What size bike do I need?

You find a bike you like. The listing says “19 inch frame”, or just “Medium”, and there is no other information. You are 5 foot 9. Is it going to fit?

I went looking for a proper answer to that, and found something I was not expecting: there is no standard. Not a loose one that everybody roughly follows. None at all. The number in that listing is whatever the company that made the bike decided it should be.

What size bike do I need?

Nobody can tell you from your height alone, because frame sizes are not standardised. Get three numbers off the seller: the seat tube, the horizontal top tube and the standover. Then stand over the bike with both feet flat on the ground and ride it up the road. That last part settles it.

Everything below is why, and what to do about it.

Is there a standard for bike frame sizes?

No. There is a safety standard, BS EN ISO 4210, which covers brakes, frame strength, steering and wheels. Nothing in the standards says where to measure a frame or what a size label means. The committee that writes them lists size designations among the things it does not cover.

This one is worth being precise about, because people mix the two up.

Every bike sold in this country is built and tested to a real international safety standard. It governs how hard the brakes have to stop the bike, how much fatigue the frame and forks have to survive, how strong the saddle and seat post have to be, and what instructions have to come in the box. That part is properly regulated.

The committee that writes it is ISO/TC 149, and its published remit covers “terminology, testing methods and requirements for performance, safety, interchangeability and maintenance”. Right underneath, in the list of things it explicitly does not cover, is the phrase “size designations”, and even there it is only talking about tyres, rims and valves.

So a safety standard, yes. A sizing standard, no. Each manufacturer makes its own up.

What does a medium bike actually mean?

Whatever the brand says it means. Across twelve UK brands and retailers, medium claims to fit a rider somewhere between 163 cm and 183 cm. Decathlon’s medium stops at 174 cm and Boardman’s road medium starts at 177 cm, so the two do not overlap at all.

Here is every one of them, read off their own published charts on the same day.

What a medium frame claims to fit, across twelve UK bike brands and retailers. The bottom of the range runs from 163 cm to 177 cm and the top from 173 cm to 183 cm.

Whose chart	Bike	Their "M" fits	Range shown as a bar160 cm to 186 cm
Cannondale Trail	Mountain	163 to 173 cm
Specialized Rockhopper	Mountain	163 to 178 cm
Liv Avail 2	Road	165 to 174 cm
Decathlon (adult, all types)	All	165 to 174 cm
Trek Marlin Gen 3	Mountain	165 to 177 cm
Trek FX 2	Hybrid	165 to 175 cm
Evans Cycles	Mountain and hybrid	168 to 178 cm
Giant Talon 1	Mountain	169 to 179 cm
Giant Escape 3	Hybrid	170 to 180 cm
Giant Contend 2	Road	171 to 181 cm
Ribble ALLROAD SL R E	Road	175 to 180 cm
Boardman (Halfords)	Road	177 to 183 cm

Look at the two highlighted rows. Somebody 175 cm tall, which is about 5 foot 9 and almost exactly the average height for a man in England, is too tall for a Decathlon medium and too short for a Boardman road medium. Both of those are sold in the UK. Both charts are the companies’ own.

And it is not only between companies. Giant’s road medium and Giant’s mountain bike medium are both for a rider of about 170 to 180 cm, and the seat tubes are 500 mm and 420 mm. That is eight centimetres apart, same brand, same letter, same year. The reach differs by nearly six centimetres too.

Halfords says as much on its own sizing page: “Similarly to how shoe sizes aren’t always universal, different frame sizes listed by different manufacturers may not be completely accurate.”

What does “19 inch frame” mean on a used bike?

This is the one that actually matters, because it is what the listing says.

Less than you would hope. A seat tube measuring about 19 inches belongs to a road bike for a rider of roughly 163 to 181 cm, or a mountain bike for a rider of roughly 183 to 195 cm. Same tape measurement, twenty centimetres of difference in who it fits.

Five bikes whose seat tube measures about 19 inches, with the rider height each manufacturer says that size fits. The road bikes suit riders of 163 to 181 cm and the mountain bikes 183 to 195 cm.

Bike	Sold as	Seat tube	The maker says it fits
Specialized AllezRoad	52	490 mm (19.3 in)	163 to 170 cm
Trek Domane SL Gen 4Road	54	500 mm (19.7 in)	168 to 174 cm
Giant Contend 2Road	M	500 mm (19.7 in)	171 to 181 cm
Giant Talon 2Mountain	XL	480 mm (18.9 in)	183 to 193 cm
Trek Marlin Gen 3Mountain	XL	500 mm (19.7 in)	188 to 195 cm

There is a reason for that, and it is recent. Mountain bikes are now designed around dropper posts, which slide the saddle down out of the way when you point the bike downhill. A dropper needs somewhere to go, so manufacturers shortened the seat tubes. The frames did not get smaller; the tube did.

