result update as you type, with metric (m³, kg) shown alongside imperial conversions.
How Much Concrete Do I Need?
The amount of concrete you need is simply the volume of the space you're filling. For a flat slab that's length × width × thickness; for a cylindrical hole or column it's the circular cross-section times the depth; for stairs and curbs it's a more irregular cross-section extruded along the run or length. This calculator handles the geometry for each shape so you only need to supply the measurements - in mm, cm, m, inches or feet, whichever suits UK or imperial drawings.
How to Calculate Concrete Volume
Measure each dimension in the same unit, multiply them together to get a volume, then convert to the unit your supplier quotes in (cubic meters or cubic yards). Ready-mix suppliers typically sell by the cubic meter or cubic yard; bagged mixes are sold by weight, which is why this tool also converts your volume into kilograms, tons and pounds using a concrete density figure.
Concrete Weight Calculator
Alongside volume, this tool doubles as a concrete weight calculator: once it has your volume and the density you've selected (2,130 kg/m³ by default for bagged concrete, or a custom figure from your supplier), it converts that into kilograms, metric tons and pounds automatically. That's the figure merchants and delivery drivers use to check a load, or that you'd use to confirm what a pallet of bags will weigh - it updates instantly if you change density, dimensions or units.
How Many Bags of Concrete Do I Need?
Bag counts come from dividing your purchase volume (structural volume plus waste allowance) by the yield of one bag - the volume of mixed concrete a single bag produces. That yield is different for every product and bag size, so this calculator lets you pick a common bag profile (20, 25, 30 or 40 kg; 40, 50, 60 or 80 lb) or enter a custom yield from your own bag's label. Bag counts are always rounded up, since you can't buy a fraction of a bag.
How Many 20kg Bags of Concrete Do I Need?
For a 20 kg bag, divide your purchase volume (structural volume plus waste allowance) by roughly 0.011 m³ - the typical yield of a single 20 kg bag - then round up, since bags can't be split. Select "20 kg bag" in the bag profile dropdown in the results panel above and the calculator does this division for you automatically, alongside the same working for 25 kg, 30 kg and 40 kg bags.
Concrete Cost Calculator UK
Turn your volume into a full concrete cost estimate: enter a price per cubic metre (or cubic yard) - the figure your local supplier or merchant has quoted you - and the calculator multiplies it by your purchase volume, then adds any flat delivery fee and pump or short-load surcharge on top for a total estimated cost. There's no single UK price of concrete per m³: as a rough planning guide, standard ready-mix typically runs somewhere in the £100–£170 per m³ range depending on region, grade and order size, with London and the South East usually higher - always confirm the exact price with your supplier before ordering. For pours above roughly 0.5–1 m³, ready-mix delivery is usually more practical and better value than mixing bags by hand; the truck estimate above shows how many loads you'd need and how full the last one would be, which helps you decide whether it's worth rounding your order up to avoid a short-load charge.
Concrete Mix Calculator: Ratio Guide (UK)
Once you know the volume from the calculator above, the concrete mix ratio tells you how to split that volume between cement, sand (fine aggregate) and gravel/stone (coarse aggregate) by parts. Ratios are written cement : sand : aggregate. Common UK mixes:
| General purpose / paths, patios | 1 : 2 : 3 |
| Foundations & footings | 1 : 3 : 6 |
| Driveways & external slabs | 1 : 1.5 : 3 |
| Fence posts / small fixings | 1 : 2 : 4 |
| Structural / high-strength (C25–C30) | 1 : 1.5 : 2.5 |
These are commonly used starting ratios, not a substitute for a specified mix grade (e.g. C20, C25, C30) on structural work - always follow your engineer's specification or your ready-mix supplier's grade recommendation where load-bearing strength matters.
Concrete Waste Allowance
Real pours rarely use exactly the calculated volume: uneven subgrade, formwork gaps and spillage during placement all use a bit more. A 5–10% allowance is typical for a well-prepared site with good formwork; rougher ground or hand-mixed small jobs often justify 15% or more. The structural volume and the waste-adjusted purchase volume are both shown separately so the extra margin is never hidden inside a single number.
How Long Does Concrete Take to Dry, Set and Cure?
Concrete moves through several stages after a pour, and each one matters for a different reason. The surface is usually firm enough to walk on lightly within 24–48 hours. It typically takes light foot traffic and equipment after about 7 days, once it has reached roughly 70% of its design strength. Full cure - the point where concrete reaches close to its rated strength - takes around 28 days under normal UK conditions, which is why 28-day strength is the figure used on most spec sheets and delivery notes. Cold weather slows every stage significantly, while hot, dry weather can dry the surface too quickly and cause cracking - covering a fresh pour matters as much as getting the mix right.
