Concrete Footing Calculator
Find how much concrete your footings or foundations need, in cubic metres or 20 kg bags.
Measure your project
Your answer
That is about 16 wheelbarrow loads. We added 10% extra.
If you buy bags
Blue Circle says each 20 kg bag makes about 0.01 m³ of concrete.
If you order a lorry (ready-mix)
Ask for 1.00 m³. Rock Solid Concrete takes home orders from 1 m³, so we price at least that.
This is a guide to the amount only. For footing sizes, check your local building rules or ask a builder or engineer.
Which costs less?
You also skip mixing 45 bags by hand. Have the edges ready before it comes.
UK middle prices from October 2026, with 20% VAT. Your supplier's price may differ.
Quick answers
- A 6 m footing, 450 mm wide, 150 mm thick
- 0.405 m³. With 10% extra, 0.45 m³, or 45 bags of 20 kg.
- Thinnest strip footing
- 150 mm, says Approved Document A, the government's guide to the building rules in England.
- How deep
- In England, at least 450 mm down to the bottom, to keep frost out. On clay that shrinks, at least 750–1,000 mm.
- Mix
- ST2 or GEN1, says Approved Document A. These are mix names a ready-mix supplier will know. Heidelberg Materials lists both.
How to work it out
- Measure the length of the trench in metres.
- Measure the footing width and thickness in mm. Divide each by 1,000.
- Multiply length × width × thickness. This gives m³.
- Add 10% extra for spills.
- Divide by 0.01 for 20 kg bags, or order the m³ from a ready-mix supplier.
6 × 0.45 × 0.15 = 0.405 m³
0.405 + 10% = 0.446 m³
0.446 ÷ 0.01 = 44.6 → 45 bags
Concrete footing size: width and depth
In England, Approved Document A gives sizes for plain concrete strip footings under small buildings. Scotland, Wales and Northern Ireland have their own rules.
- Thickness: at least 150 mm. If the footing sticks out past the wall by more than that on each side, make it that thick.
- Depth: the bottom at least 450 mm below ground, to keep frost out. This does not apply on rock.
- On clay that shrinks: at least 750 mm (low shrink), 900 mm (medium) or 1,000 mm (high). Go deeper near trees.
- Width: from the table below.
| Wall load | Medium-dense gravel or sand, or stiff clay | Firm clay | Loose sand | Soft silt or clay |
|---|---|---|---|---|
| 20 kN/m | 250 mm | 300 mm | 400 mm | 450 mm |
| 30 kN/m | 300 mm | 350 mm | 600 mm | 650 mm |
| 40 kN/m | 400 mm | 450 mm | – | – |
| 50 kN/m | 500 mm | 600 mm | – | – |
| 60 kN/m | 600 mm | 750 mm | – | – |
| 70 kN/m | 650 mm | 850 mm | – | – |
Load is in kN (kilonewtons) for each metre of wall. Your builder or engineer can give it. A dash means the table stops at 30 kN/m for that ground. On rock, the footing can be as wide as the wall. On very soft ground, Approved Document A says get specialist advice.
For any footing, check your local building rules or ask a builder or engineer.
How to calculate concrete footings
A strip footing is a long block of concrete. So the formula is length × width × thickness, all in metres. That gives the volume in m³.
Approved Document A also allows trench fill. That means filling the trench with concrete, not just the bottom 150 mm.
| Width | 150 mm strip | Bags | 450 mm trench fill | Bags |
|---|---|---|---|---|
| 300 mm | 0.045 m³ | 4.5 | 0.135 m³ | 13.5 |
| 450 mm | 0.068 m³ | 6.8 | 0.203 m³ | 20.3 |
| 600 mm | 0.09 m³ | 9 | 0.27 m³ | 27 |
20 kg bags, no extra added. Multiply by your length, then add 10%.
Steps in the footing
Some footings step up or down. Each step must overlap by the biggest of these: twice the step's height, the footing thickness, or 300 mm. That is Approved Document A's rule. For trench fill, the overlap is twice the step's height or 1 m, whichever is bigger.
