The lid must not be too small. If you are unsure, order a cover that is a few centimetres too big. The skirt that hangs down will hide it and you can be sure that you get a cover that insulates properly. Always measure your bath and make sure you use the largest measurement if the tub extends beyond the wall.
This way the skirt will hang straight and neatly.
The corner radius is measured from the point where the curvature of the lid starts (a) to the corner (b).
If possible, measure the spa. If the existing cover fits well, you can also measure it.
Choose a cover that has the corner radius you measured or the closest value that is smaller.
With a smaller corner radius, the cover will protrude slightly over the edge which does not affect the function and is barely visible due to the overhanging skirt.
If you measured a corner radius of e.g. 180 mm, choose a cover with a corner radius of 152 mm. The difference in overhang between the radius of 152 mm and 229 mm is only about 3 cm.
You can also use our template to check the corner radius. Download it and print it out.
Template for measuring the corner radius of the spalock can be found here.