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· 4 min read
How to Work Out Your Bike Frame Size
Heshan Fernando
Co-founder & COO
You’re a medium. You’ve always been a medium. You buy a medium from a different brand and it feels completely wrong — longer, or lower, or like you’re reaching for the bars.
Frame size labels aren’t standardised. Two bikes both called 56 cm can differ by two centimetres of reach, which is the difference between a bike you ride for hours and one you quietly stop using.
Why size labels don’t transfer
Manufacturers measure differently. Some quote seat tube length, which varies with how sloping the top tube is. Some quote effective top tube. Some use S/M/L with no stated measurement at all. And a compact frame with a steeply sloping top tube has a much shorter seat tube than a traditional frame that fits the same rider.
So a “56” from one brand and a “56” from another are two numbers describing different things.
Stack and reach are measured the same way everywhere
Stack is the vertical distance from the bottom bracket to the top of the head tube.
Reach is the horizontal distance between the same two points.
Together they describe where the handlebars sit relative to the pedals, which is what actually determines your riding position. Every manufacturer measures them identically, which makes them the only figures comparable across brands.
Once you know the stack and reach of a bike that fits you, you can evaluate any other bike regardless of what its size label says. That’s why fitters work in these numbers and why every geometry chart lists them.
| Figure | What it tells you | Comparable across brands? |
|---|---|---|
| Size label (M, 56) | Almost nothing | No |
| Seat tube length | Frame height, sort of | No — depends on slope |
| Stack | How high the bars sit | Yes |
| Reach | How far the bars sit | Yes |
Measuring inseam properly
The one measurement worth taking carefully. Barefoot, back against a wall, a hardback book pulled firmly up into the crotch to simulate saddle pressure. Measure from the floor to the top of the book.
“Firmly” matters — a gentle placement under-measures by several centimetres, and inseam drives saddle height, which is the fit dimension with the largest effect on comfort and power.
Between two sizes
Common, and genuinely a case for riding both.
Sizing down gives a shorter reach, which you extend with a longer stem. The result is a slightly quicker-handling bike with more weight distributed rearward.
Sizing up gives a longer reach with a shorter stem. More stable, slightly less responsive.
At the same effective reach the two feel different to ride, and which you prefer depends on the riding you do and on flexibility. This is exactly the question a test ride answers and a chart doesn’t.
Common mistakes to avoid
- Buying by size label across brands.
- Measuring inseam gently, which under-measures saddle height.
- Assuming a bike that fits your height fits your proportions — riders with the same height can have very different leg-to-torso ratios.
- Fixing a too-long reach with a very short stem, which changes handling more than most people expect.
- Skipping a professional fit on an expensive bike, where the fit costs a fraction of the frame and determines whether you ride it.
How to do it with Bike Frame Size Calculator
The Bike Frame Size Calculator gives a starting size and points at the numbers that matter.
- Measure inseam properly — barefoot, book pressed firmly up.
- Read the suggested size as a shortlist, not an answer.
- Compare stack and reach on each candidate’s geometry chart.
- Test ride before committing, particularly if you’re between sizes.
Other cycling tools are in the tools directory.
Frequently asked questions
Why are frame sizes so inconsistent?
Because manufacturers measure differently — seat tube length, effective top tube, or an arbitrary label. Stack and reach are measured the same way by everyone, which is why fitters use them.
Between two sizes, which should I choose?
It depends on flexibility and intended use, and it’s genuinely a case for a test ride. Sizing down with a longer stem handles differently from sizing up, even at the same effective reach.
Is a professional bike fit worth it?
For anything beyond casual riding, usually yes. Sizing gets you the right frame; a fit adjusts saddle height, setback, bar position and cleats, which is where comfort and power come from.
Final thought
Find the stack and reach of a bike that already fits you and carry those two numbers around. They’ll tell you more about any bike than its size label ever will.