The Complete Guide to Buying the Right Bike Size

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The Complete Guide to Buying the Right Bike Size

Why is it Important to Buy the Right Size?

All of us have probably borrowed a bike from a parent, sibling or friend when we were young.  The bike that is too big is wobbly, hard to steer and it is difficult to maintain forward momentum if the pedals are too far to reach.  A bike that is too short makes you feel like a circus clown – it’s very hard to pedal and at slow speeds hard to steer.  Either way, riding the wrong size bike is dangerous.

An experienced cyclist knows instinctively if a particular bike is the right size or not. If you are sitting on the saddle of your bike at the lowest seat setting and can’t reach the road with at least the ball of your foot, then the bike is too big.  If when you are out cycling, your thigh comes up to parallel (or beyond)with the top tube on your bike when at the top of the pedal revolution, the bike is too small.

So very simply, getting the right size bike is a matter of safety because getting the wrong size will be inefficient and distracting for the cyclist.  This is true whether you are looking for racing bikes, a mountain bike or town bike on eBay.

What is the Right Size?

The standard diamond shaped bike frame has some key components that can help determine the right size of bike:

The Head Tube
The handlebars and forks slot into the head tube
The Top Tube
The top tube runs parallel to the ground between the head tube and the seat tube
The Seat Tube
Holds the Seat Post and runs down to meet the Down Tube at the bottom bracket

These form the first triangle part of a bike linking the seat, handlebars and pedals.

The Seat Tube of a bicycle contains the Seat Post which in turn connects to the saddle.  The seat post and therefore the height of the saddle is adjustable, either by using a quick release lever or loosening the nut on a bolt and varying the length of the seat post inserted into the seat tube.
All seat posts are marked by the manufacturer with a line that shows the minimum amount of the seat post that must always be inserted in the seat tube.  On no account should the desire for extra height lead to this rule being broken for obvious reasons. The answer is to buy a bigger frame rather than compromise the safety of the cyclist.

The Frame Size of a bicycle is traditionally measured from the centre of the bottom bracket where the chain stays meet the seat tube, to the point where the seat tube meets the seat post. In other words, the length of the seat tube is given as the ‘size’ of the bike. There are, however, some bike manufacturers who measure from the bottom bracket to the point where the seat tube and top tube meet.  This can cause problems so it is worth checking with a seller on which basis the frame measurement is made.

Buying the Right Size Bicycle on eBay

Most sellers on eBay will specify the frame size of the bike they are selling in both inches and centimetres.
A good guide is to measure your inseam, from your crotch to the floor, and then multiply it by 0.65 for a road bike, or 0.59 for a mountain bike, and you will have a good estimate of the size bicycle frame that fits you. This is purely meant as a guide.  People come in all shapes and sizes; some people have quite short legs and a long torso and others long legs and a short torso and they could be the same height.

Manufacturers make frames in different sizes because we all have different size legs. The key element is the inside leg and the cyclist ought to be able to stand astride the bicycle with at least 2-5 cm clearance over the top tube. This should be achievable with the cyclist’s feet flat on the floor, not on tip toe. If you are planning to buy a mountain bike, there should be a 5-10 centimetre clearance above the top tube. This is to allow for the nature of mountain biking – the extra space is for the occasional need to dismount suddenly from the bike.

The right size frame will allow the cyclist to sit on the saddle with a pedal at the bottom of its revolution, foot flat on the pedal and the cyclists leg just a few degrees off straight. 

In this position, if buying for a young cyclist and factoring in some growth, the seat post should be close to fully inserted.

Common Mistakes

When buying a child’s bike – say for a birthday - it is easy, but wrong, to just rely on guesswork. If they already have a bike, observe how they are riding and determine whether an increase in frame size is required. Measure the inside leg of a pair of jeans that fit them well and double check on the bike frame.  If in any doubt, take the child's measurements and send them to the eBay seller who can help source the correct size bike on eBay.

If buying for a niece or nephew and you don’t have access to the child to get a proper frame measurement, then resorting to wheel size is the only option.  Average age group sizes for children approximate to the following wheel sizes:

Wheel Size

Average age group sizes for children

12" Wheel Cycles

Suitable for ages 2½ to 4

14" Wheel Cycles

Suitable for ages 3 to 5

16" Wheel Cycles

Suitable for ages 5 to 7

20" Wheel Cycles

Suitable for ages 7 to 9

24" Wheel Cycles

Suitable for ages 9 to 11

Ages above those shown in table above are moving into the territory of adult bikes and really deserve proper fitting.

Remember, if you are buying a new bike, either for yourself or any other member of the family, eBay is a great place to sell the old bike rather than simply passing something on that may be the wrong size.  The budget for your new bike can be enhanced by selling your old bike first on eBay.

A word of warning, many people confuse the measurements for a bike with the size of the wheels on the bicycle. When reviewing the results of an eBay search for a bicycle make sure that the seller has specified and does actually mean frame size where quoted in the description.

Bicycle wheels come in a much wider range of sizes from 16” to 27”.  Some of these are for speciality bikes like BMX bikes and folding bikes, normally on 20” wheels. When shopping on eBay for a folding bike it is worth noting that frame measurements are irrelevant because all of the adjustments on height come through the Seat Tube and Head Tube - so a 12” framed bike with 20” wheels will be perfectly adjustable for a tall adult.

Finally, be aware that when you step up a frame size it isn’t just the height of the bike that increases.  In the case of the standard diamond shaped frame the overall dimensions increase - so the length or wheelbase does too.  Having a frame that is too small can lead to a cramped riding position, while one that is too large can create an equally uncomfortable extended position.

Buying the Right Size Bike on eBay

When you are comfortable with the product information for the bike you have selected, you can consider how to pay for your selection and proceed to purchase. In an Auction the buyer has to place a bid in order to buy the bike at auction by the date in the listing – it may take more than one bid to ensure you are the highest bidder. Use ‘Buy It Now’ if it is offered as an option and you would like to buy the bike immediately or to put down your ‘Best Offer’. Buying through PayPal will ensure the transaction proceeds directly between the seller and buyer. eBay users are protected by the Buyer Protection Programme.

The eBay Shopping Experience

Delivery and shipping of the bike to you is often provided by the seller, but in some cases the bike may be offered on a collection only basis.
It is recommended that you only purchase from sellers which are authorised through eBay.
It is always worth having a look at a seller’s other products for the chance to pick up another bargain and also save on postage costs.

Conclusion on Buying the right size bike on eBay

When choosing to buy a bike on eBay the buyer must consider not just specific buying needs; for example, new or second hand, type of frame, purpose (racing, mountain biking etc),budget, features of the bike and any particular brand that they prefer. On top of all of these – and most importantly – the bike must be the right size for the new owner.  Through measuring, observing and testing it is critical for the cyclist’s safety that the right size is chosen. Whichever route you choose for buying the bike, searching on eBay will deliver a huge and unrivalled range of choices for you to select from in the comfort of your own home with the protection of PayPal and eBay.

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