How to do an online bike fit from a video (at home)

A professional bike fit is one of the best upgrades you can buy — but at $250–400 a session, most riders only get one, then change nothing when they buy a new bike, swap a saddle, or start getting a niggle. An online bike fit from a video closes that gap: with a phone, a trainer, and ten seconds of pedaling, you can measure your riding position accurately enough to make real, safe adjustments at home. Here's exactly how to do it well.

Why video beats a static calculator

Most free “bike fit calculators” ask for your inseam and spit out a saddle height from a formula. The problem: those formulas are wrong for a large share of riders, because they ignore how you actually pedal — your ankle motion, how far your hips rock, how much your knee bends at the bottom of the stroke. A fitter doesn't use a formula; they watch you ride and measure your joint angles in motion. Video lets you do the same thing.

How to film it (this is the part that matters)

The accuracy of everything downstream depends on the clip. Five rules:

• Side-on, dead square. The camera must look straight at the drive side of the bike, not at an angle. An off-axis camera skews every angle it measures.
• Hip height. Prop the phone at roughly the height of your hip, about 2–3 metres away, so the whole bike and your whole body stay in frame.
• On a trainer. Indoor trainer or rollers, pedaling at a steady, moderate effort — around 75–95 rpm. Settle in for a few seconds before recording.
• 10–15 seconds, landscape. That's several full pedal strokes, which is what lets the analysis average out the noise.
• Form-fitting kit, good light. Baggy clothing hides your joints and confuses pose tracking.

The angles that actually matter

A useful fit comes down to a handful of measurements, taken at specific points in the pedal stroke:

• Knee extension at the bottom of the stroke — the master number for saddle height. See our saddle height and knee angle guide.
• Knee flexion at the top of the stroke — a check on crank length and saddle fore-aft.
• Hip angle — how open or closed you are, which drives breathing, power, and comfort.
• Back angle — your torso relative to horizontal, the biggest lever for both comfort and aerodynamics.
• Elbow angle — a read on reach: too straight means you're overstretched, too bent means the cockpit is too short.

Turning angles into millimetres

Measuring an angle is only half the job — you need to know how much to move the saddle. The trick that makes video fitting genuinely useful is calibration: your crank arm is a known length (usually printed on it, e.g. 170mm), and it's visible in every frame. Because the pedal traces a circle of exactly that radius, software can convert screen pixels into real millimetres — so “your knee is over-extended” becomes “lower the saddle 6mm.” Without a length reference, any millimetre advice is a guess.

Change one thing, then re-film

The golden rule of fitting is one change at a time, in the right order: saddle height first, then saddle fore-aft, then the front end (bars and reach), then contact points. Make a single adjustment, ride for a minute to settle in, then re-film and compare. Chasing several changes at once makes it impossible to know what helped — and small saddle moves interact, so 3–5mm at a time is plenty.

What video fitting can't do

Be honest with yourself about the limits. A single side-view camera measures your position in one plane. It can't assess cleat alignment, leg-length discrepancy, or how your knee tracks side-to-side — those need a front view or a hands-on fitter. If you have persistent pain, an old injury, or you're chasing an aggressive race position, a professional fit is still worth it. For getting a new bike dialed or iterating safely on your own position, video gets you into the right window. We cover this trade-off in detail in how accurate is an AI bike fit.