LCM Tools
Concrete volume calculator
Slabs, strip foundations, pads, circular bases and post holes. Metric units, with the working shown.
1What are you pouring?
2Dimensions
Measure the runs along the trench centreline so corners are not counted twice.
no.
3Allowance and bags
%
L/bag
Use the yield printed on the bag or the merchant's listing.
Total with allowance
–
Enter the dimensions above.
Net volume
–
Allowance
–
Bags
Add yield
Working
The calculation appears here once the dimensions are entered.
Pour list
| Element | Net m³ | Allow. | Total m³ | Remove |
|---|---|---|---|---|
| Total |
Geometric volume only. Thickness, foundation size, reinforcement and concrete specification come from the drawings or the designer. Confirm the order quantity and delivery terms with your supplier.
Concrete quantity is a volume calculation. Convert every dimension to metres and multiply them, and the answer is in cubic metres. A 5m × 3m slab 100mm thick works out at 5 × 3 × 0.1 = 1.5m³ before any allowance.
The calculator above covers slabs, strip foundations, pads, circular bases and post holes, with each dimension entered in metres, centimetres or millimetres. It shows its working, keeps the net volume apart from the allowance and can total several elements in a pour list. Results are in cubic metres because that is how ready-mixed concrete is ordered in the UK.
How the calculator works
Every dimension is converted to metres before anything is multiplied. A depth of 100mm becomes 0.1m; entered as 100 with the unit set to metres, it would make the answer 1,000 times too large. The calculator flags a thickness under 20mm or over 5m, and lengths or widths far outside the normal range for the element chosen, because those figures usually mean the wrong unit is selected.
One cubic metre is 1,000 litres, so the total is shown in both. Cubic metres are what a ready-mix supplier will ask for; litres are more useful when working out bagged concrete.
| Element | Formula |
|---|---|
| Slab or single rectangular pad | Length × width × thickness |
| Strip foundation | Centreline length × width × concrete depth |
| Identical pads | Length × width × depth × number of pads |
| Circular base | π × radius² × thickness |
| Post holes | π × radius² × depth × number of holes |
Slabs, strip foundations and pads
Slabs, pads and strip foundations all use length × width × depth; only the dimensions change. For a strip foundation, enter the total length of all the runs, measured along the centreline of the trench so the corners are not counted twice, together with the width and depth of concrete to be placed.
Where several pads are the same size, enter one pad and the number of pads. Pads of different sizes can each be added to the pour list, which keeps a running total with the allowance for each element shown separately.
Circular bases and post holes
Circular volumes use the radius, which is half the diameter. A 600mm hole has a radius of 300mm (0.3m), so its volume is π × 0.3² × depth. Using the diameter in place of the radius makes the area four times too large, which is why the calculator asks for the diameter and halves it itself.
For post holes the result is the full volume of the hole, with nothing deducted for the post, so it errs slightly high. A 100mm square post set in a 300mm hole takes up about 14% of the cross-section.
Worked examples
These sizes show the arithmetic only. Thickness and foundation size for a real job come from the drawings.
| Example | Calculation | Net volume | With 5% allowance |
|---|---|---|---|
| Small slab | 2.4m × 1.8m × 0.10m | 0.432m³ | 0.454m³ |
| Shed base | 3.0m × 2.0m × 0.10m | 0.600m³ | 0.630m³ |
| Garage slab | 6.0m × 3.0m × 0.15m | 2.700m³ | 2.835m³ |
| Strip foundation | 12.0m × 0.60m × 0.25m | 1.800m³ | 1.890m³ |
| Four pads | 1.2m × 1.2m × 0.30m × 4 | 1.728m³ | 1.814m³ |
| Circular base | 0.90m diameter × 0.20m | 0.127m³ | 0.134m³ |
| Eight post holes | 0.30m diameter × 0.60m × 8 | 0.339m³ | 0.356m³ |
How much extra concrete to allow
No single percentage suits every pour. Ready-mix suppliers commonly suggest adding between 5% and 10% to the calculated volume, with the figure depending on the size, shape and complexity of the pour and the conditions on site.
