Concrete Footings Calculator

Foundation Concrete Guide

What should I know about Foundation Depth Requirements?

UK building regulations require strip foundations to be at least 150mm wide and the depth of any concrete specified by the structural engineer. For typical domestic extensions: 450mm wide × 300mm deep in stable ground, deeper in clay or shrinkable soils. Always consult building control for foundations near trees.

What's the key thing to understand about Concrete Grade?

Strip footings: GEN3 (equivalent to C20/25). Pad footings: RC28/35. Post holes: GEN1 or even just rapid-set postcrete. For reinforced concrete (RC) work, always use the specified grade — not general purpose bags.

When to Order Ready-Mix?

For volumes over 0.5m³, ready-mixed concrete is usually more economical and gives better consistency. Minimum ready-mix orders are typically 0.5–1m³. Below this, bagged rapid-set concrete is more practical and avoids minimum load charges.

What's the key thing to understand about Safety and Regulations?

Most structural and electrical DIY work in England and Wales must comply with Building Regulations and is notifiable to local Building Control. Work that is not notifiable (like-for-like replacements, cosmetic changes) can be done without notification. Notifiable work done without approval is technically illegal and can cause problems when selling the property. Electrical work in most rooms requires a Part P competent person or Building Control inspection. Gas work must always be performed by a Gas Safe registered engineer — it is illegal for anyone else to carry out gas work in the UK, and using an unregistered installer invalidates both your home insurance and any warranty on the work. For structural changes (removing load-bearing walls, altering the roof structure), always get a structural engineer's assessment before starting, even for jobs that look straightforward.

What do I need to know about Trench Fill vs Strip Foundations?

Strip foundations and trench fill foundations are the two most common approaches for a domestic footing, and the choice between them significantly affects the concrete volume this calculator estimates. A traditional strip foundation is a relatively shallow trench, often around 450-600mm deep depending on ground conditions and local building control requirements, filled with a modest depth of concrete at the base, on top of which blockwork is built up to ground level inside the trench. Trench fill, by contrast, fills the trench almost entirely with concrete right up near ground level, needing substantially more concrete volume for the same trench footprint, but removing the need for bricklaying below ground, which can save labour time and is often preferred on deeper foundations, in poor ground, or where a trench is too narrow or unstable to safely build blockwork inside. Trench fill is also commonly favoured where the foundation needs to go unusually deep, for example near trees with deep root systems, in shrinkable clay soils, or close to drains, since digging and bricklaying in a deep, narrow trench becomes increasingly impractical and unsafe as depth increases, while pouring concrete does not carry the same access difficulty.

What should I know about Curing Time Before Building On Footings?

Concrete footings need adequate time to gain strength before loads are built onto them, and rushing this stage risks cracking or settlement that's far more expensive to fix than the time saved by starting early. As a general guide, concrete reaches a significant proportion of its eventual strength within the first few days, commonly enough to tolerate light foot traffic or very light loading after around 2-3 days in reasonable weather, but full design strength isn't reached for around 28 days, which is the standard curing period concrete strength is specified against. For footings specifically, most builders and building control inspectors expect at least several days of curing, and often a week or more depending on the load that will be built on top and the weather conditions, before starting blockwork or brickwork on top of a freshly poured footing. Cold weather slows curing significantly, since the chemical reaction that hardens concrete, hydration, happens more slowly at low temperatures and can effectively stop in a hard frost, so winter pours need extra time, insulation such as hessian or insulated matting over the fresh concrete, or admixtures to maintain an adequate curing rate and protect against frost damage to the surface.

What do I need to know about Reinforcement Bar (Rebar) in Footings?

Most ordinary domestic strip footings for a single or two-storey extension don't need reinforcement if they're the correct depth and width for the ground conditions, since plain concrete in compression performs well in a footing. However, rebar becomes necessary where footings span soft spots, made ground, or variable bearing capacity, where there's a risk of differential settlement, or where the footing is unusually narrow relative to its depth and could crack under uneven loading. A typical reinforced strip footing uses two or more runs of 12-16mm deformed bar near the base, tied with spacers to keep at least 50mm of concrete cover below and around the steel — rebar sitting too close to the surface or ground is a common defect that lets corrosion start early and defeats the purpose of reinforcing. Mesh reinforcement (A142 or A193) is more commonly used in raft foundations and ground-bearing slabs than in strip footings. Whether reinforcement is required is a structural engineering decision based on ground investigation, not something to guess — your Building Control surveyor or a structural engineer will specify it based on the site conditions revealed once the trench is dug.

What's the Difference Between a Footing and a Foundation?

The terms get used loosely but mean slightly different things. A foundation is the general term for the whole substructure that transfers a building's load into the ground. A footing is the specific widened base at the bottom of that foundation — the part in direct contact with the bearing soil — whose job is to spread the load over enough ground area that the soil's bearing capacity isn't exceeded. In a strip foundation, the footing is simply the base of the trench-fill concrete. In a pad foundation (for an isolated column or post), the footing is a distinct square or rectangular concrete pad, often wider than the wall or column it supports, specifically sized to spread a concentrated point load. Getting the footing width right matters more on weaker or more variable ground, which is exactly why building control wants to see trial pit or site investigation evidence before signing off a non-standard foundation depth or width.

How Do I Calculate Concrete Volume for an L-Shaped or Stepped Footing?

Real footing layouts are rarely a single straight run. For an L-shaped footing (a common garage or extension layout), split the plan into two straight rectangular runs and sum their volumes separately: volume = (length₁ × width × depth) + (length₂ × width × depth), taking care not to double-count the corner section where the two runs overlap — measure each run's length to the centreline of the corner, not to the outer corner of the trench, to avoid that double-count. For a stepped footing (common on sloping sites, where the footing drops down in level steps rather than following the slope continuously), calculate each stepped section as its own straight rectangular volume at its own depth, then sum them — and remember that each step typically needs a short vertical 'riser' of concrete connecting one level to the next, which is often forgotten in a quick hand calculation and adds a meaningful extra volume on a steeply sloping plot.

What Reinforcement Bar Spacing Is Typical in a Domestic Strip Footing?

Where reinforcement is specified (not every domestic strip footing needs it — many simple, well-bearing-soil strip footings are unreinforced mass concrete), a common domestic arrangement uses two or more horizontal runs of 12-16mm rebar running the length of the footing, tied into a cage with vertical and horizontal links at roughly 300-450mm centres, with the steel positioned with at least 50-75mm of concrete cover on all sides to protect it from corrosion. The exact bar size, number of runs, and spacing should always come from a structural engineer's design for anything other than the simplest, most conservative footing, since reinforcement requirements depend on the specific loads, soil conditions, and footing dimensions involved — rebar in the wrong position (too close to the surface, insufficient cover) can actually weaken a footing's durability by letting moisture reach and corrode the steel, which then expands and cracks the surrounding concrete from within.