How many bags of cement do I need for 1 cubic metre of concrete?

For a standard 1:6 mix (paths and shed bases), you need approximately 12 bags of 25 kg cement per cubic metre. A stronger 1:4 mix (footings and lintels) requires about 16 bags per cubic metre. These figures include a 30% bulking factor.

What is the difference between 1:4, 1:5, and 1:6 concrete mixes?

The ratio indicates parts cement to parts ballast by volume. A 1:4 mix is stronger (suitable for structural footings and lintels), 1:5 is general purpose, and 1:6 is a standard mix for paths, shed bases, and non-structural work.

What is the bulking factor for concrete?

A bulking factor of 1.3 (30%) is standard. Dry cement and ballast occupy more volume than wet mixed concrete because of air gaps between particles. The bulking factor ensures you order enough dry material to produce the required volume of wet concrete.

How much ballast do I need per cubic metre?

For a 1:6 mix, you need approximately 1.11 m3 of ballast per cubic metre of finished concrete (after applying the 30% bulking factor). This equates to roughly 3 bulk bags (0.5 m3 each) from a builders merchant.

Ballast and Cement Calculator

Calculate how many bags of cement and ballast you need for your concrete project

Published by UltraCalculatorsLast reviewed: 29 Mar 2026v1.0Sources:NHBC Standards Chapter 5.2: Concrete BS 8500-2: Concrete specificationReport an issue

When mixing concrete by hand or ordering materials for a small to medium project, knowing exactly how many bags of cement and how much ballast you need saves time, money, and wasted trips to the builders merchant. Ballast (also called all-in aggregate or mixed aggregate) is a blend of sharp sand and gravel that is pre-mixed in the correct proportions for concrete. Combined with cement and water, it produces a versatile concrete suitable for paths, shed bases, fence post footings, and general domestic use. The three most common UK concrete mix ratios for domestic work are 1:4 (strong mix for footings and lintels), 1:5 (general purpose), and 1:6 (standard mix for paths, shed bases, and non-structural work). The first number represents parts cement and the second represents parts ballast, measured by volume. This calculator works out the exact volumes of cement and ballast needed for your required concrete volume, applies the standard 30% bulking factor, and converts the result into practical bag counts using standard UK bag sizes (25 kg cement bags and 0.5 m3 bulk bags of ballast).

How to Use This Calculator

To calculate the cement and ballast you need: 1. Enter the volume of concrete required in cubic metres (m3). If you have calculated the volume using length x width x depth, enter that figure here. For example, a shed base measuring 3 m x 2 m x 0.1 m deep requires 0.6 m3 of concrete. 2. Select the mix ratio. Use 1:6 for paths, shed bases, and non-structural work. Use 1:5 for general purpose applications. Use 1:4 for structural footings, lintels, and any work requiring extra strength. 3. Review the results. The calculator shows the volume of cement and ballast needed (in cubic metres), the number of 25 kg cement bags to buy, and the number of 0.5 m3 bulk bags of ballast to order. All figures include a 30% bulking factor to account for the difference between dry and wet volumes. 4. Always round up when buying materials. It is better to have a small surplus than to run short mid-pour, as concrete should be laid continuously to avoid cold joints.

How It Works

The calculation applies a standard bulking factor of 1.3 (30%) to account for the fact that dry materials compact when mixed with water. Without this factor, you would run short of material. For a given mix ratio (e.g. 1:6), cement makes up 1/7 of the total by volume (proportion = 0.143) and ballast makes up 6/7 (proportion = 0.857). Cement volume = concrete volume x cement proportion x 1.3 Ballast volume = concrete volume x ballast proportion x 1.3 Both volumes are rounded to 2 decimal places before calculating bag counts. Cement bags = ceil(cement volume / 0.017), where 0.017 m3 is the approximate volume of one 25 kg bag of cement. Ballast bags = ceil(ballast volume / 0.5), where 0.5 m3 is a standard bulk bag of ballast. For example, with 1 m3 of concrete at a 1:6 ratio: cement volume = 1 x 0.143 x 1.3 = 0.19 m3, requiring ceil(0.19 / 0.017) = 12 bags. Ballast volume = 1 x 0.857 x 1.3 = 1.11 m3, requiring ceil(1.11 / 0.5) = 3 bulk bags.

Worked Examples

Example 1: Shed base (1 m3 at 1:6 mix)

Inputs: Volume: 1 m3, Mix ratio: 1:6

Cement proportion = 0.143, ballast proportion = 0.857.
Cement volume = 1 x 0.143 x 1.3 = 0.19 m3 (rounded to 2dp).
Ballast volume = 1 x 0.857 x 1.3 = 1.11 m3 (rounded to 2dp).
Cement bags = ceil(0.19 / 0.017) = ceil(11.18) = 12 bags of 25 kg.
Ballast bags = ceil(1.11 / 0.5) = ceil(2.22) = 3 bulk bags.

Result: You need 12 bags of 25 kg cement and 3 bulk bags (0.5 m3 each) of ballast. This is a manageable hand-mixing project for a weekend.

Example 2: Fence post footings (0.4 m3 at 1:6 mix)

Inputs: Volume: 0.4 m3, Mix ratio: 1:6

Cement volume = 0.4 x 0.143 x 1.3 = 0.07 m3.
Ballast volume = 0.4 x 0.857 x 1.3 = 0.45 m3.
Cement bags = ceil(0.07 / 0.017) = ceil(4.12) = 5 bags.
Ballast bags = ceil(0.45 / 0.5) = ceil(0.9) = 1 bulk bag.

Result: You need 5 bags of 25 kg cement and 1 bulk bag of ballast. For fence post footings, a 1:6 mix is perfectly adequate.

Example 3: Foundation strip (2 m3 at 1:4 strong mix)

Inputs: Volume: 2 m3, Mix ratio: 1:4

Cement proportion = 0.2, ballast proportion = 0.8.
Cement volume = 2 x 0.2 x 1.3 = 0.52 m3.
Ballast volume = 2 x 0.8 x 1.3 = 2.08 m3.
Cement bags = ceil(0.52 / 0.017) = ceil(30.59) = 31 bags.
Ballast bags = ceil(2.08 / 0.5) = ceil(4.16) = 5 bulk bags.

Result: You need 31 bags of 25 kg cement and 5 bulk bags of ballast. At 2 m3 with a strong mix, this is a significant amount of material; consider ordering ready-mix concrete delivered by lorry instead.

Ballast is available from builders merchants in bulk bags (approximately 0.5 m3, also called "dumpy bags" or "tonne bags" weighing around 800-850 kg) or smaller 25 kg bags for very small projects. Cement should be stored in a dry place and used within 3 months of purchase, as it absorbs moisture and loses strength over time. For projects above 2-3 m3, ordering ready-mix concrete from a batching plant is usually more practical and cost-effective. The mix ratios in this calculator use cement and ballast only; for more precise structural mixes, consult BS 8500-2 or speak to a structural engineer. Always wear gloves, eye protection, and a dust mask when handling cement, as it is a skin and respiratory irritant.