The most important quality in any bike is that it fits you. Here's how to avoid buying one that's too big or too small.

Updated May 2019

To be comfortable to ride, your bike fit must be correct. Approximately correct is close enough because there are normally just three to six different bike sizes. You fine-tune the fit by adjusting the position of the saddle and handlebar, but it needs to be in the right ballpark to begin with.

There are three basic criteria to consider when finding your ideal bike size, all of which you can determine without expert knowledge:

1. The top tube must be low enough for you to stand over it.

2. The saddle's position must allow you to pedal comfortably.

3. The handlebar's position must suit you.

There is some leeway with these dimensions, as we all have a 'fit window' within which we can be comfortable. The dimensions will vary between bike types – for example, a road bike has a different handlebar position from a roadster.

A Formula for Bike Fitting?

Take with a pinch of salt any bicycle fitting formulas that depend on static body measurements, such as your inside leg length. These formulas tend to be extrapolations from bike dimensions that seemed to work for young professional racing cyclists on road bikes. Middle-aged cycle commuters on hybrids were not consulted.

There is a better, but not infallible, way to estimate whether a given bike should fit. It's based on the fact that a manufacturer's medium size bike in any range is designed to fit Mr Average or (if it's a women's specific bike) Ms Average. Manufacturers can sell more bikes if their average size model fits the average cyclist, because there are more people in the middle of the height bell curve.

Bike size means seat tube length. While the effective top tube length is a better indication of whether a bike will fit, seat tube length is the de facto standard. So a 54cm bike is one whose seat tube is 54cm long.

You and Mr Average

Bike sizes can get confusing, so we’ve explained bike frame sizes here. The average size bike in a range is the one listed as Size M or, if not designated as such, the arithmetic mean of the different sizes.

To find that, add the sizes of the different models together and divide by the number of different sizes. So if a bike comes in 50, 52, 54, 56, 58 and 60cm sizes, the mean is 55cm. Mr Average should fit size M or a notional 55cm model.

To determine which bike frame size fits you, you need to compare yourself against Mr Average – unless you're buying a women's specific bike, in which case you'll compare yourself to Ms Average.

In the UK, Mr Average is about 176cm (5ft 9in), while Ms Average is about 162cm (5ft 4in). If you are of average height, simply pick size M or the nearest size to the mean. When the mean is the middle of two sizes, either should fit, given some fine-tuning.

If you are not Mr (or Ms) Average, the important thing to know is that bike sizes vary half as much as height. To find the size that should fit you, add or subtract half the difference you are taller or shorter than average height to the size of the average bike. So if you are 180cm tall and the mean or size M model is 55cm, you want the 57cm model; 180cm minus 176cm, divided by two, is 2cm, which you're adding because you're taller. If you are 160cm tall and are buying from the same range, you want the 47cm model.

This method will get you a bike that's approximately the right size. Depending on your proportions, flexibility, or preferences, you might be happier on a bike that's one size either side.

Shopping for the Perfect Bike Fit

You can't beat visiting a bike shop in person to try a bike for size. Many shops will let you have a short test ride and some will offer a bike-fitting service. If you can't leave the shop with the bike, or don't want to pay for a bike fit, you can still get a good idea of whether a bike will fit you.

Stand over the bike first. Got clearance? Good.

If you're buying online you won't be able to try bikes out for size, so check what the online shop's policy is regarding returns.

Can you easily swap it for a different size if you get the wrong one? Even if you get the right size, you will probably need to fine tune the fit yourself.

Online shopping is a better option if you already know your size and you're confident about making adjustments to the bike.

How to Get The Right Size Bike

Buying the right sized bike and setting it up correctly is vital if you’re going to be comfortable, cycle efficiently and avoid injury. Here’s our cycle size guide…

1. Stem length

Stems come in various different lengths and this can have a big effect on the way you feel on your bike. Changing your stem is usually a 5-minute job. A very rough guide to setting your reach on a road bike is to put your elbow on the nose of your saddle with your forearm horizontal - the tip of your middle finger should be 2-3cm from your handlebar. Some people prefer a less-stretched, more upright position, particularly on a flat-handlebarred bike.

2. Brake lever reach

Many brake levers come with a reach adjustment screw to bring them closer into the handlebars for smaller hands. Setting it is an easy job with an Allen key.

3. Saddle height

A good rule of thumb is to adjust the saddle height so that your knee is straight when your heel is on the pedal and the crank is in the 6 o'clock position. That way, when you cycle normally with the ball of your foot on the pedal there will be a slight bend in your knee.

If you're riding a mountain bike on off-road trails, you'll probably want to reduce your saddle height a little - about 1cm.

4. Saddle setback

When your crank is in the 3 o'clock position, the front of your kneecap should be directly above the end of the crank arm. Loosen the seatpost bolts and shift the saddle backwards or forwards on its rails to achieve this.

5. Handlebar height

This is a tricky one that varies hugely between riders.

A low handlebar height gives a flatter, more aerodynamic body position for sports riders, but higher handlebars put less strain on your back, provide easier control, and give you a better view of the road ahead.

In the end, it comes down to comfort and personal preference. The easiest ways to adjust your handlebar position are by swapping your stem for one with a different rise (measured in degrees), or by altering the height of your headset spacers.

6. Frame size

Like clothes and shoe sizes, bike sizes vary greatly between manufacturers - plus there's the complication that some are given in inches, some in centimetres, and some are described as small, medium, large and so on.

The best advice is to check out each manufacturer's website - they'll usually give their own sizing guidance.

When choosing your frame size, first make sure you can get the saddle height you need, and the reach that you're after. Then make sure there's clearance between your crotch and the top tube when you stand astride the bike with your feet flat on the ground. Ideally, you want 5cm of clearance or more, to give you a bit of leeway if you have to get your feet down in a hurry.

7. Toeclip overlap

On some bikes, especially smaller ones with large wheels, it's possible for your toe to hit the front wheel or mudguard when you turn. You have to be turning pretty sharply, though, and that only happens when you're going very slowly and it's rarely dangerous.

If it bothers you, check for it before you buy or remove the toeclips. You're less likely to get toeclip overlap on a bike with smaller wheels.

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