Concrete calculator
Work out the cubic metres of concrete for a base, footing or post holes, and how many bags of premixed concrete to buy.
From the top of the compacted hardcore to the top of the formwork.
Concrete to order
- Concrete to order
- 0.99 m³990 L, with 10% extra
- Volume without extra
- 0.9 m³900 L
- 20 kg bags
- 1242,480 kg in all
Waste allowance and bag yield
Allow 4 to 10% over the measured volume, as the US ready-mix industry advises.
The default is the yield on the maker’s data sheet for its MaxiPack20. The sheet does not print the weight, so the calculator reads the pack as 20 kg. If your bag gives another figure, type it in litres: 0.008 m³ is 8 litres.
Optional. Fill in both fields to add a row for a bag that is not listed above, for example a 25 kg bag.
Estimate only. Check the product label and the building regulations where you live before you buy or build.
Quick answer
Multiply length × width × thickness in metres to get cubic metres, then add 4 to 10% for waste. A 3 × 3 m base 100 mm thick is 0.9 m³; with 10% extra that is 0.99 m³, or 124 bags of premixed concrete at 0.008 m³ (8 litres) per bag, the yield Heidelberg Materials gives for its 40N Concrete.
How to use the concrete calculator
- Pick what you are pouring: a base (shed base, patio, path), a footing (a strip or pad under a wall, steps or a garden building), or post holes and round columns.
- Enter the size. Lengths are in metres; thickness, footing width and depth, and hole diameter and depth are in centimetres, so 100 mm is 10 cm. Switch to imperial to type feet and inches; the results stay in cubic metres and kilograms.
- Read the concrete to order in cubic metres and the number of bags of premixed concrete. The bag count includes the extra for waste and is rounded up to whole bags.
- Open ‘Waste allowance and bag yield’ to change the extra from 10%, or to type the yield printed on the bag you plan to buy.
How do you work out how much concrete you need?
Multiply length by width by thickness, with every measurement in metres, to get cubic metres. A 3 × 3 m base 100 mm thick is 3 × 3 × 0.1 = 0.9 m³, which is 900 litres.
For a round hole or column, the cross-section is π × diameter² ÷ 4. Multiply it by the depth and the number of holes, after taking off the part of each hole that a post fills. Then add the waste allowance and divide by the yield of one bag, rounding up to a whole bag.
base volume = length × width × thicknessfooting volume = length × width × depth × number of footingshole volume = (π × diameter² ÷ 4 − post width²) × depth × number of holesconcrete to order = volume × (1 + waste % ÷ 100)bags = concrete to order ÷ yield of one bag, rounded up1 m³ = 1,000 litresHow much concrete for a shed base?
A base for an 8 × 6 ft shed, 2.4 × 1.8 m and 100 mm thick, takes 0.475 m³ of concrete with 10% extra, which is 60 bags. The table gives the concrete to order for a base 100 mm thick under common shed sizes, with 10% extra. Base sizes are in metres, rounded from the shed size in feet. Bag counts use the yield on Heidelberg Materials’ 40N Concrete data sheet: 0.008 m³ (8 litres) per bag.
| Shed | Base (m) | To order | Bags |
|---|---|---|---|
| 6 × 4 ft | 1.8 × 1.2 | 0.238 m³ | 30 |
| 8 × 6 ft | 2.4 × 1.8 | 0.475 m³ | 60 |
| 10 × 8 ft | 3.0 × 2.4 | 0.792 m³ | 99 |
| 12 × 8 ft | 3.6 × 2.4 | 0.95 m³ | 119 |
| 10 × 10 ft | 3.0 × 3.0 | 0.99 m³ | 124 |
| 16 × 10 ft | 4.8 × 3.0 | 1.58 m³ | 198 |
For a base 75 mm thick, multiply the cubic metres by 0.75; for 150 mm, multiply by 1.5. Divide the result by 0.008 and round up for the number of bags.
Worked examples
Bags of concrete for a 2.4 × 1.8 m shed base, 100 mm thick
The base is 2.4 × 1.8 × 0.1 = 0.432 m³. With 10% extra, plan on 0.475 m³, or 475 litres.
One bag of 40N premixed concrete yields 0.008 m³ (8 litres), so 475 ÷ 8 = 59.4, rounded up to 60 bags. At about 2 litres of water per bag, the figure on the data sheet, that is 120 litres of water to add.
Concrete for six fence posts
The posts are 75 mm square, set in holes 200 mm across with 600 mm of concrete, one of the hole sizes in Heidelberg Materials’ guidance for setting posts.
