Concrete Calculator Guide

Bagged Concrete

Pre-mixed dry concrete in bags. Just add water and mix.

Standard Bag Sizes

When to Use Bagged Concrete

• Small jobs under 0.2 m³ (~16 × 25kg bags)
• Fence posts, small paths, step repairs
• DIY projects where you can mix in batches
• No vehicle access for ready-mix lorry

Postcrete (Rapid-Set)

Special bagged concrete for fence posts. Sets in 10 minutes without mixing (just pour dry and add water).

• 20kg bag = 1 fence post
• £7-9 per bag
• Only for posts, not slabs/footings

Ready-Mix Concrete Delivery

Wet concrete delivered by lorry. Poured directly where needed.

Minimum Orders

• Most suppliers: 1 m³ minimum (some charge extra under 2 m³)
• Price: £90-130 per m³ depending on mix
• Delivery charge: £0-50 (free over minimum order)

When to Use Ready-Mix

• Jobs over 1 m³ (saves time and money vs bagged)
• Driveways, large patios, garage bases
• You need to pour it all at once (can't do in batches)
• Vehicle access available (lorry needs to get close)

Cost Comparison

Small Path (0.5 m³)

• Bagged: 40 × 25kg bags = £200-240
• Ready-mix: 1 m³ minimum order = £100-130 (you'd waste 0.5 m³)
• Winner: Bagged (if you can use the 0.5 m³ elsewhere, ready-mix wins)

Patio (2 m³)

• Bagged: 160 × 25kg bags = £800-960 + huge mixing effort
• Ready-mix: 2 m³ delivery = £180-260
• Winner: Ready-mix (massively cheaper and faster)

Concrete Mixes

General Purpose (C20)

• Paths, light-use patios, sheds
• Most bagged concrete
• Ready-mix: ~£95/m³

Domestic (C25)

• Driveways, heavy-use patios
• Ready-mix: ~£105/m³

Foundations (C30)

• Strip footings, structural bases
• Needs Building Control approval
• Ready-mix: ~£115/m³

Calculating Concrete Volume

Slabs and Bases

Volume (m³) = length (m) × width (m) × depth (m)

Example: 5m × 3m patio, 100mm (0.1m) thick = 5 × 3 × 0.1 = 1.5 m³

Footings

Strip footing volume = length × width × depth

Example: 10m long footing, 450mm (0.45m) wide, 200mm (0.2m) deep = 10 × 0.45 × 0.2 = 0.9 m³

Fence Posts

Circular hole volume = π × radius² × depth

Example: 300mm (0.3m) diameter hole, 600mm (0.6m) deep = 3.14 × 0.15² × 0.6 = 0.042 m³ per hole

For 8 posts: 0.042 × 8 = 0.34 m³ (27 × 25kg bags, or use Postcrete)

Always Add Wastage

Add 10% to your calculation:

• Uneven ground (dips and holes use more)
• Spillage during pouring
• Slightly out measurements

Running short mid-pour is a disaster (visible join lines, weak spots).

Mix-It-Yourself Alternative

Buy separate cement, sand, and aggregate (ballast). Cheaper per m³ than bagged concrete, but much more work.

• Ratio: 1 part cement : 2 parts sand : 3 parts aggregate
• Cost: ~£60-80 per m³ in materials
• Effort: Mixing 1 m³ by hand = several hours hard work

Only worth it for large DIY projects (2-5 m³) if you have a cement mixer.