Keep in mind for tub designs that are not straight and are either offset or curved across the front of the tub, the insert will not curve along those lines. The insert does slope into the tub, but the front is a straight edge. Once the tub to shower kit insert is installed there may be sections where it extends past the tub wall more at some places than others. This is also why it is important when taking the rail measurement to be sure you take it at the widest point where you will be installing the tub to shower kit.
It is also possible that in some styles of tubs that in order to accommodate a larger rail measurement or to avoid cutting into the curve of the tub that the installed insert may rise a few inches higher than the tub rail. Be aware of the aesthetic changes that can result due to the placement of the insert and choose the best place to make the tub cut prior to taking the measurements.
2. Measure the Tub Rail
The most important factor in determining which size tub to shower kit to order is the measurement of the tub rail. This is the wall portion of the tub that you are currently stepping over and will be cutting out. This is the indicator for which size tub to shower kit to order.
Please note that most tub rail designs will widen and curve as they reach the floor of the tub. Therefore, you will want to make sure you measure the rail from the widest point at which the tub to shower kit will be placed; not simply across the top of the tub rail.
Use a tape measure or ruler to measure down 10" on the INSIDE of the tub rail wall and mark it.
Then use a level and tape measure together to determine the rail measurement at the widest point.
A. Place one end of the level on the mark you made.
B. Align the top bubble to make sure you are holding it level
C. Run the tape measure from the aligned level to the opposite edge of the tub rail and read the measurement
Some tubs may have little to no variance between the measurement at the top of the tub rail and the widest rail measurement.
On this example, however; it reveals an offset of 1/2"; which could indicate a larger size tub to shower kit is required than what taking the measurement simply across the top of the rail would have indicated
3. Select the Correct Size Kit
The small kit is for a maximum rail size of 4 3/4
The medium kit is for a maximum rail size of 5 3/4