Ordering foundation concrete requires knowing exactly how many cubic metres you need to fill your trenches properly.
Running low can stall the job mid-pour, creating vulnerable seams where fresh material meets portions that have already begun to cure. Overestimating brings its own issues, as surplus mix sits unused and hardens, driving up unnecessary costs.
Calculating foundation concrete volume is not difficult once you understand what you are measuring. You need three measurements, and multiplying them together gives you your answer in cubic metres. This guide explains what to measure and how to work out the right quantity for your project.
The Formula Behind Every Foundation Calculation
Every foundation concrete calculation uses the same formula:
Length × Width × Depth = Cubic Metres (m³)
You take three measurements, multiply them together, and the result tells you how much concrete you need to order. The formula works for strip foundations, trench fill, pad foundations, and any other type you might be working with.
What makes foundation calculations different from slab calculations is understanding what those three measurements refer to when you are filling trenches rather than covering flat ground.
Pro-Mix Concrete’s Online Calculator gives you fast, accurate results the moment you enter your measurements. It keeps the process simple, saves time, and helps you plan each step with confidence.
Understanding What Each Measurement Means
Length Refers To Total Trench Distance
When calculating foundation concrete, length means the combined measurement of all trenches that need filling. You measure along the trenches themselves rather than across the building footprint.
A 5m × 4m extension, for example, has trenches running beneath all four external walls. Adding these together gives you your total length:
5 + 5 + 4 + 4 = 18 metres
If your project includes internal load-bearing walls that also require foundations, you need to add those trench lengths to your total as well. An internal supporting wall running 4 metres would bring your combined length to 22 metres.
Pro Tip:
Walk the actual trenches with a tape measure rather than relying solely on architectural drawings. Real excavations often vary slightly from plans, and small differences across multiple wall runs add up quickly.
Width Refers To The Trench Itself
Foundation width is determined by your structural engineer based on the loads your walls will carry and the bearing capacity of the ground beneath. Most domestic foundations in the UK fall into one of three standard width categories.
| Structure Type | Typical Trench Width |
| Garden walls and lighter structures | 450mm |
| Standard houses and extensions | 600mm |
| Heavier loads or weaker ground | 750mm |
Before running your calculation, convert the width to metres so all your measurements use the same unit. A 600mm trench becomes 0.6m in your formula.
Foundation widths must be designed to distribute building loads safely to the ground without excessive settlement. The minimum width depends on the total load from walls, floors, and roof, combined with the bearing capacity of the soil beneath your site.
Depth Refers To Concrete Fill Rather Than Total Excavation
This measurement causes the most confusion and leads to the most expensive mistakes. The depth of concrete you need is often quite different from the depth of the trench your groundworkers have dug.
Strip foundations place a layer of concrete at the bottom of the trench, typically 150mm for single-storey buildings or 200mm for two-storey structures. Blockwork then builds up from this concrete base to reach ground level, meaning most of the trench depth contains blocks rather than concrete.
Trench fill foundations take the opposite approach, pouring concrete almost to the top of the trench and leaving only about 150mm of space for a course or two of blocks beneath the damp-proof course.
Did You Know?
Trench fill foundations became popular in the UK during the 1980s because they significantly reduce construction time. Pouring concrete to near ground level eliminates days of blockwork below the damp-proof course, though it does require substantially more concrete than traditional strip foundations.
The difference in concrete requirements between these two foundation types is significant:
| Foundation Type | Trench Depth | Concrete Required |
| Strip foundation | 1,200mm | 150mm (0.15m) |
| Trench fill | 1,200mm | 1,050mm (1.05m) |
The same trench requires seven times more concrete when using trench fill compared to strip foundations. Confirming which foundation type your project uses before calculating prevents costly ordering mistakes.
Not certain which foundation type you need?
Our team can help you work it out and recommend the right concrete grade for your ground conditions.
How To Calculate Using The Pro-Mix Calculator
The Pro-Mix Concrete Calculator handles the arithmetic for you and automatically includes a waste allowance for real-world conditions. Here is how to use it for foundation projects:
Step 1: Enter The Length
Find the length field and type in your total trench measurement. This should be the combined length of all foundation trenches on your project.
Before entering your figure, make sure you have measured each external wall trench, added any internal load-bearing wall foundations, and included corners and returns in your total.
For a 6m × 4m extension with one 4m internal supporting wall, your calculation would be: 6 + 6 + 4 + 4 + 4 = 24 metres.
Choose your preferred measurement unit from the dropdown menu. Metres, millimetres, yards, feet, and inches all work, though metres suit most foundation projects best.
Step 2: Enter The Width
Move to the width field and enter your trench width. For standard 600mm domestic trenches, you would type 0.6 when working in metres or 600 when using millimetres.
Select your preferred unit from the dropdown menu and use the same unit consistently throughout your calculation.
Always measure perpendicular to your length line, and remember to confirm the width with your engineer if trenches vary across different sections of the project.
