Soil volume calculations (or How much topsoil should I buy?)

It’s very hard to estimate the amount of topsoil you will need for a landscaping project.  This article shows you how professionals work out how much topsoil to buy.

Calculating how much topsoil to buy

When it comes to buying topsoil or turf, guestimating just isn’t good enough.  If you don’t have enough topsoil in place before your Meadowmat or turf arrives, it will delay laying.  That increases the risk of sod heating.

On the other hand, if you have a lot of topsoil left over – what do you do with it?  It’s not easy to transport and its bulky to store – particularly if it’s loose.

Here’s how to calculate the amount of soil you need for a project and work out how much topsoil to buy.

First: Measure the area

TAKE ALL MEASUREMENTS IN METRES – it makes it easier to convert the final quantity into either bagfuls or lorry loads.

Mark out the area you need to top up with topsoil.    Measure the length and the width of a rectangle, the diameter of a circle or the height and width of a triangle.  Just as you would if you were measuring for turf.

Now calculate the area:

For a rectangle:

Area = Length x Width

For a circle

Divide the diameter measurement by two to find out the radius of the circle.

Area = 3.14 x radius x radius

Example, the diameter is 15 metres.

Area = 3.14 x 15 x15 = 706.5 square metres

For a triangle

Multiply the height of the triangle by the base and then divide the answer by two.

Work out the depth of soil needed

If you are filling a raised bed, it’s easy to work out the depth of soil you’ll need.  Simply measure from the bottom of planter to about 5cm below the top.  This will allow you plenty of room to add plants.

For a living green roof, treat it as if it were a huge container.  Measure the depth of the edgings and subtract 5cm to allow for the drainage mat and the sedum mat.   Be wary of the weight bearing capacity of your building though.   If you’re not sure, check with your architect before ordering substrate.

If you are topping up the growing medium for a lawn or a wild flower area, you will need a total soil depth of 15cm or more.    Decide how deep the existing soil is and then work out how many more cm deep it needs to be.

To work out how much soil to buy

Multiply the area you calculated earlier, by the depth of growing medium needed.

In simple terms you will be working out width, x length x height

Make sure all of the measurements you used were in metres.   (that means 15cm would be expressed as 0.15 metres in your sums)

The final answer is the volume – this is the figure you need.

Divide the final answer (the volume) by 0.86, this will tell you how many big bags of topsoil you need.

For composite shaped gardens

If your project is an irregular shape, maybe it’s curvy, maybe it has lots of angles, perhaps the height of it varies?  The easiest way to calculate areas and volumes is to divide it into simple shapes.  Work out the area and volume of each shape separately and then add them all together to get your final requirement.

Sounds complicated?

Here’s an example.

Picture an “L” shaped raised bed.  It’s in two sections.  The foot of the “L” is 15 cm deeper than the tail.

First I’m going to measure each section separately.

The tail is 30cm deep, 0.9 metres wide and 2 metres long.  I only want to fill it to within 5cm of the top, so I’m going to count the depth as 25cm.

That section will need 0.25 x 0.9 x 2 = 0.45 cubic metres

The foot of the “L” measures 45cm deep, 90cm wide and 1.5 metres long.  Again, I’m going to take 5cm off the depth measurement because it won’t be filled to the brim

0.4 x 0.9 x 1.5 = 0.54 cubic metres

Adding the two sections together

0.45 + 0.54 = 0.94

0.94 cubic metres is slightly more than one bag of topsoil.   1 big bag holds 0.86 cubic metres so now I have the choice to either buy 1 bag and top the planters up with mulch or compost from the garden centre. Or I could buy 2 bags and use the remainder for another project.

What would you do?

Help with calculations

If you are confused by maths, areas, volumes and all that.  Don’t panic.  Just take the measurements as best you can and then call one of our team – we’ll help you with the calculations and quote you for the best soil for your project.

Contact the Turfonline team

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