Right, so you’re planning a garden project and trying to work out how much soil to buy.
The thing is calculating soil isn’t rocket science but there’s a few tricks to getting it right. I will talk you through the basics so you know what youre doing.
Before you start measuring though, you need to be clear with what youre trying to achieve with this project. Are you filling up some raised garden beds? Creating new garden areas from scratch? Laying down a new lawn? Each one needs different amounts of soil depth, which obviously affects how much you’ll need to buy.
General depth requirements:
- Lawns need about 10-15 centimetres of soil depth
- Flower beds work well with 20-30 centimetres
- Vegetable gardens should be 30-45 centimetres deep
- Raised beds can go deeper if you want – 40-50 centimetres
- Potted plants obviously depend on pot size
The depth thing is pretty important actually. You don’t need to mulch lawns but if you are making a veggie patch then you may want to go a little bit deeper because the veggies need lots of space to grow their roots.
Measuring rectangular or square areas
This is the easy bit. Get your tape measure out and measure the length then the width then work out how deep you want the soil. Convert everything to metres because that’s how soil suppliers work out their measurements.
Here’s the basic process:
- Measure length in metres
- Measure width in metres
- Work out depth needed (convert centimetres to metres)
- Multiply length x width x depth = cubic metres needed
- Double-check your maths because mistakes are expensive
So if you’ve got a garden bed that’s 5 metres long by 2 metres wide and you want the soil 30 centimetres deep, you just multiply it all together. That’s 5 x 2 x 0.3 which gives you 3 cubic metres. Pretty straightforward really.
The tricky bit is remembering to convert your depth to metres. 30 centimetres is 0.3 metres not 30 metres!
Round areas are a bit trickier
If you’re doing circular garden beds or round planters you may need to pull out some highschool maths skills. Measure from the centre to the edge, that’s your radius. Then you use that formula for the area of a circle, which is pi times radius squared, then multiply by your depth.
For circular areas you need:
- Radius measurement (centre to edge)
- Depth requirement in metres
- Formula: 3.14 x radius x radius x depth
- Calculator helps because the numbers get messy
- Round up to nearest 0.1 cubic metres for ordering
So a circular bed with a 2 metre radius and 30 centimetres deep works out to roughly 3.77 cubic metres. The exact number is 3.14 x 4 x 0.3 if you want to check my maths.
Weird shaped areas
Let’s be honest here most gardens aren’t perfect rectangles or circles. Most gardens are made up f unequal shapes with trees and bushes randomly throughout, the key is to section off your garden into different spaces you can measure then combine everything at the end.
Tips for irregular shapes:
- Break complex areas into simple rectangles and circles
- Measure each section separately
- Calculate volume for each section
- Add all the volumes together
- Allow extra for the bits you couldn’t measure exactly
- Draw a rough sketch to help visualise it
Converting to what suppliers actually sell
Some suppliers sell soil by the cubic metre, others by the litre and some still work in cubic yards.
The conversions you need to know:
- 1 cubic metre = 1000 litres
- 1 cubic metre = roughly 1.3 cubic yards
- 1000 litres = about 1.3 cubic yards
- Always round up when converting for orders
- Check what units your supplier uses before calculating
Cubic metres to litres is easy just multiply by 1000. So 3 cubic metres is 3000 litres. Cubic metres to cubic yards is about 1.31 so 3 cubic metres is roughly 4 cubic yards.
The compaction problem
This part is genuinely confusing so I feel for you here. When your soil gets delivered its probably light and fluffy, but once you spread it out and it settles then it compacts down and you are left with way less volume than you started with.
Account for compaction by:
- Adding 10-15% extra to your calculated amount
- Ordering slightly more rather than exactly what you calculated
- Considering soil type some compact more than others
- Factoring in weather conditions when it’s delivered
- Better to have leftovers than run short
What type of soil?
There’s heaps of different types of soil available and they vary in prices
Different soil types and uses:
- Topsoil – cheapest option, good for lawns and basic gardens
- Garden soil – has compost mixed in, better for most plantings
- Potting mix – expensive but great for containers and raised beds
- Sandy soil – drains well, good for natives and Mediterranean plants
- Clay soil – holds moisture, can be heavy to work with
- Organic compost – expensive but fantastic for improving existing soil
For most projects in the garden using good quality soil or topsoil mixed with compost works well. If you’re doing raised beds potting mix might be worth the extra cost. For lawns topsoil’s usually good if its decent quality.
The bottom line
Once you know what youre doing, calculating soil volumes isn’t that hard but theres definitely some techniques that make it easier. Measure carefully, add a bit extra for compaction and don’t be afraid to ask a professional about what soil type they recommend.
If you’re not confident about the calculations or just want someone else to worry about it, most decent landscapers can handle the whole thing. Costs more obviously but sometimes it’s worth it for the peace of mind.
Give us a call if you want to chat about your project. We’ve done heaps of soil calculations over the years and can probably save you some headaches.