A Dive into the Obsessed Garage Approach to Pressure Washer Nozzle specifications and selection.
Any time you search the web for pressure washer fittings, you get bombarded with industry jargon, inconsistent terminology, and confusing naming schemes. Even on Obsessed Garage, the vast number of options can get a little overwhelming. If you're having trouble sorting through the mess, this article should be exactly what you need.
To help take some of the guesswork out of the process, we have also done a huge amount of testing on 70+ pressure washers in order to find the best nozzle size for each machine.
Be sure to check that out here: The Pressure Washer Project
Nozzle Specifications
There are three main specifications on all nozzles:
1. Fan Spread, described in degrees.
2. Nozzle Orifice Size, described in a single digit followed by a decimal and another digit.
3. Connection type, threaded or 1/4 QD plug.
Example:
You can have 0°, 15°, 25°, and 40° nozzles in a 4.0 size.
You can also have 0°, 15°, 25°, and 40° in a 2.0 size or a 6.0 size, etc.
The numbers are stamped on the side of the nozzle like so. The first two digits are the fan spread; the last two digits are the orifice size: 
The nozzle above has a 1/4 plug-style connection that fits into the 1/4 coupler on a sprayer or wand. Some nozzles, however, have a fixed style with 1/8" male NPT threads that fix directly into a wand or nozzle assembly.
Nozzle Selection
When selecting the correct nozzle for your pressure washer, water flow is the main number you need to be looking at. A pressure washer does not build pressure, so to speak; it merely pushes through a certain amount of water. The nozzle, which restricts the flow of water, is what builds up the pressure. Because of this, the nozzle you're using has a huge impact on pressure washer performance. Too large of a nozzle, and you won't have enough pressure. Too small of nozzle and pressure may build up too high, and flow will be too restricted. While balance is good, we try to shoot for between 1000-1200PSI and as much flow as we can muster out of a particular pressure washer. This is ideal for washing vehicles.
To help with selection, here's a quick little chart based on MTM Hydro's calculator that may help. First, start with the rated GPM (gallons per minute) of your machine, and then find your desired pressure. Where those two intersect is the nozzle size you should go with. Now, this isn't totally dead-set and foolproof. There are times when a slightly larger or smaller nozzle works better with a particular machine, even if the chart should say otherwise. Keep in mind that you should NEVER shoot for a PSI rating higher than the rated work PSI of your machine.