Measuring saddle height with an app

What the familiar formulas are worth

If you want to calculate saddle height or set it from your inseam, you'll run into one of these formulas:

Almost every saddle-height calculator online multiplies your inseam by a factor. The best known:

• LeMond — inseam × 0.883 and Hügi — inseam × 0.885, both measured from the centre of the bottom bracket to the top of the saddle. Two formulas that arose independently and get lumped together; the difference is under 2 mm. Both still usable starting points.
• The Holmes method — via knee angle, traditionally taken with a goniometer while stationary. Closer to the truth because it measures the angle instead of guessing it — but static. Note: Holmes gives 25–35° as the target, a narrower and lower window than the dynamic 30–40°. Two methods, two windows — don't mix them.
• Heel-on-pedal check — leg just straight with the heel on the pedal at its lowest. Quick, rough, no tools. Fine as a first orientation.

All three share one blind spot: they describe a body at rest. Two people with identical inseams sit completely differently once they pedal — different pelvic rotation, different foot angle, different mobility. A factor cannot know that.

Saddle fore/aft (setback): what the app shows — and what it doesn't

Many people search for a millimetre measurement for the classic KOPS rule ("knee over pedal axle"). RoadFitter doesn't give you one — deliberately.

What the app shows: your knee plumb line at a horizontal crank — a line dropped from the knee joint, drawn on your own photo. You move it yourself with two arrow buttons until it sits over the ball of your foot (roughly the pedal axle) or doesn't. This is a visual reference, not automatic detection: RoadFitter has no pedal or toe keypoint in the image, only the knee joint and ankle. You decide, not the app.

What the app does not do: it does not grade the position with a traffic-light score, it names no target value, and it does not measure setback in millimetres. After a new measurement it can show, at most, a roughly estimated, "~"-marked relative change from your own previous run — calculated using an assumed typical thigh length, not your actually measured one. The feature is part of Pro.

And for context: KOPS itself is a historical rule of thumb, not a scientifically validated method. RoadFitter labels it accordingly, as "a historical guide, not a hard rule" — a starting point for your own judgement or a conversation with a fitter, never an automatic verdict.

What a measurement sees on top of that

Saddle height is the one part of a riding position with scientific consensus behind it — and that consensus is not expressed in centimetres but in the knee angle at the bottom of the stroke: 30 to 40 degrees.

That angle is measurable while you pedal — where it actually happens. The pelvis rotates under load, the foot drops, the heel moves. A value taken seated and a value taken pedalling therefore regularly disagree. Only the second describes the position you actually ride in.

And because a single moment says nothing about a movement, what counts is the median across many crank revolutions. Outliers are filtered out automatically.

From a number to an action

The value is not the point; what follows from it is. "Your knee is at 46 degrees" helps nobody. "Raise the saddle by about 15 mm, roughly 48 mm in total — in steps, measuring again after each one" does.

That is what the app does: it converts the angular deviation into millimetres of saddle adjustment, suggests larger steps when the deviation is large, and lets you measure again after each change until the value sits inside the target range. One run takes about two minutes.

How a measurement works

• No turbo trainer needed. Lean the bike against a wall or have someone hold it and pedal backwards — bottom-of-stroke detection works in both directions.
• Camera side-on, roughly hip height, whole body in frame, 3–5 metres away. On iPad the frame is a little tighter, so stand slightly further back.
• Three routes, one result: a live fit with a voice coach telling you when to pedal; a video scan of a clip you already have; or manual measurement with live angles while you adjust the bike.
• Everything stays on the device. No account, no upload, no trackers.

Common questions

How do I measure saddle height correctly in the first place?

The convention is: from the centre of the bottom bracket, along the seat tube, to the top of the saddle. What matters most is measuring the same way every time — otherwise your next comparison is meaningless. In the app you can note the value alongside the measurement, so you know later what the result referred to.

How much may I change at once?

Start small. Five millimetres is already clearly noticeable at the knee. For large deviations the app suggests bigger steps, but still recommends going in stages and measuring in between — your body needs a few rides to settle into a new height.

Does this replace a professional bike fit?

No. It does the part that can be measured objectively from the side. Knees tracking sideways, cleat position, leg length differences: a side-on camera cannot see those. A studio fit typically costs €150–450 in Germany (more on what a bike fit costs) and is worth the money when something genuinely hurts. For persistent pain, see a professional.

Does the app run on iPad?

Yes, since version 1.4 — and not as a stretched iPhone: the report reads in two columns there, photo and millimetre instruction on the left, every angle and the measurement quality on the right. The measurement itself is identical, with the same target ranges.