Sizing vs fitting

Bike sizing vs bike fitting: what's the difference?

They sound like the same thing. They solve opposite problems at opposite ends of the purchase, and confusing them is why a lot of people end up on the wrong bike.

The short version

A bike fit optimises the bike you already own.
Bike sizing decides which bike to own.

A fit is about adjustment. Sizing is about selection. If you already have the bike, you want a fit. If you're about to buy, you want sizing first, then a fit later once you have the frame.

What each one can change

A bike fit can change:

  • Saddle height, fore-aft position and angle
  • Stem length and angle
  • Spacer stack under the stem
  • Handlebar width, reach and rotation
  • Cleat position
  • Crank length, if you're replacing them

A bike fit cannot change:

  • Frame stack
  • Frame reach
  • Wheelbase, head angle, seat tube angle
  • The bike's fundamental proportions

The second list is the one that matters. Stack and reach decide where your adjustment range sits, and they're fixed the moment you pay. Between adjacent sizes stack steps 12 to 25mm and reach only 4 to 12, depending on the maker. A fitter's own range is generous, roughly 95mm of reach and 59mm of stack, but every millimetre of it moves you around inside the frame's range rather than moving the range itself.

Buy a size out and the fitter spends their session, and your money, clawing back ground you gave away at the checkout.

Frame stack and reach measured from the bottom bracket to the top of the head tube A road frame drawn side on. A dashed vertical line runs from the bottom bracket up to the height of the head tube top: that is stack. A dashed horizontal line runs forward from there to the head tube top: that is reach. Neither can be changed after purchase. Between adjacent sizes stack steps 12 to 25 mm and reach only 4 to 12, and how much varies by manufacturer. FIXED THE DAY YOU PAY Stack and reach cannot be adjusted afterwards. Between sizes stack steps 12–25 mm and reach only 4–12, and how much you get varies sharply by manufacturer. STACK REACH Head tube top Measured across 833 dropbar size steps in the BikeSelect geometry database Bottom bracket
How far a bike fit can move your handlebars, in millimetres A bar chart of the range each cockpit component offers. For reach: stem length gives 70 mm, handlebar reach 25 mm, and spacers pull the bars back by 12 mm. For stack: a spacer stack gives 38 mm and flipping the stem 21 mm. That is roughly 95 mm of reach and 59 mm of stack in total. Frame reach and frame stack both show zero, because a fit cannot change them at all. All of the available adjustment moves the rider around inside the frame's range rather than moving the range itself. WHAT A FIT CAN MOVE YOUR HANDLEBARS, IN MILLIMETRES Everything in a fitter's toolkit, and the two numbers none of it touches. 010203040506070 MILLIMETRES OF MOVEMENT AVAILABLE REACH · HOW FAR FORWARD THE BARS SIT 70 mm Stem length 70 to 140 mm 25 mm Handlebar reach 70 to 95 mm bars 12 mm Spacers, pulling back 40 mm of spacer, leaned back STACK · HOW HIGH THE BARS SIT 38 mm Spacer stack 0 to 40 mm under the stem 21 mm Stem angle a 6° stem, flipped NEITHER · FIXED WHEN YOU PAY 0 mm Frame reach chosen at the checkout 0 mm Frame stack chosen at the checkout ROUGHLY 95 MM OF REACH AND 59 MM OF STACK A generous range, and every millimetre of it moves you around inside the frame's. None of it moves the frame's.
Ranges for a typical road cockpit. Yours will differ a little; the two zeroes will not.

Why the industry does this backwards

The normal sequence: pick a bike from a size chart → buy it → ride it for a few months → notice something isn't right → pay for a fit → have the fitter do their best inside the constraints of a frame that was chosen by a chart.

Nobody designed that sequence. It's what happens when the person advising you is also the person selling you the bike, in a shop that stocks what it stocks.

The rational sequence: establish your position → find the frames that reach it → buy one → get it fitted properly.

Sizing costs less than the fit and constrains a decision worth thousands. It should come first. It almost never does.

What we deliberately don't do

The scope is narrow on purpose.

We don't diagnose pain or injury.

One camera, one side, forty-five minutes. Enough to measure where your body is. Nowhere near enough to explain why your knee hurts. If something hurts, see a physiotherapist, and go before you buy a bike rather than after.

We don't analyse asymmetry or pedalling technique.

Both need equipment we don't have and time we don't take. A good in-person fitter does this properly. We won't pretend to.

We don't adjust your current bike.

We'll measure it, because it tells us about you. We won't change it.

We don't replace a fit.

When your new bike arrives, get it fitted. You'll be starting from a frame that can accommodate you, which is the point.

So which do you need?

You want sizing if:

  • You're buying a new bike in the next few months
  • You're moving discipline, road to gravel or endurance to race
  • You're buying online and can't test ride
  • You're between sizes on a chart and nobody can tell you which way to go
  • You've owned bikes that never felt right and you want to know why before you buy another

You want a fit if:

  • Something hurts
  • You've just bought a bike and want it dialled in
  • Your position feels wrong and the frame is definitely right
  • You want cleat setup, technique work or a full biomechanical assessment

You want both if:

You're buying a bike you intend to keep for years. Size it before you buy, fit it after it arrives. In that order.

Buying rather than adjusting?

45 minutes, one professional fitter, one report you keep.

Book a sizing session — $125