Meanwhile the inch charts on retailer websites were worked out before any of that happened, and never moved. So they are describing bikes that are no longer being made.

Which means the first question to ask about a second-hand listing is not what size it is, it is what kind of bike it is. If you are not sure which kind you want in the first place, that is a separate question with a short answer.

Separately, those charts do not even agree with each other. Across UK retailers, “19 inch” is given as suiting anything from 168 cm to 193 cm, and Halfords’ main mountain bike ladder has no 19 inch size in it at all, jumping straight from 18 to 20.

Why doesn’t the frame number mean anything any more?

Two reasons, and both are the industry’s doing rather than anybody’s fault.

Nobody agrees where to measure from. At least five reference points are in use on UK websites right now: the top of the seat tube, the top of the seat clamp, the centre of the top tube, the centre-to-centre measurement, and a pretend measurement taken as if the top tube were level when it is not. Evans Cycles says so on its own page. Decathlon’s page says flatly that “the exact measuring point varies by brand.” And none of the biggest brands tells you which one it uses.

The number stopped describing the frame. Road bikes used to have a level top tube, so the seat tube length was also roughly your standover and roughly your fit. Then sloping top tubes arrived in the late nineties, the seat tube shrank while the bike stayed the same size, and the label stayed where it was.

The proof of that is quite funny when you see it. Three different size 56 road bikes, from Specialized, Trek and Cannondale. Not one of them has a 56 cm seat tube. They measure 53 cm, 52.5 cm and 52 cm. “56” is a label, not a measurement.

What should you measure instead?

Three numbers, and a seller can get all of them with a tape measure in about two minutes. The seat tube, from the centre of the bottom bracket to the top of the tube. The top tube, measured level from the seat tube to the head tube. And the standover, from the ground to the top of the top tube.

• ASeat tube

  Centre of the bottom bracket to the top of the seat tube. This is the number listings quote, and the one that tells you least.

• BTop tube, horizontal

  Measured level, from the seat tube to the head tube, however much the real top tube slopes. This is the one that decides whether you are stretched out.

• CStandover

  Ground to the top of the top tube. Tells you whether you can get off safely. It does not tell you whether the bike fits.

Of those three, the horizontal top tube is the one that tells you whether the bike will fit, because it decides how stretched out you are. The seat tube is the one everybody quotes and the one that tells you least. Cycling UK puts it plainly: “the seat-tube length alone will tell you little about whether a bike will fit.”

If you already have a bike that fits you, measure that one and compare. That is the single most useful thing in this whole article, and it costs nothing.

How much room do you need over the crossbar?

A couple of centimetres is the usual advice, and more on a mountain bike. Published figures disagree by about threefold for the same kind of bike, and no standard or study sits behind any of them. What matters is that you can stand flat footed over the bike without touching it.

Cycling UK’s version is my favourite because it says why: you want “a gap of a couple of centimetres or more so you won’t catch your undercarriage when you dismount.”

But I would not lean on standover too hard, and here is why. On a 2025 Giant Talon, the standover heights for medium, large and extra large are 709 mm, 709 mm and 710 mm. Three sizes, one millimetre between them. On the other model in the same family the extra extra large actually has a lower standover than the extra large. So standover tells you whether you can get off the bike safely. It does not tell you whether the bike fits.

Does saddle height decide what size bike you need?

No, because the saddle moves. It is the one part of the fit you can change in ten seconds with an Allen key, so it is not a sizing question at all. Get the frame right and set the saddle afterwards.

I only go into this because every sizing guide puts a formula in, and the formulas have a stranger history than you would think.

You will be told to multiply your inside leg by 0.883. Cycling UK publishes it. The number comes from Greg LeMond, and the peer-reviewed literature that cites it describes it as his “empirical experience”, referenced to a coaching book from 2002. There is no study behind it.

The other one, inside leg times 1.09, does trace to something published. It is a one or two page communication from 1967, and it is about calibrating an exercise bike in a laboratory. When researchers later tested both against the knee angle they are supposed to produce, the two formulas gave statistically identical answers, and the 1967 one landed outside the target range 63% and 73% of the time in two separate studies.

So use a formula as a starting point if you like, then ignore it and adjust. The best evidence available, a 2022 review of 41 studies, found that changes of under 4% of your leg length make no measurable difference to how hard you are working. On an 80 cm inside leg that is about three centimetres of wriggle room, which is far more than you will be out by.

Set it the simple way. Sit on the saddle, put your heel on the pedal at the bottom, and your leg should be just about straight. Then ride normally, on the ball of your foot, and you will have a slight bend.

What if you are between two sizes?

There is no evidence either way, and two large UK sellers give opposite advice about what the smaller frame even does to your position. Go and sit on both if you can. If you can only try one, take the one you can stand over comfortably.

