Guide to Glass Joint Sizes and Nomenclature

By Carlton Hoyt, 18 December 2015

As you well know, rotary evaporators have a lot of different glass components. Most of those components connect to each other by ground glass joints, but glass joints come in many different sizes and styles and many people aren't aware of the glass joint sizing nomenclature and what it means, so we wanted to explain it to you.

Most times you see a ground glass joint size listed it will be in a format like this: XX ##/##. For instance, NS 24/40. But what does all that mean?

The two letters at the beginning of the size represent the type of joint. There are a number of different types of joints, but on rotary evaporator glass you'll usually see two: NS and KS. NS represents a conically tapered joint. KS represents a ball-and-socket kind of joint. Both are illustrated below.

NS and KS ground glass joints

 In rotary evaporator setups, evaporating flasks most often have NS joints and receiving flasks (sometimes called collecting flasks) most often have KS joints. You can see the examples in the images of IKA rotovap flasks below.

Receiving flask and evaporating flask 


The number before the slash signifies the outer diameter (OD) in millimeters (mm) at the widest part (the base) of the inner (male) joint. The number after the slash tell us the length of the ground glass joint, again in millimeters. So, for instance, an NS 24/40 joint would be a conical joint where the base of the male joint was 24 mm in diameter and the joint was 40 mm in length.

Joint sizes for rotary evaporator glass are only semi-standardized, and glass joint standards are different in the US and Europe, so be sure to check the joint sizes when purchasing glassware if you need them to fit into existing glassware that you own.

One other point about glassware sizing that sometimes is a cause of curiosity - the small symbol with a T overlayed on top of an S that appears on some glassware. This stands for "standard taper." Each type of joint has its own standard. On conical "NS" joints, for instance, the standard taper is 1:10 - for each 10 millimeters of length, the diameter decreases by 1 millimeter. As a general rule all laboratory glassware uses standard tapers, but it's possible that it does not so if you don't see the symbol it can't hurt to ask.

Standard taper symbol on the next of a receiving flask

Note the small "TS" symbol to the right of the joint size on the next of this Scilogex receiving flask.

If you have questions about any rotary evaporator components, feel free to contact us.