Each hole has a cross-section of π × 0.2² ÷ 4 = 0.0314 m², less 0.075 × 0.075 = 0.0056 m² for the post, which leaves 0.0258 m². Times 0.6 m and six holes, that is 0.093 m³. With 10% extra it is 0.102 m³: 13 bags of premixed concrete at 8 litres a bag.
How thick should a concrete base be?
In England, a concrete ground-supported floor can be built with concrete at least 100 mm thick, thicker if the structural design requires, on a well-compacted bed of hardcore no more than 600 mm deep and with a damp-proof membrane, under paragraph 4.7 of Approved Document C. The calculator flags a base thinner than 100 mm. That guidance is for floors of buildings; a path or a shed base is not one, so treat the warning as a reference point.
Wales, Scotland and Northern Ireland publish their own guidance. Foundations, extensions and anything that carries a building or a vehicle need a proper design, so check with building control or a structural engineer before you pour. Their requirements take priority over any figure on this page.
How much extra concrete should I order?
Order 4 to 10% more than the volume of the formwork, as the US National Ready Mixed Concrete Association advises; the calculator adds 10% unless you change it. Small errors add up: the association notes that a 100 mm slab poured 3 mm thicker than planned uses 3% more concrete.
Ready-mixed concrete is sold by the volume of fresh concrete as it is discharged from the mixer lorry, so give the supplier the ‘concrete to order’ figure, not the volume of the formwork. Once hardened, concrete can take up about 2% less volume than it did fresh.
Questions
How many bags of concrete make a cubic metre?
It takes 125 bags of premixed concrete to make one cubic metre, using the yield of 0.008 m³ (8 litres) that Heidelberg Materials gives for a bag of its 40N Concrete, a high-strength mix. With 10% extra for waste, a cubic metre calls for 138 bags. For any other product, divide the volume you need by the yield printed on the bag and round up.
How many bags of concrete do I need per fence post?
A 75 mm square post in a hole 200 mm across with 450 mm of concrete takes 2 bags of general-purpose premixed concrete at 8 litres a bag, with 10% extra. The hole holds 0.012 m³ (12 litres) once the post is taken off, or 13 litres with the extra. Heidelberg Materials’ guidance for its fast-setting post mix, a different product, gives 1 bag for this hole, so follow the post mix bag if you use one.
How many square metres does a bag of concrete cover?
One bag of 40N premixed concrete covers about 0.08 m² at 100 mm thick and 0.10 m² at 75 mm, according to Heidelberg Materials’ data sheet. Put the other way, a square metre of base 100 mm thick takes 13 bags before waste (12.5 rounded up), or 14 bags with 10% extra. At 75 mm thick, a square metre takes 11 bags with the extra.
How do I work out concrete for a strip footing?
Multiply the length of the trench by its width and depth, all in metres. A strip 5 m long, 500 mm wide and 300 mm deep is 5 × 0.5 × 0.3 = 0.75 m³, or 0.825 m³ with 10% extra, which is 104 bags of premixed concrete. For a foundation around a building, measure along the centre line so the corners are not counted twice, and check the design with building control first.
How much water does a bag of premixed concrete need?
A bag of 40N premixed concrete needs about 2 litres of clean water, according to Heidelberg Materials’ data sheet. Add the water slowly until the mix is as workable as you need, because too much water weakens the concrete and affects its setting time and durability. Mix the whole bag, as the contents can separate in transport, and use the concrete within 30 minutes of adding water.
Sources
- Heidelberg Materials UK, 40N Concrete technical data sheet (checked 19 October 2026)MaxiPack20: about 2.0 litres of water, yield 0.008 m³, coverage 0.10 m² at 75 mm and 0.08 m² at 100 mm; mix the whole bag; use within 30 minutes.
- Heidelberg Materials UK, PostFix technical data sheet (checked 19 October 2026)Rate of usage guidance: 75 mm square post in a 200 × 450 mm hole, 1 bag; hole sizes 200 × 450, 600 and 750 mm.
- HM Government, Approved Document C: site preparation and resistance to contaminants and moisture (2004 edition incorporating 2010 and 2013 amendments, for use in England) (checked 19 October 2026)Paragraph 4.7: concrete ground supported floor at least 100 mm thick on hardcore no greater than 600 mm deep, with a damp-proof membrane.
- National Ready Mixed Concrete Association (US), CIP 8: Discrepancies in Yield (checked 19 October 2026)Allow 4 to 10% over plan dimensions; 3 mm on a 100 mm slab is a 3% shortage; ready-mix is sold by fresh volume; hardened volume about 2% less.
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