The calculator starts at 5% and the figure can be changed for each element. A pour into accurate formwork carries less uncertainty than a trench dug into irregular ground, where overbreak, an uneven formation, grout loss through the formwork and spillage all add to what is used. The allowance is applied after the net volume is calculated, so both figures stay visible.
How many bags of concrete you need
Bag yield varies by product, so the calculator asks for the manufacturer's stated yield in litres per bag, divides the total by it and rounds up to a whole bag.
Wickes states that a 20kg bag of Blue Circle Multi-Purpose Ready To Use Concrete typically makes around 0.01m³, or about 10 litres. At exactly 10 litres a bag, 1m³ takes 100 bags before any allowance, which is two tonnes of bags to lift and mix; at 9 litres a bag it takes 112. Check the figure printed on the product you are buying.
Ready-mixed concrete being placed into reinforced slab formwork. Illustrative image.
Before ordering ready-mixed concrete
Before placing an order, recheck the measured dimensions and the allowance, then the practical side of the pour: access for the mixer, the distance from the truck to the pour, whether a pump or skip is needed, formwork, labour, weather and the delivery slot. Strength class, exposure class and the rest of the specification come from the drawings or the designer.
Delivery terms vary between suppliers and regions. Standard mixer trucks typically carry six or eight cubic metres, some suppliers run smaller mini-mix vehicles for loads of a few cubic metres, and part loads, waiting time on site and returned concrete are often charged separately.
Ask your own supplier for its minimum order, the increment it sells in, and its part-load and waiting-time charges before rounding the calculated figure up.
For price context, LCM's UK construction material prices 2026 analysis found the national ready-mixed concrete price index 1.1% lower in the year to June 2026, with cement down 4.5%.
Common calculation mistakes
Most errors come from units and geometry: entering 100mm as 100m, confusing square metres with cubic metres, leaving out the depth, using the diameter as the radius, or forgetting to multiply one pad or post hole by the number needed.
Excavated dimensions need checking too. Trenches often end up wider or deeper than drawn because of overbreak or a soft formation, while a formed pour is set by the inside faces of the formwork. Either way, measure the space the concrete will actually fill.
A correct volume says nothing about how well the concrete is supported, placed or compacted. LCM's investigation into concrete slab voids and hidden defects covers the problems that show up after the pour.
Frequently asked questions
How much concrete do I need for a 10m² slab?
It depends on the thickness. At 100mm, 10m² needs 1.0m³ before allowance; at 150mm it needs 1.5m³. Take the thickness from the drawings or specification.
It depends on the thickness. At 100mm, 10m² needs 1.0m³ before allowance; at 150mm it needs 1.5m³. Take the thickness from the drawings or specification.
How much extra concrete should I order?
Between 5% and 10% is a common range. The calculator starts at 5%; increase it for trenches in irregular ground or awkward placement.
Between 5% and 10% is a common range. The calculator starts at 5%; increase it for trenches in irregular ground or awkward placement.
How many bags make 1m³ of concrete?
That depends on the product's yield. At 10 litres a bag, 1m³ needs 100 bags before allowance; at 9 litres, 112.
That depends on the product's yield. At 10 litres a bag, 1m³ needs 100 bags before allowance; at 9 litres, 112.
How much does a cubic metre of concrete weigh?
Normal-weight concrete is about 2.4 tonnes per cubic metre, or around 2.5 tonnes once reinforced, and additions such as steel fibres change the figure. That load matters when wet concrete is placed on, or barrowed across, a suspended floor, so the floor and any propping need checking first.
Normal-weight concrete is about 2.4 tonnes per cubic metre, or around 2.5 tonnes once reinforced, and additions such as steel fibres change the figure. That load matters when wet concrete is placed on, or barrowed across, a suspended floor, so the floor and any propping need checking first.
Expert Verification & Authorship
Founder & Author, London Construction Magazine | Construction Testing & Investigation Specialist