Concrete Volume Calculator: Slabs, Footings & Mix Ratios

Summary

Accurate concrete volume calculation prevents two costly problems: ordering too little (requiring an additional truck delivery, with associated waiting time and minimum delivery charges) and ordering too much (paying for unused concrete that must be disposed of, which is also an environmental offence if not handled correctly). Ready-mix concrete is ordered in m³ and has a minimum order volume (typically 0.3–0.5 m³ for small-load concrete from specialist suppliers, or 6 m³ minimum for standard drum trucks).

The relationship between dry materials and finished concrete is counterintuitive — a nominal 1:2:4 mix by volume does not yield 7 units of concrete; the cement and water fill the voids in the aggregate. Approximately 1 m³ of dry materials (cement + sand + aggregate combined) yields approximately 0.67 m³ of finished concrete. This "bulking factor" must be understood for site-mix calculations.

For UK domestic work, understanding the correct concrete mix for each application is as important as the volume calculation. A foundation footing requires structural concrete (minimum C20, BS 8500 designation RC20/25), whereas a garden path can use a weaker decorative mix. Using the wrong mix — particularly too weak a mix for structural applications — can cause premature failure under load and is a defect that building control will identify on inspection.

Key Facts

• Basic formula — Volume (m³) = Length (m) × Width (m) × Depth (m); add 10% for waste and form over-dig
• Ready-mix ordering — ordered in m³; minimum order typically 3–6 m³ for standard drum truck; 0.3–0.5 m³ minimum for volumetric (Minimix) trucks
• BS 8500 concrete designations — C20 (RC20/25), C25 (RC25/30), C30 (RC30/37) are the most common domestic grades; higher number = higher strength
• Site mix ratio 1:2:4 — C20 equivalent; 1 cement : 2 sharp sand : 4 coarse aggregate by volume; suitable for slabs, paths, and general purpose
• Site mix ratio 1:3 — ballast mix (cement : all-in ballast); convenient for small domestic pours; equivalent to approximately C15–C20 depending on ballast quality
• Cement bag yield — one 25kg bag of cement mixed 1:2:4 yields approximately 0.015 m³ of finished concrete; 1 tonne bag of ballast mixed 1:3 yields approximately 0.5 m³ of finished concrete
• Slab depth minimums — pedestrian path: 75mm; domestic patio: 100mm; domestic driveway (light vehicles): 150mm; garage floor: 150mm minimum
• Foundation depth — strip foundation minimum 150mm thick, minimum 1.0m depth (or to frost-free depth on shrinkable clays); trench fill: pour to within 150mm of finished ground level
• Steel reinforcement — domestic slabs for light use do not normally require steel reinforcement; suspended slabs, driveways with regular HGV access, and any structural element require structural engineer specification
• Mix water quantity — typical water:cement ratio 0.45–0.55 for site-mixed concrete; too much water weakens the mix; slump (workability) should be 75–100mm for slab pours
• Curing time — concrete reaches approximately 70% strength at 7 days; 95%+ at 28 days; do not load or traffic the slab before 28-day cure (or allow 7 days minimum for light foot traffic)
• Frost protection — do not pour concrete when the temperature has been below 2°C for the preceding 24 hours, or when air temperature is expected to drop below 2°C within 24 hours of pouring

Quick Reference Table — Slab Thickness by Application

Worked Examples

Example 1: Patio Slab

Job: 5m × 4m patio, 100mm deep

Calculation:

• Volume = 5.0 × 4.0 × 0.10 = 2.0 m³
• Plus 10% waste: 2.0 × 1.10 = 2.2 m³

Ready-mix option: Order 2.25 m³ (round up to nearest 0.25 m³ for most suppliers)

Site-mix option (1:2:4 ratio by volume): Dry materials needed:

• Using the rule that 1 m³ dry materials → 0.67 m³ concrete
• Dry materials needed = 2.2 ÷ 0.67 = 3.28 m³ of dry mix
• In a 1:2:4 mix, total parts = 1+2+4 = 7; cement fraction = 1/7
• Cement volume = 3.28 ÷ 7 = 0.469 m³
• At 0.015 m³ per 25kg bag: 0.469 ÷ 0.015 = 31.3 bags → 32 × 25kg bags cement
• Sand volume = 3.28 × 2/7 = 0.937 m³ → approximately 1.5 tonnes of sharp sand
• Aggregate volume = 3.28 × 4/7 = 1.875 m³ → approximately 3 tonnes of 20mm gravel

For this quantity (2.2 m³), ready-mix from a volumetric truck is significantly more cost-effective and less labour-intensive than site-mixing.

