Stem Length Calculator — Adjust Your Reach

Stem Length Adjustment

New Stem = Current Stem + (Target Reach − Current Reach)

Stem length maps roughly 1:1 to horizontal reach, so a 10 mm longer stem adds about 10 mm of reach. This calculator keeps the same stem angle.

Stem angle changes both reach and bar height, and how much depends on the head-tube angle (HTA). A stem angle is measured from the perpendicular to the steerer, so a 0° stem already points (90 − HTA)° above horizontal. Horizontal reach = stem length × cos(90 − HTA + angle) and bar height = stem length × sin(90 − HTA + angle). On a 100 mm stem at 73° HTA, a +6° stem shortens reach by about 3.6 mm and raises the bar about 9.8 mm versus a 0° stem; flipping a ±6° stem from −6° to +6° changes reach by about 6 mm and height by about 20 mm. Because cosine is steeper away from horizontal, +6° and −6° stems do NOT give the same reach.

Headset spacers stack along the steerer, which is tilted at the head-tube angle. A spacer stack of height h raises the bar by h × sin(HTA) and pulls it back by h × cos(HTA) — about 6 mm of lost reach per 20 mm of spacers at a 73° head angle.