When planning a landscaping project, it can be very hard to estimate the amount of topsoil you need. Happily, there are certain secrets the professionals use to work out how much topsoil to buy.
When it comes to buying topsoil or turf, guesstimating just doesn’t do the job well enough. If you don’t have enough topsoil in place before your Meadowmat or garden turf arrives, it will delay laying and increase the risk of sod heating.
On the other hand, if you have a lot of topsoil left over, you’re stuck wondering what to do with it, as it’s not easy to transport and bulky to store. Here’s how to figure out how much topsoil per square metre you need for your garden.
Calculating How Much Soil You Need
Here’s how to calculate the amount of top dressing you could need for a project or to calculate how much topsoil to buy.
Measure the Area
Firstly, measure the area you wish to spread topsoil over. Make sure that you take all measurements in metres, as this makes it easier to convert the final quantity into either bagfuls or lorry loads.
Mark out your desired area, and carry out measurements depending on its shape, just as you would if you were measuring for turf:
- If it’s a rectangle, measure the length and width.
- If it’s a circle, measure the diameter.
- If it’s a triangle measure the height and width.
How to Calculate the Top Dressing Area
For a rectangle, use the following formula:
Area = Length x Width
For a circle, divide the diameter measurement by two to find out the radius of the circle, then use the following formula:
Area = 3.14 x Radius x Radius
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 the planter to about 5cm below the top, which 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 wildflower turf 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.
How Much Soil Do I Need To Buy?
Working out the amount of soil that you need to buy doesn’t need to be complicated but it is important. 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 will give you the volume of your area – 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 More Complicated Gardens
Suppose your project is an irregular shape. Maybe it’s curvy, or has lots of angles. Perhaps the height of it varies? The easiest way to calculate areas and volumes is to divide them 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 soil bed. It’s in two sections. The foot of the “L” is 15 cm deeper than the tail.
First, let’s measure each section separately. The tail is 30cm deep, 0.9 metres wide and 2 metres long. You only want to fill it to within 5cm of the top, so the depth is going to be 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, let’s take 5cm off of 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 you have the choice to either buy 1 bag and top the planters up with mulch or compost from the garden centre. Or you could buy 2 bags and use the remainder for another project.
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 teams – we’ll help you with the calculations and quote you for the best soil for your project.