A neck finish number specifies the outer diameter (in millimeters) and thread pitch of a bottle neck; the most common sizes are 18/415, 20/410, 24/410, and 28/410, and a mismatch causes leaks, loose caps, and applicator failure.
What a neck finish number actually means: the two numbers, what each measures, why both matter. The first number is the outer diameter of the bottle neck in millimeters. The second number refers to the thread configuration—pitch, profile, and dimensions. Both must match between bottle and closure.
18/415 is one of the most common sizes for fragrance and wellness bottles. 20/410 is standard for many dropper and spray applications. 24/410 and 28/410 appear in larger formats. Best Bottles lists compatible closures for each neck finish on every product page.
Why mismatch causes real problems: leaks, looseness, returns. A cap rated for 20/410 will not seal properly on an 18/415 neck. Thread engagement depth matters. What happens when a cap threads on two turns instead of four: the seal is incomplete. Product leaks. Caps work loose in transit. Customers return product.
How to read a spec sheet: where to find the neck finish and how to cross-reference with closure suppliers. The neck finish is usually listed under "finish" or "neck" or "thread." Closure suppliers publish compatibility charts. Cross-reference before you order.
Best Bottles offers a filter by neck finish so buyers can find compatible closures in one step. Every product page includes the neck finish spec and a list of compatible closures. Use the compatibility filter at bestbottles.com to build your complete packaging set.