For DIY'ers, threaded rod can be cut to length with a cheap hacksaw. A jigsaw, bandsaw or reciprocating saw will work too, if you don't mind the sparks. But the problem with all of these approaches is that you must thread the nut on first, as the saw damages the threads at the cut.
For professionals who install overhead conveyor belts or exposed pipes, a quicker, cleaner solution is needed.
Nuts may need to be removed down the line for maintenance, throwing sparks presents a safety hazard, and the installer needs a device they can handily wield overhead for dozens or hundreds of cuts at a time.
The right tool for the job is a cordless threaded rod cutter. One example is Milwaukee's 2872-21:
Tools like this are meant to be used upright and overhead, as well as flat on their sides, for benchtop applications, as seen with competitor DeWalt's DSC-350:
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Both will cut through 1/4", 3/8" and 1/2" mild steel coarse thread rods. For stainless steel coarse thread rods you're limited to 1/4" and 3/8".
If you want a closer look at Milwaukee's tool, they've got this product video:
Which one to buy will probably be a function of which manufacturer's battery system you've already committed to. But it's worth nothing that Milwaukee's 18-volt offering weighs 8.8 pounds including the battery, whereas DeWalt's 20-volt tool is a bit heavier at 10.4 pounds including the battery. You'd need to calculate what will add up more, over the course of a workday: The power difference of two volts, or the weight of 1.6 pounds.