Pro Tip:
If your trenches have slightly irregular widths due to ground conditions, measure at both ends and use the larger number. This ensures you order enough concrete rather than running short mid-pour.
Step 3: Enter The Depth
The depth field is where you enter your concrete fill measurement rather than your total trench excavation depth.
For a strip foundation requiring 150mm of concrete, enter 0.15 in metres. For a trench fill foundation requiring 1,050mm of concrete, enter 1.05 in metres.
Your depth choice directly affects structural performance. Strip foundations rely on blockwork for most of the below-ground structure, while trench fill uses concrete throughout but requires significantly more material.
Step 4: Click Calculate
Press the calculate button to see your result. The calculator displays your required volume in cubic metres and automatically includes a 10% allowance for spillage, uneven trench bottoms, and normal waste.
This removes guesswork entirely and gives you an exact figure based on your measurements, eliminating manual calculation mistakes.
Step 5: Reset For Multiple Sections
If your project has trenches with different widths or depths, click the reset button and calculate each section separately. Once you have volumes for each section, add them together to get your total order quantity.
Practical Examples You Can Follow
Example 1: Single-Storey Kitchen Extension With Strip Foundations
Project measurements:
- Combined trench length: 4 + 4 + 3 + 3 = 14 metres
- Trench width: 600mm (0.6m)
- Concrete depth: 150mm (0.15m)
Calculation: 14 × 0.6 × 0.15 = 1.26m³
With 10% waste allowance: 1.39m³
Recommended order: 1.5m³
Example 2: Two-Storey Side Extension With Trench Fill
Project measurements:
- Combined trench length: 6 + 6 + 4 + 4 = 20 metres
- Trench width: 600mm (0.6m)
- Concrete depth: 1,050mm (1.05m)
Calculation: 20 × 0.6 × 1.05 = 12.6m³
With 10% waste allowance: 13.86m³
Recommended order: 14m³
Example 3: Detached Garage With Centre Post Pad
Perimeter foundation trenches:
- Combined length: 6 + 6 + 3 + 3 = 18 metres
- Width: 450mm (0.45m)
- Depth: 150mm (0.15m)
- Volume: 18 × 0.45 × 0.15 = 1.215m³
Centre post pad:
- Dimensions: 600mm × 600mm × 300mm
- Volume: 0.6 × 0.6 × 0.3 = 0.108m³
Combined total: 1.323m³
Recommended order: 1.5m³
Selecting The Appropriate Concrete Grade
Knowing your volume tells you how much concrete to order, but you also need to specify the right strength grade for your foundation type.
| Grade | Recommended Applications |
| C25 | Domestic strip foundations on stable ground |
| C30 | Trench fill foundations and heavier residential loads |
| C35 | Commercial projects, challenging ground, or engineer-specified requirements |
Foundation concrete must achieve adequate strength to support structural loads and resist deterioration from ground conditions. The appropriate grade depends on your foundation type, building loads, and exposure to sulfates or other aggressive chemicals in the soil.
All concrete Pro-Mix Concrete supplies meet BS EN 206 and BS 8500 UK standards for strength and durability. If you’re unsure about the right concrete grade or depth for your project, our team is on hand to guide you based on UK Building Regulations and your specific ground conditions.
What happens if Building Control requires deeper trenches than originally planned?
This situation arises frequently, particularly on clay soils or sites near mature trees. You can prepare a rough estimate for initial budgeting purposes, but it makes sense to finalise your concrete order after Building Control has inspected and approved the excavation depth.
Should I order extra concrete to account for steel reinforcement?
Rebar and mesh occupy very little volume within the concrete mass. The standard 10% waste allowance already covers this displacement, so you do not need to add anything extra.
Checklist Before Placing Your Order
Use this list to confirm you have everything ready:
- Measured the total length of all foundation trenches
- Confirmed trench width from the engineer or drawings
- Identified the correct concrete depth for your foundation type
- Converted all measurements to metres
- Calculated volume using length × width × depth
- Added 10% allowance for waste and spillage
- Rounded total up to the nearest 0.5m³
- Confirmed appropriate concrete grade
Alternatively, use our Concrete Calculator to handle the calculations automatically.
Order Your Foundation Concrete
Calculating concrete should not slow you down. The Pro-Mix Concrete Calculator gives you clear, accurate measurements in cubic metres within seconds, so there is no second-guessing and no risk of costly missteps.
Once the numbers are right, everything else falls into place. Your ready mix concrete arrives across London and nearby areas with same-day and next-day options, keeping the pace steady and the project moving without interruptions.
It all comes together in one smooth flow. The right volume, the right grade, and delivery that fits your schedule. Strong foundations begin with smart planning, and the right support keeps every step aligned until the job is done.
- Dennis Broderick is the founder and owner of Pro-Mix Concrete Company, a trusted name in ready-mix concrete solutions across the UK. With over 20 years of hands-on experience in the construction and concrete industry, Dennis brings unmatched expertise, practical insights, and a commitment to quality on every project - from residential driveways to large-scale commercial developments.
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