Measure your nails.
Please be aware that there is more room for error when measuring your nails versus using a sizing kit, and could lead to you ordering nails that do not fit correctly. 
How to Measure your Nails in Milimeters:
Step 1
Lay a soft measuring tape over the widest part of your nail pulling down tight to account for any curve of your nail. 
Note the width of each nail in milimeters for each finger, at the widest part.
OR
Stick a piece of tape across the widest part of your nail pressing down to account for any curvature.
Pull the tape off your nail and measure the distance between your marks of each nail (thumb, index, middle, ring and pinky) with a measuring tape or ruler in millimeters.
Step 2
Now that you've recorded the width (accounting for any curve) of your nails in millimeters, use the below chart to determine your individual nail numbers (00-9) and the corresponding size kit to select (XS, S, M, L, XL).
*If you are between sizes it is safer to go up one size. 
*If none of the standard sizes fit your nail measurements, select "custom" and leave your measurements at checkout in the instruction box.
(Thumb, Index, Middle, Ring, Pinky) - Example: Right: 16mm, 12mm, 15mm, 12mm, 10mm. Left: 16mm, 12mm, 15mm, 12mm, 10mm.