With a tape, a level and a plumb line you measure most numbers yourself: wheelbase, chainstay and bottom bracket height are easy; the angles you get with an inclinometer app. The hard ones are reach and stack: they're distances between two points in the air, so you need the bike perfectly leveled — for those two, trust the manufacturer's table unless you can plumb it properly.
Useful for checking that your bike matches the table, rebuilding a geometry you can't find, or understanding where each number comes from by feeling it with your hands.
The secret to all of it: level the bike first. Almost every home measurement error comes from measuring with the bike tilted or on uneven ground. With the bike plumb, half the work is done.
Flat, level ground, a tape measure, a level (or your phone's inclinometer app) and a plumb line (a weight hung from a string works). Nothing expensive or weird. Stand the bike upright, both wheels touching the ground and the frame plumb — that's the starting point for every measurement.
| Number | How | Difficulty |
|---|---|---|
| Wheelbase | Tape from front axle to rear axle. | Easy |
| Chainstay length | From the bottom bracket center to the rear axle. | Easy |
| Bottom bracket height | From the ground to the center of the bottom bracket axle (bike leveled). | Easy |
| Head angle | Inclinometer rested on the head tube / fork stanchion. | Medium |
| Seat angle (actual) | Inclinometer on the seat tube. The effective one depends on your saddle height. | Medium |
| Stack | Vertical height from the bottom bracket center to the head tube center (plumb line). | Hard |
| Reach | Horizontal distance between those same two points (plumb line + bike very well leveled). | Hard |
If you insist on measuring them: with the bike very well leveled, hang the plumb line from the center of the bottom bracket axle and mark the point on the ground; do the same from the center of the head tube (where the headset axle would be, at the top). The horizontal distance between the two marks is the reach; the height difference between the two origin points is the stack. Measure two or three times: if the millimeters wander, the bike isn't fully plumb.
Yes but it's the hardest: it connects two points in the air, needs the bike leveled and a plumb line. For reach and stack, better to use the manufacturer's table.
With the bike leveled, rest an inclinometer (phone app) on the head tube and read the angle. It's one of the most reliable home measurements.
Flat ground, tape, level (or app) and plumb line. With that you get wheelbase, chainstay, bottom bracket height and angles.
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