Geometry Calculator vs AI Video Bike Fit: Which One Do You Need?

Two very different tools get called "bike fit" online, and choosing the wrong one wastes either your money or your afternoon.

• A geometry calculator works from measurements and the bike's chart. It answers: will this bike fit me, and what should I change?
• An AI video fit films you pedalling on a trainer and measures your joint angles. It answers: how am I actually sitting on the bike I have?

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What a Geometry Calculator Does

Stamptivity Bike Fit is the first kind. You enter your height, inseam and — if you have them — torso, arm and shoulder measurements, then a bike's geometry chart. It gives you:

• how the bike fits: standover, seatpost, where your hands land, how big a stem change closing the gap takes;
• a starting setup: saddle height, setback, stem, spacers, bar width;
• what to change if you enter the setup you ride now;
• the uncertainty in all of it, because a tape measure cannot tell how upright you like to sit.

It works before you own the bike. That is the point: comparing two frames in a shop, or two sizes on a website, is something no camera can do for you.

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What an AI Video Fit Does

MyVeloFit is the best-known of these. You film yourself pedalling on a trainer, upload the clip, and computer vision tracks your joints through the pedal stroke. It reports angles — knee extension, hip, elbow, torso — and suggests changes like "raise the saddle 8 mm".

It costs about $35 for a fit, or roughly $75 a year for unlimited analyses. Independent testing has put its saddle height and setback recommendations within about 5 mm of a professional fit, which is genuinely good for the price. It also assesses mobility, which a calculator cannot.

What it needs from you: a trainer, a phone on a tripod, decent light, and a few minutes of setup. What it cannot see: a bike you do not own yet.

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Side-by-Side

	Geometry calculator	AI video fit
Answers	Will this bike fit, what to change	How you actually sit on this bike
Works before you buy	Yes	No
Needs a trainer and camera	No	Yes
Sees your flexibility	No	Partly, through your range of motion
Sees your pedalling	No	Yes
Compares two frames	Yes	No
Cost	Free in Stamptivity	~$35, or ~$75/year
Time	A couple of minutes	15–30 minutes

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The Honest Order

Geometry first. Frame size, then saddle height and setback, then the stem and spacers that put your hands in a sensible place. These are the big, cheap moves, and they are the ones a chart can decide.

Video second, if you want to refine. Once the position is roughly right, a video fit is good at telling you whether your knee extends too far, whether you are reaching, and whether the setup suits the body you have today rather than the one the formulas assume.

A fitter when something hurts. Neither tool sees pain, cleat position or a leg-length difference. Both say so; believe them.

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Where They Overlap

One thing a calculator can do surprisingly well: use a bike you already like as the reference. In Bike Fit's Adjust & Compare, you can put a new frame next to the one that already feels right and find the stem and spacers that reproduce the position you have. That is the same underlying idea as a video fit — trust the position you know works — without needing a camera.

The difference is what evidence you feed it. A video fit measures your body on the bike. A calculator measures the bike against your body. Most riders need the second one first, and only some need the first at all.

Start with the free one: Bike Fit, or one of the single-question pages — saddle height, stem length, stack and reach, handlebar width.