Knee angle in bike fitting: the 25–35° rule
Knee angle at the bottom of the pedal stroke is the single most useful number in a bike fit. It tells you whether your saddle height is right, and it is easy to measure at home from one side-on photo.
By Maxence Poutord·Last updated
What knee angle means
In bike fitting, knee angle usually means knee flexion: how far your knee is from fully straight when the pedal is at the very bottom of the stroke (the 6 o'clock crank position). A completely straight leg is 0° of flexion; a right angle at the knee is 90°.
Fitters measure it at 6 o'clock because that is where your leg is most extended. If the knee works in a good range there, saddle height is close to right for the rest of the stroke too.
The target range: 25–35° of flexion
Most published fitting systems (Holmes, Retül-style dynamic fits, and the ranges Pedalyze uses) put the target at 25–35° of knee flexion at the bottom of the stroke, for both road and TT positions. A band of roughly 20–40° is tolerable; outside that, most riders lose power or load the knee.
There is no bonus for hitting exactly 30°. Anywhere inside the range is fine — pick the spot that feels stable and pain-free.
| Knee flexion at 6 o'clock | Verdict | What it usually means |
|---|---|---|
| 25–35° | In range | Saddle height is close to right |
| 20–25° or 35–40° | Borderline | Adjust the saddle 5–8 mm and re-check |
| Below 20° | Too straight | Saddle too high — lower it 10–15 mm, then fine-tune |
| Above 40° | Too bent | Saddle too low — raise it 10–15 mm, then fine-tune |
Leg too straight: saddle too high
Under about 20° of flexion, the leg is nearly locked out at the bottom of every stroke. Typical signs on the bike: rocking hips, pain behind the knee or in the hamstrings, numb feet, and reaching for the pedal with a pointed toe.
Lower the saddle in 5 mm steps and re-measure after each change. Small steps matter — 5 mm at the saddle changes the knee angle by roughly 2–3°.
Leg too bent: saddle too low
Above about 40° of flexion, the knee stays deeply bent through the power phase. This overloads the front of the knee (a common cause of pain around the kneecap) and shortens your effective stroke, so you produce less power at the same effort.
Raise the saddle in 5 mm steps. If you have been riding low for years, spread the change over a few rides — your muscles adapt to the position you give them.
How to measure it from a photo
You do not need a goniometer. Sit on the bike on an indoor trainer, put the crank at 6 o'clock, and have someone take a photo from directly side-on at hip height. The angle between hip–knee and knee–ankle lines, subtracted from 180°, is your knee flexion.
Pedalyze does this automatically: upload the photo and it detects the hip, knee and ankle, measures the flexion, and checks it against the 25–35° range. Photo quality drives accuracy, so shoot it carefully — see the photo guide below.
Frequently asked questions
- Is 25–35° the same target for road and TT bikes?
- Yes. Saddle height relative to the pedals follows the same rule in both positions. What changes on a TT bike is the hip, torso and elbow geometry, not the knee flexion target.
- How much does saddle height change the knee angle?
- As a rule of thumb, 5 mm of saddle height changes knee flexion at the bottom of the stroke by about 2–3°. That is why fitters adjust in 5 mm steps and re-measure.
- Should I measure with the crank in line with the seat tube instead?
- Some methods measure with the crank pointing along the seat tube (maximum leg extension). The 6 o'clock convention is more common and easier to reproduce in a photo; just be consistent — do not mix targets from one method with measurements from the other.
- My knee angle is in range but my knee still hurts. Why?
- Saddle height is only one variable. Fore-aft position (see the KOPS guide), cleat placement, saddle tilt and crank length all load the knee. Pain at the front of the knee often points to a low saddle or a knee far forward; pain at the back points to a high saddle.
Related guides
Related tools
- Saddle height calculator — Estimate your saddle height from inseam with the LeMond method, measured from the bottom bracket to the top of the saddle.
- 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.
Pedalyze measures every angle on this page from two side-on photos of you on your bike — and tells you what to adjust. The first analysis is free.