How Much Does a Bulk Bag of Topsoil Cover? UK Coverage Calculator

An 850-litre bulk bag of topsoil covers approximately 17m² at 5cm deep, 8.5m² at 10cm, or 5.7m² at 15cm. These are calculated coverage figures before allowing for settlement, uneven ground or spreading losses.

The important detail is the amount of soil inside the bag. “Bulk bag” describes the packaging, so always check the stated volume before working out how far it will go.

Kelvin Compost lists its Top Soil bulk bags at 850 litres, equivalent to 0.85 cubic metres. This guide uses that capacity for Kelvin examples and shows how to calculate coverage for other bag sizes.

How much ground will one bulk bag cover?

The deeper you spread the soil, the smaller the area it covers. Double the depth and you halve the coverage.

For an 850L bag:

Figures are rounded where necessary. They assume an even layer and use the stated bag volume without a settlement allowance.

These depths are calculation examples, rather than recommendations for every garden. A shallow layer added to existing soil is a different job from creating an entire growing layer.

UK topsoil coverage calculator: work out your order

You need three things: the area, the depth you want to add and the bag’s stated capacity.

Calculate how much soil you need

For a rectangular area:

Length (m) × width (m) × added depth (cm) × 10 = litres required

Then:

Litres required ÷ litres per bag = number of bags

Round the final result up to the next whole bag if you are buying only that bag size.

For example, a 6m × 4m area needing an additional 7cm of soil requires:

6 × 4 × 7 × 10 = 1,680 litres

With 850L bags:

1,680 ÷ 850 = 1.98 bags

You would need two bulk bags, supplying 1,700 litres. That leaves only 20 litres above the calculated requirement, so check your measurements and any allowance for settlement before ordering.

Calculate how far one bag will go

If you already know the bag capacity:

Bag volume (litres) ÷ [depth (cm) × 10] = coverage in m²

For an 850L bag spread 10cm deep:

850 ÷ (10 × 10) = 8.5m²

If you prefer millimetres, there is a useful shortcut:

Bag volume (litres) ÷ depth (mm) = coverage in m²

Calculate using cubic metres

You can also use:

Area (m²) × depth (m) = volume in m³

Remember:

• 5cm = 0.05m

• 10cm = 0.10m

• 15cm = 0.15m

• 1m³ = 1,000 litres

Keep the units consistent throughout. Entering 10 instead of 0.10 for a 10cm depth would make the result 100 times too large.

Why different bulk bags cover different areas

Two bags can both be described as “bulk bags” while containing different volumes.

For example, Rolawn’s Beds & Borders Topsoil specification lists approximately 500 litres when packed. Kelvin’s listed Top Soil bag capacity is 850 litres.

Here is how different stated capacities compare:

These are volume comparisons, not a list of bag sizes sold by Kelvin Compost.

Use the supplier’s declared contents. The outside dimensions of a flexible bag do not reliably tell you how much soil has been supplied.

Is a bulk bag the same as a tonne of topsoil?

No. Litres and cubic metres measure volume. Kilograms and tonnes measure weight.

An 850L bag contains 0.85m³ by volume. That does not mean it weighs 850kg.

Likewise, a bag described as 850kg does not necessarily contain 850 litres.

Topsoil’s weight per cubic metre depends on its composition, moisture and how densely it is packed. You therefore need the supplier’s volume information or bulk-density figure to convert a weight-based order.

The calculation is:

Volume (m³) = weight (tonnes) ÷ bulk density (tonnes per m³)

For illustration only, if a supplier states a density of 1.4 tonnes per cubic metre:

1 tonne ÷ 1.4 = approximately 0.714m³

At 10cm deep, that would cover:

0.714 ÷ 0.10 = approximately 7.14m²

The 1.4 figure is an example, not a specification for Kelvin’s topsoil. Ask for the conversion appropriate to the material you are ordering.

