Bike Fit Calculator (Reach, Stack, Stem & Spacers)

How it works

Bar reach = frame reach − (spacers + clamp) × cos(HTA) + stem × cos(90° − HTA + stem angle); bar stack = frame stack + (spacers + clamp) × sin(HTA) + stem × sin(90° − HTA + stem angle)

Frame reach and stack — the horizontal and vertical distance from the bottom bracket to the top of the head tube — describe a frame's size independently of seat-tube length or top-tube slope, but a rider feels the position of the handlebars, which sits at the end of a stack of spacers along the slanted steerer and a stem angled off it. Adding those two vectors converts frame numbers into handlebar reach and stack; because the steerer leans back at the head angle, every spacer added to raise the bars also shortens reach slightly, and a flipped stem trades a couple of centimetres of height for a centimetre of length. Once a rider knows their handlebar coordinates from a fit, any candidate frame can be checked for whether a sensible stem and spacer stack will reproduce them.

Worked example

A frame with 385 mm reach and 560 mm stack, 73° head angle, 25 mm of spacers and a 100 mm −6° stem puts the bar clamp at about 470 mm reach and 622 mm stack: the 45 mm of steerer above the head tube adds 43 mm of stack but takes 13 mm of reach, and the stem adds 98 mm of reach and 19 mm of stack. Flipping the stem to +6° would raise the bars about 20 mm and shorten reach 6 mm.

Frequently asked questions