Example 2: Strip Foundation

Job: 18m run of strip foundation, 450mm wide, 200mm deep

Calculation:

• Volume = 18.0 × 0.45 × 0.20 = 1.62 m³
• Plus 10%: 1.62 × 1.10 = 1.78 m³

Ready-mix order: 2.0 m³ (rounded to nearest 0.25 m³)

Mix specification: C20P (C20 air-entrained) or RC20/25 per Building Regs Approved Document A

Example 3: Fence Post Bases (5 posts)

Job: 5 fence posts, 100mm round post, 300mm diameter base hole, 600mm deep

Per post:

• Volume = π × r² × depth = π × 0.15² × 0.60 = 3.1416 × 0.0225 × 0.60 = 0.0424 m³
• Less post volume (100mm round, 600mm deep) = π × 0.05² × 0.60 = 0.0047 m³
• Net concrete per post = 0.0424 - 0.0047 = 0.0377 m³

5 posts: 0.0377 × 5 = 0.189 m³

Site-mix option (1:3 ballast mix):

• Dry materials: 0.189 ÷ 0.67 = 0.282 m³
• At 1:3 ratio: cement = 0.282 ÷ 4 = 0.071 m³ → 5 × 25kg bags cement
• Ballast = 0.282 × 3/4 = 0.211 m³ → approximately 400kg (0.4 tonne) ballast

This is a classic small job: buy 6 bags of cement and a half-tonne bag of ballast. The surplus can be used for minor repairs.

Example 4: Trench Fill Foundation for Extension

Job: Rear extension, 7m × 4m footprint; 4 sides of trench fill; average depth 1.0m, width 600mm

Perimeter of trench (for a new foundation, all four sides):

• Total length = (7.0 × 2) + (4.0 × 2) = 14 + 8 = 22m

Volume:

• 22m × 0.60m × 1.0m = 13.2 m³
• Plus 10% for over-dig and waste: 13.2 × 1.10 = 14.52 m³

Ready-mix order: 15 m³ (this will require 2–3 drum trucks on the day)

Logistics notes:

• A standard 6 m³ drum truck takes approximately 40 minutes to discharge
• Ensure adequate site access for trucks (width 3m minimum, height 4m)
• Have sufficient labour to spread and compact concrete as it arrives
• Specify C20P mix for trench fill (air-entrained for frost resistance)

Detailed Guidance

Site Mix Ratios — Which Mix for Which Job

UK concrete mixes are specified by BS 8500 in two ways: prescribed mixes (where the materials ratio is specified) and designated mixes (where the performance is specified and the supplier chooses the mix). For site-mixed concrete, prescribed mixes are used.

All-in ballast mixes: For convenience, many small domestic pours use "all-in ballast" (a pre-mixed aggregate containing sand and gravel) at a ratio of 1 part cement : 3 parts all-in ballast. This approximately produces C15–C20 concrete depending on ballast quality. This is acceptable for garden paths and non-structural slab work.

Estimating Water Requirement

Adding the correct amount of water is critical. Too much water increases workability (easier to pour) but reduces strength significantly. The water:cement ratio (W/C ratio) governs strength:

• W/C = 0.45: strong concrete, stiff mix, harder to work
• W/C = 0.55: moderate strength, workable mix — recommended for domestic work
• W/C = 0.65+: weak concrete, prone to shrinkage cracking — avoid

Water per batch (approximate for 1:2:4 mix per 25kg cement bag):

• 1 bag cement (25kg) at W/C = 0.55: 25 × 0.55 = 13.75L → approximately 13–14 litres of water
• Adjust for aggregate moisture content (wet sand needs less added water)

The "wrist test" for workability: lift a handful of concrete and squeeze. It should hold its shape without slumping. If it flows through the fingers, it is too wet. If it crumbles and won't hold shape, it is too dry.