Names such as “tonne bag”, “jumbo bag” and “dumpy bag” are not enough to calculate coverage on their own.

How many 850L bags do you need?

This table gives the number of whole 850L bags, rounded up after calculating the required volume.

No extra allowance is included. Rounding up sometimes leaves plenty of spare material and sometimes very little.

For instance, 40m² at 10cm requires 4,000 litres. Five 850L bags supply 4,250 litres, leaving 250 litres above the calculated volume.

Where the result runs into several cubic metres, compare the practical options in our bulk bag versus loose-load delivery guide.

Will one bag fill a raised bed?

Measure the bed’s internal length and width, then the depth you actually need to fill.

A bed measuring 2.4m × 1.2m internally, with 30cm left to fill, needs:

2.4 × 1.2 × 0.30 = 0.864m³

That is 864 litres.

One 850L bag is therefore 14 litres short, even before settlement. It is close, but it is not a complete fill.

You could source the shortfall separately or order two bulk bags if that is the only available format, allowing for the substantial surplus. Do not round the volume down simply because it is nearly one bag.

If you are combining topsoil with other materials, 864 litres is the total growing-medium requirement, not automatically the amount of topsoil alone. Our raised-bed filling guide explains the wider planning considerations.

Choose the depth before choosing the number of bags

It is tempting to start with “I’ll order one bag” and spread it until it runs out. That can leave a layer too shallow for the job.

Work out the additional depth first. Consider how much suitable soil is already there and where the finished surface needs to sit.

The RHS topsoil buying guide gives starting points of at least 10cm for laying lawns and around 20cm or more for new beds. Those figures need to be considered alongside the existing soil; they do not mean every garden needs that much imported material.

For project-specific advice, read how deep topsoil should be for lawns, beds and gardens.

Allow for uneven ground and settlement

A calculation gives you the volume of the space you want to fill. The finished result also depends on the ground and how the soil settles after spreading and firming.

For uneven areas, measure the required fill depth at several points. Divide markedly different sections into separate areas, calculate each one and add the volumes together.

If your supplier recommends an ordering allowance, apply it before rounding to whole bags.

For example, with a chosen 10% allowance:

1,680L × 1.10 = 1,848L

1,848 ÷ 850 = 2.17 bags → three whole bags

That is a different order from the two bags required by the original calculation.

Ten per cent is an illustrative allowance here, not a guaranteed settlement rate. Check what is appropriate for the soil and project, particularly where finished levels matter.

Frequently asked questions

How much does an 850L bulk bag cover at 5cm deep?

Approximately 17m², before settlement or spreading losses.

How much does a bulk bag cover at 10cm deep?

An 850L bag covers 8.5m². A 500L bag covers 5m², while a 1,000L bag covers 10m². Check the actual capacity before using a coverage figure.

How many bulk bags make one cubic metre?

One cubic metre is 1,000 litres. That equals approximately 1.18 bags of 850L. If you can buy only whole 850L bags, you would need two, supplying 1,700 litres.

How much topsoil do I need for 20 square metres?

At an added depth of 5cm, you need 1,000 litres. At 10cm, you need 2,000 litres. At 15cm, you need 3,000 litres.

Those quantities require two, three or four whole 850L bags respectively, before any extra allowance.

How much does one tonne of topsoil cover?

There is no single reliable answer without knowing the soil’s volume or bulk density and the depth you intend to spread it. Ask the supplier for the volume represented by the quoted weight.

Order the right amount of topsoil

Before ordering, note your area, the additional depth required and the bag capacity. Those three figures are much more useful than guessing from the size of the garden.

Browse Kelvin Compost’s Top Soil, or contact our team with your measurements for help choosing the quantity and arranging delivery in Glasgow, East Kilbride, South Lanarkshire and surrounding areas, subject to availability and access.

Call +44 7561 251350 or email info@kelvincompost.co.uk. For delivery planning, see our Glasgow topsoil delivery guide.