What Size Bike Do I Need? A Comprehensive Guide to Finding the Perfect Fit

Finding the right bike size is key to a comfortable and enjoyable cycling experience. The universal rule for knowing what size to buy is simple: Don't go by age. Go by the inside leg measurement.

But there are a few things to bear in mind for children's bikes that are different from adults' bikes. We'll get into that.

Selecting the correct bike size ensures proper ergonomics, efficient pedalling, and optimal control. In this guide, we’ll walk you through the key factors to consider. Bonus: we will point you in the direction of some quality bikes from our very own store - Bobbin!

 

 

How to measure yourself for a bike

1. Measure your inside leg

You'll need a hardback book and a tape measure. Wear the shoes the rider will normally cycle in — bare feet measurements are inaccurate because nobody cycles barefoot.

Stand against a wall, feet about 15cm apart. Place the spine of the book between the legs, pulled up snugly against the crotch as if it were a bike saddle. Measure from the floor to the top of the book. That's the inside leg measurement.

For a child, do this with the child standing normally — not stretched up on tiptoes, not slouching. If they wriggle (and they will), do it twice and average.

2. Match the leg measurement to the bike wheel size

Your inside leg should be roughly 5–10cm larger than the wheel diameter for the bike to fit comfortably (and with room to grow for kids).

Wheel diameters in centimetres:

• 12-inch wheel = 30.5cm
• 16-inch wheel = 40.6cm
• 20-inch wheel = 50.8cm
• 24-inch wheel = 61cm
• 26-inch wheel = 66cm

So if a child's inside leg is 47cm, the wheel size that puts them in the sweet spot is 16-inch (40.6cm wheel, child's inside leg is 6.4cm above it). If their inside leg is 55cm, they're at the upper end of the 16-inch range and the bottom end of the 20-inch range — time to size up.

The reason it's a 5–10cm window rather than a single number is that bikes are designed to fit a range of riders, not one rider. The bottom of the range is the smallest child who can safely stand over the top tube and reach the ground at the saddle's lowest position. The top of the range is the largest child for whom the saddle can be raised enough to give proper leg extension without the riding position becoming hunched.

If your inside leg lands between two wheel sizes — say, 50cm, which is at the top of the 16-inch range and the bottom of the 20-inch range — go smaller. A bike a child can confidently put their feet down on is a bike they'll ride. A bike that's too big is a bike that sits in the garage.

 

Where to find the exact size for the bike you want

The 5–10cm rule of thumb gets you to the right wheel size. The exact inside-leg range for each specific Bobbin bike is published on every product page, and the full range is laid out in our Bobbin Size Chart — covering kids' bikes, adult bikes, and helmets.

That's where to go once you know your measurement and want to pin down the specific model. The rest of this guide explains the principles behind the chart — why inside leg matters more than age, how growth changes the fit, and why the riding position shifts as a child grows into a bike.

Why height and age are unreliable

Height is closer to useful than age, but still misleads. People have different proportions — some have long legs for their height, some have short. A 170cm rider with long legs and a 170cm rider with short legs need different frame sizes. Inside leg cuts straight to the measurement that determines whether the rider can plant their feet at a standstill and clear the top tube safely.

Age is even more variable. The "average" 5 year old has an inside leg somewhere between 44cm and 54cm — a 10cm range that crosses two wheel sizes. The chart-based recommendation is always a guess for an average child who may or may not exist in your house. The measurement is exact.

This is why every Bobbin product page lists the inside-leg range a bike fits, not the age. Age guides are a convenience for parents browsing — but if you're between sizes, inside leg is what you trust.

Growth room: how a child's bike grows with them

A bike is a significant purchase, and parents reasonably want to know how long it will last before the child outgrows it. Bobbin's bikes are designed with adjustable saddles so a single wheel size covers a meaningful inside-leg range — typically 8–12cm of growth from minimum to maximum.

In practice, that's usually 18 months to two years on a single bike, depending on how fast the child is growing.

Here's how that works in real terms. When a child first gets the bike, the saddle is at its lowest position. They can comfortably stand over the top tube with both feet flat on the ground — important for confidence and for stopping safely. As the child grows, you raise the saddle gradually. At the maximum saddle height, the child has grown into the upper end of the inside-leg range, and it's time to think about the next size up.

The handlebar constraint — why you can't just keep raising the saddle

There's a complication worth knowing about: raising the saddle changes the rider's position relative to the handlebars.

When the bike fits at the lower end of the inside-leg range, the rider sits upright with the saddle low. As the saddle goes up, the rider's hips rise — but the handlebars stay roughly where they are. The rider ends up leaning forward more, in a slightly more sport-oriented position.

For a small amount of growth this is fine. Most bikes do have some handlebar adjustment, usually a couple of centimetres on the stem. But handlebars can't be raised indefinitely. At some point the riding position becomes uncomfortable enough that the child should move up a wheel size even if the saddle could technically still go higher.

The practical rule: when the child's posture starts to feel hunched, it's time for the next bike — regardless of whether the saddle has room left at the top.

This is the real reason "buy big and grow into it" has limits. Buying a wheel size too large means the child can't reach the ground at the bottom of the saddle range, which kills their confidence. Buying within the right range and growing through it means the bike fits properly throughout its useful life with you — sat upright at the start, slightly forward at the end, with the next bike on the horizon before the position becomes a problem.

Adult bike sizing — the principle still applies

The same rule applies to adult bikes: inside leg matters more than height. The difference is that adults are within a smaller range of wheel sizes than children, so adult bikes are sized by frame measurement (in cm) rather than by wheel.

Bobbin's adult bikes come in either a single size that suits a wide inside-leg range, or in two frame sizes (typically marked S/M and M/L) for models that benefit from the distinction. The published inside-leg range for each specific model lives on the product page and in the Bobbin Size Guide.

For adults, the growth question doesn't apply — you're not going to outgrow a bike. What changes is comfort over distance. If your inside leg falls between two frame sizes, the smaller frame is usually more comfortable for shorter rides; the larger frame gives more leg extension for longer ones. Use the published ranges on each product page to confirm.

Read more about each wheel size

We've covered each wheel size in detail in its own guide:

• 12-inch wheel bikes — first pedal bikes, typical inside leg 33–43cm
• 16-inch wheel bikes — confident pedaller, inside leg 40–54cm
• 20-inch wheel bikes — independent rider, inside leg 52–62cm
• 24-inch wheel bikes — pre-teen rider, inside leg 60–72cm
• 26-inch wheel bikes — teens and petite adults

For the full size chart across the Bobbin range, including each specific model's inside-leg range and helmet sizing, see the Bobbin Size Guide.

When to ask for help

If you've measured inside leg and you're still uncertain — particularly if the rider falls exactly on the boundary between two sizes, or if you're buying for someone whose proportions are unusual — contact us. We size hundreds of bikes a year and we can talk you through the choice for your specific case.