How to Measure Bike Frame Size

Riding the wrong size frame causes discomfort, poor handling, and can lead to injury. Here's how to measure your current frame, work out what size you need, and understand the sizing systems used by different bike types.

How frame size is measured

Frame size is the length of the seat tube — the tube that runs from the bottom bracket (where the cranks attach) up to where the seatpost enters the frame. It's measured in either centimetres or inches, depending on the bike type.

Road bikes: Usually measured in centimetres (e.g., 54cm, 56cm)
Mountain bikes: Usually measured in inches (e.g., 17”, 19”) or S/M/L sizing
Hybrid bikes: Either cm, inches, or S/M/L depending on the brand

How to measure your current bike

Find the centre of the bottom bracket (the axle where the cranks rotate)
Measure in a straight line from the centre of the bottom bracket to the top of the seat tube (where the seatpost enters)
This measurement is your frame size

Note: Some manufacturers measure to the top of the top tube instead (centre-to-top vs centre-to-centre). Check the manufacturer's specification if comparing sizes between brands.

Finding the right frame size for your height

The most reliable method uses your inside leg measurement (inseam), not just your height, because people of the same height can have very different leg and torso proportions.

How to measure your inseam

Stand barefoot against a wall
Place a book between your legs, spine up, pressed firmly against your crotch
Measure from the floor to the top of the book
This is your inseam length

Road bike frame size

Multiply your inseam (in cm) by 0.70 to get your approximate road frame size.

Example: 82cm inseam × 0.70 = 57.4cm → a 56cm or 57cm frame.

Road bike size guide by height

Height	Frame Size
5'0” – 5'3” (152–160cm)	47–49cm (XXS/XS)
5'3” – 5'6” (160–168cm)	50–52cm (S)
5'6” – 5'9” (168–175cm)	53–55cm (M)
5'9” – 6'0” (175–183cm)	56–58cm (L)
6'0” – 6'3” (183–191cm)	58–61cm (XL)
6'3”+ (191cm+)	61–63cm (XXL)

Mountain bike frame size

Multiply your inseam (in cm) by 0.57 to get your approximate MTB frame size in cm, then convert to inches if needed (divide by 2.54).

Mountain bike size guide by height

Height	Frame Size
5'0” – 5'4” (152–163cm)	13”–15” (XS/S)
5'4” – 5'8” (163–173cm)	15”–17” (S/M)
5'8” – 6'0” (173–183cm)	17”–19” (M/L)
6'0” – 6'3” (183–191cm)	19”–21” (L/XL)
6'3”+ (191cm+)	21”–23” (XL/XXL)

Hybrid bike frame size

Hybrid sizing typically falls between road and mountain bike sizes. Most brands use S/M/L sizing. Use the manufacturer's size chart for the specific model you're looking at.

Between sizes?

If you're between two frame sizes:

Comfort and leisure riding: Go for the larger size. A more upright position is more comfortable
Performance and racing: Go for the smaller size. A smaller frame is more responsive and allows a more aggressive riding position
Long torso, short legs: The larger size usually fits better
Short torso, long legs: The smaller size usually fits better

Frame size isn't everything

Frame size gives you a starting point, but the final fit depends on:

Saddle height — adjustable within a range. The most important fit adjustment
Stem length — controls your reach to the handlebars. Can be swapped to fine-tune fit
Handlebar width — should roughly match your shoulder width
Saddle fore/aft position — adjustable on the seatpost clamp

A bike that's one size off can sometimes be made to fit with stem and seatpost adjustments. But a bike that's two sizes wrong will never feel right.

Buying second-hand?

If you're buying a second-hand bike, knowing your correct frame size prevents expensive mistakes. Measure the bike before you buy, and ideally test ride it. A bike that's too small or too big is a poor buy at any price.

Not sure about your bike's fit?

We check saddle height and basic fit as part of every service. If something's off, we'll let you know.

Call 07530 176721 Send an enquiry