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Frame size gets you close; reach and stack tell you how a bike actually fits. Saddle height is the single most important adjustment: start at 0.883 times inseam in cm for saddle-to-pedal length, then refine by feel.
Size and fit are two different problems, and most riders only discover the second one after buying. Sizing is picking the right frame from a size chart. Fit is what you adjust afterwards so the bike does not hurt. The guides on this page cover both: how to read frame sizes across brands that disagree with each other, why reach and stack compare used bikes better than a size label does, how to size a kids bike by age and inseam, and which adjustments are worth making at home before you pay for a professional fit.
Readiness for a pedal bike is behavioural, not chronological. When a child glides with both feet up for several seconds, steers while gliding and can stop themselves without panicking, they are ready, whether that happens at two and a half or four and a half. Stabilisers work against this: they hold the bike upright so balance is never learned, and they teach leaning away from a turn rather than into it. Size the first pedal bike by inside leg and set the saddle low enough for flat feet.
9 min
Reach and stack are two frame measurements, given in millimetres, that describe bike fit regardless of the size label. Stack is the vertical height from the bottom bracket to the top of the head tube; reach is the horizontal distance forwards. Compare them against a bike that already fits you.
7 min
Inseam matters more than height. Stand against a wall with a hardback book pressed up into your crotch and measure to the floor. That is your inseam. Road bikes use cm (50, 52, 54, 56, 58, 60), MTBs use S/M/L/XL. Between sizes, smaller rides sportier, larger rides more stable. Test ride before buying.
10 min
Saddle height is the single most important fit measurement. Use 0.883 times inseam in cm as a starting saddle-to-pedal length, then refine by feel. Knee pain in front means saddle too low; pain behind means too high. A 90-minute professional fit costs £100-£200 and prevents most overuse injuries.
28 min
Height gets you to a starting size and inseam refines it, but sizing varies enough between brands that a medium in one is a large in another. Compare reach and stack rather than the size label whenever the numbers are published. Between two sizes, the smaller frame feels sportier and more responsive, the larger feels more stable over distance.
If you ride more than a few hours a week, or anything hurts after an hour, yes. Below that, saddle height, saddle fore-aft and bar height cover most of the benefit and cost nothing but an afternoon. Book a fit before buying an expensive bike rather than after, so the numbers inform the purchase.
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