Concrete Calculator UK – Free Volume & Bag Estimator

Project Shape & Metrics

Step 1: Select Shape

Step 2: Enter Dimensions

Length (m):

Width (m):

Depth (mm):

Step 2: Enter Dimensions

Diameter (mm):

Depth/Height (m):

Quantity:

Step 2: Enter Dimensions

Outer Dia (mm):

Inner Dia (mm):

Length (m):

Quantity:

Step 2: Enter Dimensions

Curb Height (mm):

Curb Depth (mm):

Gutter Width (mm):

Gutter Thick (mm):

Total Length (m):

Step 2: Enter Dimensions

Step Run (mm):

Step Rise (mm):

Width (mm):

Platform (mm):

No. of Steps:

Concrete Mix (optional):

Wastage Allowance (%) 5%

5% is standard. Circular or complex pours often need 10%.

Curing Science Interactive Strength Chart

Days from Pouring: 7 Days 65% Strength

Standard concrete reaches 65% strength at 7 days and 99% at 28 days. Avoid heavy loads for the first 48 hours.

Results

Cubic Metres Needed: 0.00 m³

25kg Bags Required (Pre-Mixed): 0

Lorry Loads (6m³ Mixer): 0

Note: Ready-mix suppliers typically have a 1.0m³ minimum order and apply part-load charges for small volumes.

Estimated Labour: 0.0 hrs

Expert Note: Results include your selected wastage allowance. For ready-mix orders, round up to the nearest 0.1m³.

Estimated Material Cost: £0

(Based on UK averages)

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How to Calculate Concrete for Slabs, Columns, and Stairs

Whether you're pouring a simple shed base, setting circular fence posts, or forming complex concrete stairs, getting your volume calculation exactly right is crucial. Our Multi-Shape Concrete Calculator instantly works out the cubic metres (m³) you need and translates that into the exact number of standard 25kg bags required.

Concrete Volume Formulas by Shape

While our calculator handles the heavy lifting, here is the underlying math used by civil engineers and builders:

Rectangular Slabs & Footings:
Volume (m³) = Length (m) × Width (m) × Depth (m)
Round Columns & Post Holes:
Volume (m³) = π × Radius² (m) × Depth (m)
Circular Tubes & Rings:
Volume = [π × (Outer Radius)² - π × (Inner Radius)²] × Depth
Concrete Stairs: Calculated by treating each step as a solid rectangular block (Run × Width × Rise) stacked on top of each other, plus the top landing platform.
Curb & Gutter: Calculated as a composite shape: (Curb Height × Curb Depth) + (Gutter Width × Flag Thickness), multiplied by the total length.

Why Choose a Wastage Allowance?

It is an industry standard to order slightly more ready-mix concrete than your mathematical volume. Ground excavations are rarely perfectly flat, and formwork can bow under the immense weight of wet concrete.

5% (Standard): Ideal for simple square slabs on well-compacted, level sub-bases.
10% (Recommended): Best for digging circular post holes or column footings where the sides may be uneven.
15% (Complex Shapes): Highly recommended for intricate formwork like concrete stairs or curbs where spillage and movement are common.

Frequently Asked Questions

What is the standard ratio for mixing concrete?

A standard general-purpose concrete mix in the UK (C20) is usually 1 part cement, 2 parts sharp sand, and 4 parts 20mm gravel (1:2:4). Add water until the mix is workable but not sloppy.

How deep should a concrete base be?

For a garden shed base or patio, 100mm (4 inches) of concrete is usually sufficient. For a driveway taking vehicles, you should aim for a minimum of 150mm (6 inches) on a firm sub-base.

Do I need a sub-base under concrete?

Yes, for any load-bearing slab like a driveway or large shed, a compacted MOT Type 1 aggregate sub-base of at least 100mm is strongly recommended to prevent cracking and subsidence.

How long does concrete take to cure?

Concrete takes 24 to 48 hours to set enough to walk on, but it takes 28 days to reach its full structural strength. Keep it damp during the first week to prevent cracking. Use our interactive Curing Science chart above to see strength percentage over time.

What is the 'slump' of concrete?

The slump is a measure of the consistency or 'wetness' of the concrete. A low slump means a stiff mix (good for foundations), while a high slump means a more fluid mix (easier to level for slabs). Most DIY projects use a medium 'S3' slump.

How do I calculate the volume for concrete stairs?

Concrete stairs are calculated by modeling them as solid rectangular blocks. You calculate the volume of the top landing platform, and then calculate each step tread as a solid block extending down to the ground. Our calculator automates this complex geometry: just enter the Run, Rise, Width, and Number of Steps.

How much concrete do I need for fence post holes?

For standard round fence post holes, select the "Round Column or Footing" option. Enter your hole diameter (usually 300mm to 400mm depending on the post) and the depth of the hole. We recommend setting a 10% wastage allowance for hand-dug holes, as the sides are rarely perfectly vertical.

How many 25kg bags are in a cubic metre (m³)?

Typically, there are roughly 100 to 108 bags of 25kg pre-mixed concrete in one cubic metre, depending on the density and how tightly it is compacted.