I am not being evasive. Ribble says a smaller frame gives you a “stretched-out riding position” and a larger one a more upright one. Decathlon says the smaller size gives you a “straighter back” and the larger one a “more stretched-out position”. Those are two large UK retailers stating opposite things as fact, on their own websites, at the same time.

Nobody has studied it. The research that exists is about bike fit, which is where the saddle and bars end up, not about frame size. “If in doubt, size down” is a saying, not a finding.

What everyone does agree on is Cycling UK’s version: “Pick which feels best when you sit on them.”

Can you make a bike that is slightly too long fit?

On a road bike, usually yes. One whole frame size is worth only 3 to 9 mm of reach, and one step of stem length is 10 mm, so a shorter stem genuinely bridges a size. On a mountain bike, no. One size there is worth 20 to 30 mm, which is two or three stem steps.

That is from the manufacturers’ own geometry tables, and it surprised me. On a Giant Contend road bike the reach across six sizes goes 372, 375, 378, 384, 393, 401 mm. The gaps are tiny. On a Giant Talon mountain bike the five sizes go 385, 415, 436, 456, 476 mm, and those gaps are enormous by comparison.

So the old advice to just fit a shorter stem is true for road bikes and false for mountain bikes. Trying it on a mountain bike pushes the stem outside the 50 to 80 mm range the bike was designed around, and it will handle oddly.

One thing not to do, from a bike fitter quoted by BikeRadar: “You should never think: ‘My reach is too long, I’ll move my saddle forward’.” The saddle position is for your pedalling, not for your arms.

Are women’s bikes a different size?

Mostly they are the same bikes with the size range shifted down, plus a slightly shorter top tube. A step-through frame is genuinely different, because taking the crossbar out removes the standover problem entirely. The big brands disagree about whether anything else needs to change.

Specialized stopped making women’s specific road frames and published why: “with data from over 8,000 Retül Fits, we’ve learned that all riders are unique”, and “Body proportions vary as much within a gender as between them”. Liv, which is Giant’s women’s brand, says the opposite, that its research shows “significant anthropometrical differences”.

Both of them are selling something, and I could not find an independent study settling it. What I can tell you is what the charts show, which is that Liv’s medium sits about 6 to 7 cm lower than Giant’s medium, and Whyte’s women’s hybrid is its unisex hybrid moved one size step down. That is a shift, not a different shape.

For buying second-hand, the practical bit is simpler. A step-through frame is easier to get on and off and takes standover out of the equation, which is a real advantage for a lot of people and has nothing to do with gender.

What size bike do most adults need?

The average man in England is 175.8 cm and the average woman 162.1 cm, measured rather than self-reported. That puts most men on a medium or a 56, and most women on a small, which is why those are the sizes you see most often second-hand.

Those figures are from the Health Survey for England for 2024, published in January 2026, where a trained interviewer measures people rather than asking them. Worth knowing: the ONS does not publish adult height, and has said so in writing in response to a freedom of information request, so any article telling you “the ONS says the average UK height is” has got its source wrong.

One caveat on my own claim there. That is a mean, not a spread, and I did not get hold of the percentile tables. So “most men” is me reasoning from an average across the brand charts above, not a figure anybody publishes.

What about a child’s bike?

Completely different system, and much more sensible. Children’s bikes are sized by wheel diameter rather than frame, and the thing to measure is the inside leg rather than the age. I have written that up separately: what size bike does my child need?

The short version

• There is no sizing standard. There is a safety standard, BS EN ISO 4210, and it says nothing about size labels
• Medium means 163 to 183 cm depending on whose chart you read. Decathlon’s medium and Boardman’s road medium do not overlap at all
• “19 inch frame” suits a 163 to 181 cm rider on a road bike and a 183 to 195 cm rider on a mountain bike. Ask what kind of bike it is before you ask anything else
• No size 56 road bike has a 56 cm seat tube. Three of them measure 53, 52.5 and 52 cm
• Ask the seller for the seat tube, the horizontal top tube and the standover. The top tube is the one that decides whether it fits
• Standover tells you that you can get off safely, nothing more. Three sizes of one mountain bike are within a millimetre of each other
• Saddle height is not a sizing question. It adjusts, and anything within about 4% of ideal makes no measurable difference to how hard you are working
• A short stem can bridge a size on a road bike and cannot on a mountain bike
• If you are between two sizes, nobody knows. Go and sit on them

The honest summary is that a size label on a second-hand listing is a hint, not a fact, and the only test that has never let anybody down is getting on the thing and riding it up the road.

Which is, conveniently, how we sell bikes anyway. Come and try it, stand over it, ride it to the end of the road and back, and only pay if it is right. Here is what we have in, and here is what we do to every one before it goes up. If you are buying from somebody else instead, which is fine, here is how to not get stung.

And if the reason you are reading this is that the bike in your shed turned out to be the wrong size all along, we buy good second-hand bikes across North Devon and we collect. Here is how that works.