Ready-Mix vs Site-Mix — When to Choose Each

Site-mix is more economical for:

• Very small volumes (under 0.3–0.5 m³)
• Locations without good vehicle access
• Jobs where pours can be spread over multiple sessions (e.g., fence posts over several days)

Ready-mix is more economical for:

• Volumes over 1 m³ (labour cost of site-mixing becomes significant)
• Pours that must be completed in one pour (slabs, foundations — construction joints are weakness)
• When volumetric trucks (Minimix) can access the site
• When time is critical

Volumetric (Minimix) trucks carry separate ingredients and mix on-site to the specified volume. They allow exact quantities (minimum typically 0.25–0.5 m³) and the driver can adjust consistency on-site. They cost more per m³ than drum trucks but are the most practical option for small domestic pours.

Formwork Considerations

Concrete expands laterally under its own weight (hydraulic pressure). Formwork must be strong enough to contain this pressure. A 200mm deep foundation pour exerts approximately 500 Pa of pressure at the base — manageable with standard timber formwork. A 1m deep trench fill exerts much higher pressure and the soil sides of the trench provide containment in most cases.

Formwork for slabs:

• Use 150mm × 50mm (6" × 2") C24 timber for slab edging
• Stake at 600mm centres for a 100mm slab; 400mm centres for a 150mm slab
• Set the top edge of formwork to the finished slab level with a spirit level

After pouring, allow a minimum of 24 hours before stripping formwork (48 hours in cold weather). The concrete surface is still green at 24 hours and must not be loaded.

Frequently Asked Questions

How do I convert the result from m³ to bags of cement?

From the example above: for a 1:2:4 mix, one 25kg bag of cement produces approximately 0.015 m³ of finished concrete (including all the other materials). So: required m³ ÷ 0.015 = number of cement bags. For a 1 m³ pour: 1 ÷ 0.015 ≈ 67 bags of 25kg cement (plus corresponding sand and aggregate). At this volume, ready-mix is almost always more economical.

What happens if I use too weak a mix for a foundation?

An undersized mix (e.g., C10 where C20 is specified) produces concrete that does not achieve the required compressive strength. Under the loads of a house or extension, it can crush or crack over time, leading to structural settlement. Building control will carry out foundation inspections for any notifiable work — using an incorrect mix specification is a Building Regulations failure.

Can I pour concrete in cold weather?

Concrete can be poured in cold weather with precautions. The risks are: freezing before sufficient hydration (strength gain) occurs, and frost heave damage to unhardened concrete. Do not pour when the slab or ground is frozen. If temperature will drop below 2°C within 24 hours: use accelerator admixtures, heated mix water, polythene sheet and straw covering after the pour, or postpone. Air-entrained concrete (contains tiny air bubbles) is more frost-resistant — specify C20P for external pours in UK conditions.

How long do I need to wait before building on a fresh concrete foundation?

28 days is the standard curing period for concrete to reach design strength. In practice, Building Regulations allow construction to proceed after concrete has been inspected and cleared by the building inspector — this is typically at 7 days when concrete has reached approximately 70% strength. For low-load structures (fences, sheds), 7 days is usually acceptable. For load-bearing walls, wait the full 28 days where practicable.

What's the difference between concrete and mortar?

Concrete contains coarse aggregate (gravel, 10mm or 20mm stones) and is used for structural and slab work. Mortar contains only sand (and lime or admixtures) — no coarse aggregate — and is used for bedding and jointing bricks, blocks, and paving. The same cement is used in both; the aggregate is the key difference.

Regulations & Standards

• BS 8500:2015+A2:2019 — concrete: specification, performance, production and conformity; defines concrete designations

• Building Regulations Approved Document A — structural requirements; foundation minimum depths and concrete specifications for strip and trench fill foundations

• BS 8204:2003 (Parts 1–6) — screeds, bases and in-situ flooring; screed thickness and concrete slab specifications

• Concrete Centre: Mix design guidance — UK concrete specification and mix design guidance

• The Concrete Society — technical guidance on concrete production and quality

• MPA The Concrete Industry — concrete production standards and guidance

• BRE: Foundation design — guidance on foundation types and concrete specifications

• foundations — foundation types: strip, trench fill, raft, piled

• patio laying — patio sub-base and mortar bed specifications

• block paving — sub-base depths and concrete edge restraints

• timber quantities — timber joist calculator for comparison