Most bikes include adjustable parts to fine-tune your riding position. However, before buying, make sure none are already maxed out—otherwise, you might struggle to make further tweaks later, which could lead to discomfort or fit issues down the road.

A bike that fits well:

• Feels good from the get-go
• Prevents injuries
• Makes you faster (and happier)
• Helps you actually want to ride again
• So, if you’re buying a second-hand bike on Shifta (good call by the way), nailing the size is step one. Luckily, this guide’s got your back.

Before we get technical, here’s a quick look at rough frame sizes based on rider height.

📝 Note: This is just a starting point—geometry and riding style play a big role too. Keep reading, legend.

Bike sizing is mainly based on your height and inseam, but don’t be fooled: frame size numbers can mean different things depending on the bike type. Don’t just rely on a bike size guide as they do vary, test riding is always recommended.

• Road bikes use cm measurements (like 54cm)
• MTBs use S, M, L, XL sizing (but vary wildly across brands)
• Hybrids and commuters are often S/M/L too, but more relaxed
• Gravel bikes blur the lines – road-style sizing with off-road comfort
• Kids bikes? You size by wheel diameter, not frame!

All you need is a tape measure, a book, and your dignity (optional).

1. Height: Stand against a wall, feet flat, back straight.
2. Inseam: Measure from the floor to your crotch—yes, really.
3. Arm span (optional): Helpful if you’ve got orangutan limbs or T-Rex arms.
4. Torso length (if you’re fussy): From your neck base to top of your hips.
5. Once you’ve got those, plug ‘em into a size chart or match them to a brand’s bikes size guide.

Let’s break it down by category so you’re not mixing apples and carbon fibre.

🚴 Road Bikes
Fit is critical. Too big = overstretched. Too small = cramped.
Look at stack (vertical) and reach (horizontal) as well as frame size.
Performance-focused? Go slightly smaller.
Comfort-focused? A taller stack or endurance geometry helps.

⛰️ Mountain Bikes
More forgiving, but still important.
Focus on reach and standover height (can you straddle it comfortably?).
Long legs? Size up. Short torso? Size down.
Some riders downsize for agility, others upsize for stability.

🚴‍♀️ Hybrid & Commuter Bikes
Prioritise comfort and control over pure performance.
These often come in general S/M/L sizing.
Saddle height and handlebar position can be adjusted easily.
If you’re between sizes, go for the one that feels more upright.

🚲 Gravel Bikes
Sit between road and MTB in sizing.
Often similar to road sizing, but slightly more relaxed.
Choose based on what terrain you’ll ride most—more road? Size like a roadie. More off-road? Aim for comfort.

⚡ E-Bikes
Get this wrong, and it’s like riding a fridge downhill.
Heavier frames = sizing is crucial for safety and control.
Step-through frames are easier to size and mount for shorter riders.

👶 Kids Bikes
Size by wheel diameter, not frame.
Inseam is more important than age or height.
If they can’t put their feet flat on the ground, it’s too big.
Don’t size up “so they grow into it”—you’ll ruin their fun (and your wallet if they crash).

Found a bike with no sizing labels? No stress—it’s easy to work it out yourself with a tape measure and a bit of know-how.

Here’s how to do it:

📏 Step 1: Measure the Seat Tube Length

Grab a tape measure.
Measure from the centre of the bottom bracket (the middle of the crank axle) up to the top of the seat tube (where the seat post enters the frame).
This number (in centimetres for road bikes or inches for mountain bikes) is your rough frame size.

📐 Step 2: Double-Check the Top Tube Length (Optional)

For a better fit check, measure the top tube from the centre of the seat tube to the centre of the head tube.
This helps you compare bikes across brands with different shapes.

🔎 Step 3: Match It to a Bike Size Guide

Now compare your seat tube length to a size chart (like the one above).
If it’s in between, check reach, standover, and personal comfort.

💡 Top tip: Always round down if you’re unsure. It’s easier to adjust a smaller frame than to fight a big one.

Here’s the golden rule:

• Size down for agility, lower standover, and more fun on trails
• Size up for stability, long legs, or more upright posture
• Still unsure? Test ride some bikes before you buy to see what feels right for you.

In other words, Go smaller if you like it nimble and go bigger if you want a stable cruiser.

Most bikes have some parts you can adjust to alter the riding position but always remember to check when buying that none of the parts are at their maximum position as this may hinder you down the line. Here’s some of the parts you can usually tweak:

• Saddle height and fore/aft position
• Stem length
• Handlebar width or rise
• Crank length (if you’re really picky)
• But if the frame itself is the wrong size, no amount of tweaking will save the ride.

• Measure inseam AND height – You’ll get a much more accurate fit than using height alone.
• Check brand-specific charts – Sizes can vary a lot between manufacturers.
• Don’t rely on size names (S/M/L) – A Medium in one brand can feel like a Large in another.
• Look at geometry (reach and stack) – Especially important for road, gravel, and MTB riders.
• Try before you buy, if possible – Fit matters more than colour or components.

Most bikes come with a few adjustable parts that help you dial in your ideal riding position — including the saddle height, stem length, and handlebar angle. For the most part, these adjustments can make a big difference to your comfort and control on the bike.

However, before you buy (or ride), it’s important to check that none of these parts are already at their limit. If they’re fully extended or maxed out, you won’t have much flexibility to tweak the fit later on. As a result, even small changes in your riding style or body position could become harder to accommodate.

Over time, this lack of adjustability can lead to discomfort, poor posture, or issues that are tricky (and sometimes costly) to fix.

To stay on the safe side, always take a close look at these key areas — especially if you’re buying second-hand. By doing this early, you’ll avoid nasty surprises and make sure the bike actually fits your needs both now and down the road.

If you’re still unsure about sizing, don’t worry—most major bike brands offer their own dedicated size guides. In fact, these brand-specific charts can be more accurate than generic guides, especially since sizing can vary widely between manufacturers. Therefore, we’ve rounded up 10 of the most popular bike brands in the UK and linked directly to their official sizing pages. As a result, you can easily compare recommendations, check geometry charts, and make a more confident decision. Even if you’re buying second-hand, it’s still worth using these tools to cross-reference your fit. That way, you avoid guessing and get a frame that actually suits your height and riding style. So before you pull the trigger on your next bike, take a few minutes to scroll through the links below—it could save you a lot of hassle later on.

1. Specialized

2. Trek

3. Giant

4. Cannondale

5. Cube

6. Scott

7. Canyon (Direct-to-consumer – has a size calculator)

8. Orbea

9. Santa Cruz

10. Raleigh