Concrete Calculator Formula
Each shape uses its own formula, shown live under the diagram as you enter values:
- Slab, wall, pad footing: Volume = Length × Width × Thickness × Quantity
- Round hole or column: Volume = π × (Diameter / 2)² × Depth × Quantity
- Rectangular column: Volume = Length × Width × Height × Quantity
- Circular slab or tube: Volume = π × (Outer Diameter² − Inner Diameter²) / 4 × Height × Quantity
- Curb & gutter: Volume = (Curb Depth × Curb Height + Gutter Width × Gutter Depth) × Length × Quantity
- Stairs: Volume = Width × [Risers × (Risers + 1) × Run × Rise / 2 + Risers × Run × Platform Depth]
- Strip footing: Volume = Total Length × Width × Depth × Quantity
Concrete Calculator Examples
| Length × Width × Thickness | 4 m × 3 m × 0.10 m |
| Structural volume | 1.20 m³ (42.4 ft³ · 1.57 yd³) |
| With 10% waste | 1.32 m³ |
| Weight at 2,130 kg/m³ | ≈ 2,809 kg |
| 25 kg bags needed | 110 bags |
| Diameter × Depth | 0.30 m × 0.60 m |
| Structural volume | 0.042 m³ |
| With 10% waste | 0.047 m³ |
| 20 kg bags needed | 5 bags |
Frequently Asked Questions
What density should I use for concrete?
2,130 kg/m³ is a common default for pre-mixed bag concrete and is used as the default here. Poured ready-mix concrete is often closer to 2,400 kg/m³, and lightweight mixes can be as low as 1,600–1,900 kg/m³. If your supplier gives you a specific figure, switch the density selector to "Custom" and enter it.
Why does the bag count depend on the bag profile I pick?
Different bag weights and brands yield different volumes of mixed concrete per bag, so a 25 kg bag and a 20 kg bag from different manufacturers won't scale linearly. Pick the closest profile to what you're buying, or type the exact yield printed on your bag into the custom yield field for the most accurate count.
How much waste allowance should I add?
5% is reasonable for a precise, well-formed pour; 10% is a safer general-purpose default; 15% or more suits rough ground, hand mixing, or a first-time DIY pour where some spillage is likely.
How long does concrete take to dry?
The surface is usually dry enough to walk on lightly within 24–48 hours, but that's not the same as being cured. Concrete keeps gaining strength for weeks after it looks dry, so treat "dry" and "ready to build on" as two different milestones.
How long does concrete take to set and cure fully?
Concrete typically takes on light loads after about 7 days and reaches close to its full design strength at around 28 days under normal UK conditions - the figure most suppliers quote as "28-day strength". Cold weather extends this timeline; ask your supplier about cold-weather mixes if pouring in winter.
Is this a structural design tool?
No. This calculator estimates material quantities only. It does not check load-bearing capacity, reinforcement, frost depth, or local building code requirements - consult a structural engineer or your local code for anything load-bearing.
How much concrete do I need?
Pick the shape closest to your pour above, enter the dimensions, and the calculator works out the volume automatically. As a rule of thumb, volume is simply length × width × depth for a slab-shaped pour, converted into cubic metres (m³).
How many 20kg bags of concrete do I need?
Divide your purchase volume (structural volume plus waste allowance) by the yield of a 20 kg bag - roughly 0.011 m³ per bag - then round up. Select "20 kg bag" in the bag profile dropdown in the results panel above and this calculator does it for you automatically.
How much concrete do I need UK?
UK suppliers typically quote ready-mix concrete by the cubic metre (m³) and bagged concrete by weight in kilograms. Enter your dimensions in mm, cm or m above and this calculator converts everything into m³, kg and the number of UK-sized bags for you.
How much does concrete cost per m³ in the UK?
There's no single fixed price - it depends on region, mix grade, delivery distance and order size - but standard ready-mix typically falls somewhere in the £100–£170 per m³ range, with London and the South East usually towards the higher end. Enter your supplier's actual quoted price into the cost section above to get an estimate for your specific job, including delivery.
Is this a tarmac calculator?
No - this tool calculates poured concrete quantities. Tarmac (asphalt) uses a different density and is usually quoted by tonnage per square metre at a given depth, so it needs a separate calculation.
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