Stem length calculator

Stem length is printed on most stems, or measure from the centre of the steerer to the centre of the bar. Stem angle is printed too, usually as 6° or 17°; if the stem is flipped, use the opposite sign. Head-tube angle is on your frame's geometry chart. For spacers, measure the whole stack under the stem, including the headset top cap.

A stem is bolted at a right angle to the steerer, not to the ground, and the steerer leans back. So a stem sold as −6° on a 73° head tube actually sits 11° above horizontal: 90 − 73 + (−6). That is why a stem that looks flat still rises.

Everything else follows. The stem adds length × cos(angle) of reach and length × sin(angle) of height. Spacers move the bar along the tilted steerer, so they add height but also pull the bar backwards by spacer × cos(head angle). Twenty millimetres of spacers on a 73° head tube costs about 6 mm of reach.

• The result is trigonometry on the steerer and the stem, so it is exact for the numbers you enter.
• Stem angles are quoted relative to the perpendicular of the steerer, which is the convention stem makers use.
• Handlebar reach and drop are not included. They are added on top of the bar centre and differ by bar model.

How much does 10 mm of stem length change my reach?

Slightly less than 10 mm. The stem rises above horizontal once fitted, so part of the extra length becomes height. On a typical road setup, 10 mm of stem gives about 9.8 mm of reach and 2 mm of height.

What happens if I flip my stem?

Flipping a −6° stem to +6° raises the bar and shortens the reach at the same time. On a 100 mm stem that is roughly 20 mm up and 2 mm back. It is the cheapest way to raise a bar that is already at the spacer limit.

Do spacers change my reach?

Yes. The steerer leans back, so every spacer moves the bar up and backwards. Twenty millimetres of spacers on a 73° head tube costs about 6 mm of reach, which is why riders often go one stem length longer when they raise the bar.

Should I change stem length to fix an uncomfortable position?

Only after the saddle is right. Saddle height and setback decide where you sit; the stem decides where you reach. Changing the stem first hides a saddle problem instead of fixing it.

• Road bike fit angles — The complete reference chart for a road position: knee, hip, torso, elbow and shoulder angles plus KOPS, with target ranges and what to change when a number is off.
• TT & triathlon bike fit — The aero position by the numbers: TT target ranges for torso, hip, elbow and knee angles, how they differ from road, and how to get low sustainably.

• Stack and reach calculator — Carry frame stack and reach up through your stem and spacers, and compare where two bikes really put the handlebar.
• Saddle setback calculator — See how far behind the bottom bracket your saddle sits, and how that compares with the range road and time-trial riders use.