How to calculate gravel needed
To calculate how many cubic metres (m3) of gravel you will need to use, firstly measure the area of the garden to be covered.
The width x length will give you the area in square metres (m2). eg. an area measuring 8m x 5m = 40 m2. Then multiple your square metre measurement by 0.05 ie. 40 x 0.05 = 2m3 of product required for good surface coverage.
Area of garden (m2) x 0.05 = volume of gravel required (m3)
This will cover your garden in gravel to a depth of 50mm which is recommended to retqin valuable soil moisture and inhibit weed growth.
The laying of weedmat first is a good idea to keep the gravel separate to the soil. Weedmat is woven to enable water to pass through and will degrade in sunlight and break up over time.
Organic or gravel mulch will control weed growth, retain moisture in the earth for plants and protect the soil against erosion.
How to calculate packaged rock needed
A 20kg bag of gravel will cover approx 0.5m2 of garden to a depth of 50mm. Remember there are lots of variables, so it is probably better to underestimate. You can always come back for one more bag.