New bicycles come with stems (the component that holds the handlebars) the stem length usually relates directly to the size of the frame.
Consequently, there’s an excellent chance that if your bike fits, your stem length is correct and you don’t need to worry about it.
The exception is if you’re experiencing problems such as neck, back, hand and shoulder soreness during or after rides. And/or if you find that you have difficulty maintaining bent elbows while riding or feel the need to move forward or backward on the seat all the time. These are signs that the reach might be wrong and that swapping stems could be a good move.
In my experience, stem length is more about comfort. i.e. your riding style and flexibility will dictate your reach to the handlebars. Most bike fitters will look at this part last, once everything else is sorted. So don’t be surprised if end up changing your stem during this process.
A quick test test is to hit the road, warm up until you’re comfortable and then look down at your front hub (the centre of the wheel). If you can’t see the hub because the handlebars are blocking your view, it’s an indication that the stem is probably the right size. If you see the hub in front of the bars, it’s a sign that a longer stem may be needed (behind the bars, a shorter stem).
A more accurate way to gauge stem length is to video yourself riding on a trainer (shot from the side), or get someone to watch. You can then look at the video (or ask your helper questions) to find out how your position looks. Ideally, your back will be flat (no hump), your head and neck will rest at a comfortable angle, your shoulders will be dropped (not hunched), your arms will be slightly bent, and an imaginary plumb line dropped from the tip of your nose would fall about an inch behind the handlebars.
Tip: If you’re not sure what to look for, you might check side view photos of professional riders in books and magazines. Generally, competitive cyclists use extreme positions that are lower than what you’ll prefer. Other than that, though, the body position should be very similar, so you can compare.
Remember, this part of the bike fit is about riding style (sportive, time trial, etc) and flexibility. Other parts of the bike fit e.g. saddle position relative to the pedals, etc, remain pretty much the same.
How stems are measured
Stems are measured from the centre of the steering clamp (or steering tube) to the centre of the handlebars. It’s close enough to measure along the top, by laying a scale with the end at the centre of the top cap bolt and measuring to the centre of the bars, but the measurement will usually be 3-5mm longer than the nominal size, using this method (if the stem measures 103mm, it’s a 100mm stem).
Road bike stem angles are usually measured from the centreline of the steering tube. A 90 degree stem would be perpendicular to the steering tube. Common angles are 73, 80, 82, and 84 degrees. All of these stems can be flipped to produce greater angles of 107, 100, 98 and 96 degrees, respectively. Occasionally, you will see a road bike stem listed as a -17 (73), -10 (80), -8 (82) or -6 (84).
73 is the old standard for quill-type stems, which makes the stem extension approximately horizontal to the ground. All of the others have a “rise” (from the horizontal). An 80 would have a 7 degree rise, an 84 degree an 11 degree rise, etc. (subtract 73 from the angle to get the rise).
Mountain bike stems and some road stem may be listed with the degree of rise. This is where it gets real confusing. In MTB terms, a 90 degree stem has 0 degrees of rise, hence a 6 degree rise stem is really 96 degrees from the centreline of the steering tube and a 10 degree has an angle of 100 degrees.
The following chart from “Habanero Cycles” should clarify the above and also give relative reach and height (rise) measurements;