To calculate the appropriate size of your polytunnel cover, you’ll need to determine both the width (C) and length (B) of your polytunnel. Use the illustration to ensure you collect the right measurement.
How do you you calculate polythene cover size?
Our calculator takes the stress out of calculating. But if you’d like to see how we calculate polythene size here’s how we do it. First take your polytunnel width (C) measurement and match it against the first column of the table below. If your width lands between two breakpoints always round up to the next size. For example, if your polytunnel width is 9ft (or 2.8m), use the 10ft (3.05m) row. Now you’ve got correct row, select the roll width size that matches your polytunnel cover fixing method.
|
Width of Polytunnel (C)
|
Trenching into the ground
|
Fixing to a base rail
|
Fixing to a side rail
|
|
8ft (2.44m)
|
Use 9.2m roll width
|
Use 7.3m roll width
|
Use 7.3m roll width
|
|
10ft (3.05m)
|
Use 9.2m roll width
|
Use 7.3m roll width
|
Use 7.3m roll width
|
|
12ft (3.66m)
|
Use 9.2m roll width
|
Use 9.2m roll width
|
Use 7.3m roll width
|
|
14ft (4.27m)
|
Use 9.2m roll width
|
Use 9.2m roll width
|
Use 7.3m roll width
|
|
18ft (5.49m)
|
Use 11.1m roll width
|
Use 11.1m roll width
|
Use 9.2m roll width
|
|
21ft (6.40m)
|
Use 14m roll width
|
Use 11.1m roll width
|
Use 9.2m roll width
|
|
24ft (7.32m)
|
Use 14m roll width
|
Use 14m roll width
|
Use 11.1m roll width
|
|
27ft (8.23m)
|
Use 14m roll width
|
Use 14m roll width
|
Use 11.1m roll width
|
|
30ft (9.14m)
|
N/A
|
Use 14m roll width
|
Use 14m roll width
|
Now you know the required polythene roll width you can calculate the polythene length.
For EasyBuild structures, take your polytunnel length (B) and add your polytunnel width (C) then add a further 0.3m and round up to the nearest metre. For example, if your tunnel width is 3m and the length is 3m you’ll need a 9.2m roll width x 7m long.
For commercial single span structures, simply take your polytunnel length (B) and add 3m to each end. For example, if your tunnel length is 3m you’ll need a 9m length of polythene.
For multispan structures, please get in touch with our team for a price – 01282 873120.
What's the difference between diffused and clear polythene?
Diffused and clear polythene are two types of polytunnel coverings, each offering distinct benefits based on your cultivation needs.
Clear Polythene: This type allows approximately 92% light transmission with about 35% light diffusion. The high clarity permits more light and heat to enter the tunnel, which can be advantageous during the early and late months of the growing season, especially in regions with lower natural light levels. This increased light exposure can lead to earlier harvesting and extended post-harvest shelf life. Additionally, clear polythene allows UVB light to penetrate, enhancing the fragrance, taste, and colour of both edible and ornamental crops, while promoting stronger plant development.
Diffused Polythene: This variant allows around 90% light transmission and is designed to scatter sunlight as it passes through the material. The diffusing process deflects more light downwards into the crop, reducing shading and promoting uniform growth. This scattering effect reduces the risk of leaf burn and crop deformation, encourages more uniform ripening and colouring of fruits, and can lower mid-day summertime temperatures by 2-4°C. Diffused polythene is particularly beneficial in regions with intense sunlight, as it mitigates the stressful effects of direct sunlight on plants.
In summary, the choice between diffused and clear polythene depends on your specific growing conditions and objectives. Clear polythene is ideal for maximising light and heat, beneficial in cooler, low-light regions. In contrast, diffused polythene offers more even light distribution, reducing plant stress and promoting uniform growth, making it suitable for areas with intense, direct sunlight.