Add each straight run as its own part in the calculator. For round pads, such as deck post footings, pick the round shape.
Concrete mix and cement for footings
Approved Document A names ST2 or GEN1. Heidelberg Materials also names ST2 for foundations in its merchant guide. Rock Solid Concrete says C25/30 suits most house footings.
Mixing your own
The British Standard table gives 265 kg of cement and 1,895 kg of ballast for each m³ of ST2.
Mixing by volume instead? The Concrete Society's DIY guide uses 1 : 6 ballast for footings. Our cement calculator counts that as ST3, about 12 bags per m³, a little richer than ST2.
For the 6 m footing above, that is 5 bags of 25 kg cement. Add 844 kg of ballast, or 2 bulk bags of 800 kg.
Approved Document A also allows a mix by weight. It is 50 kg of cement to at most 200 kg of sand and 400 kg of stone. This is for soil that does not attack concrete (non-aggressive soil). Our cement calculator works out other mixes.
Bags or a lorry for footings
Footings soon add up. The 6 m example takes 45 bags, which is 0.9 tonnes to lift and mix.
At our middle prices with VAT, the bags cost about £495. A lorry with 1 m³ costs about £150, before any delivery fee or minimum charge.
Ask the supplier about part loads and how long the lorry can wait. Our concrete cost calculator adds those in.
Questions people ask
How big should a concrete footing be?
In England, Approved Document A asks for at least 150 mm thick. The width depends on the ground and the wall's load. On firm clay it goes from 300 to 850 mm. Check your local building rules or ask a builder or engineer.
How deep should footings be in the UK?
In England, Approved Document A says at least 450 mm to the bottom, so frost can't reach under. On clay that shrinks, 750–1,000 mm, and deeper near trees. Scotland, Wales and Northern Ireland have their own rules. Check your local building rules or ask a builder or engineer.
How do I calculate concrete for footings?
Multiply length × width × thickness in metres, then add 10%. A 6 m footing, 450 mm wide and 150 mm thick, takes 0.45 m³.
How much cement do I need for footings?
For 1 m³ of ST2 mix, about 11 bags of 25 kg cement and 1,895 kg of ballast. That is from the British Standard table.
What concrete mix is best for footings?
Approved Document A names ST2 or GEN1. Heidelberg lists GEN1 for house footings with no steel, and RC25/30 for footings with steel in normal soil.
Do concrete footings need rebar?
Approved Document A's sizes are for plain concrete, with no steel, under small buildings. Whether yours needs steel is a design question. Check your local building rules or ask a builder or engineer.
How long does a concrete footing (footer) need to cure?
Cemex says concrete takes 28 days to cure to full strength. Blue Circle's bagged concrete sheet says to cover it for at least 7 days. We found no UK figure for when to build on it, so ask your builder.
Where our numbers come from
- Footings: Approved Document A (450 mm deep, 150 mm thick)
- Bag size: Blue Circle Multi-Purpose Concrete data sheet (20 kg makes about 0.01 m³)
- Bag price: the middle of 4 prices from 3 UK stores for 20 kg concrete (Oct 2026)
- Ready-mix price: the middle of 7 prices from 4 UK suppliers (Oct 2026, with VAT)
- Rock Solid Concrete price guide (Midlands ready-mix)
- Full loads, barrows and mixes: Heidelberg Materials UK merchant guide
- Heidelberg Materials UK: types of concrete
- Rock Solid Concrete: C25/30 mix uses
- Cement and stone per m³: BS 8500-2 Table 10 (as copied by Pavingexpert)
- DIY mixes: Concrete Society, Batching by volume
- Mix ratios and 25 kg bags: Heidelberg General Purpose cement data sheet
- Ballast weight: Wickes ballast bulk bag (800 kg covers 9.7 m² at 50 mm)
- Shed base: Cemex guide (about 3 in concrete, 3 in hardcore)
- Extra for spills: Sakrete slab calculator (10%)
Made by BuildersCount. Last checked 4 Oct 2026.
How we work out our numbers About us Found a mistake? Tell us