How to find your bike frame size: height, inseam, standover, reach and stack

A frame size starts with two body measurements, height and inseam, and ends with a check against the geometry numbers of the exact bike you are buying. This guide covers each step, with the figures that BikeRadar, Road Bike Rider, REI, Cycling UK and GearJunkie publish.

Start with height, then inseam

Height gets you into the right size band. Inseam tells you whether your legs are long or short for that height, which is the first sign that you should move up or down a size. GearJunkie calls height the first step and inseam the recommended second measurement.

The inseam that matters for bikes is not the one on your trousers. Road Bike Rider says to take off your shoes and bulky trousers, stand with your feet about 8 inches apart, and press a hardcover book spine-up into your crotch as if it were the saddle. Keep the book parallel to the floor, measure from its top edge to the floor, and repeat two or three times. Average the results. REI and GearJunkie describe a one-person version: stand against a wall in bike shoes, straddle the book, mark the wall at the spine, and measure from the mark to the floor.

Write the number down in both inches and centimetres. Brand charts mix the two, and you will end up converting (1 inch is 2.54 cm).

Frame size by height and inseam

Road Bike Rider's table matches both measurements to a size label. Its frame size for road bikes is measured from the centre of the bottom bracket to the top of the seat tube. Road and gravel run in centimetres, mountain bikes in inches.

The mountain column is Road Bike Rider's own height band for each label, which it prints as slightly different cutoffs (for example 5'3" to 5'7" for S). BikeRadar's mountain bike overview runs 13–14 in for riders 5'0" to 5'4", 16–18 in for 5'7" to 5'10", and 20–22 in at 6'1" and up. For a faster lookup use the bike size calculator.

Why a 56 cm is not always a 56 cm

The labels do not line up between makers. BikeRadar notes that road frames are mainly sized by seat tube length in centimetres, some brands use S, M and L, and Colnago and Pinarello have their own systems. Mountain frames are usually in inches, often with T-shirt sizes, and the frame numbers are much smaller than road numbers, so the two should not be compared. Brands that sell both may switch conventions between them.

GearJunkie states it plainly: there is no industry standard for measuring a bike or fitting it to the rider. A 56 cm from one brand may not match a 56 cm from another. Cycling UK adds that seat tube length alone is a weak guide on modern bikes, because a 51 cm and a 54 cm frame can both fit the same person.

Standover: the clearance check

Standover is the height of the top tube where you straddle it. Cycling UK suggests a gap of a couple of centimetres or more when you stand over the top tube. REI is more specific for mountain bikes: at least 2 inches of clearance between the tyres and the ground when you lift the bike, and 3 to 5 inches for aggressive riders, with cycling shoes on. REI also gives an online check: compare the listed standover height with your inseam. Its example pairs a 30 inch inseam with a standover height around 28 inches.

Standover has limits as a test. REI points out that top tube slope changes it a great deal, so it cannot confirm fit by itself. GearJunkie says modern bikes usually have plenty of standover at each size, and that it matters most on mountain and commuter bikes.

Reach and stack: the numbers that describe the frame

Reach is the horizontal distance from the centre of the bottom bracket to the top of the head tube. Stack is the vertical distance between the same two points (road.cc defines both this way, and GearJunkie gives the same definitions). Neither depends on the seat tube, the saddle or the stem, so they describe the frame alone.

Road.cc compares two Specialized road bikes in the 56 cm size. The Tarmac has a stack of 565 mm and a reach of 395 mm. The Roubaix has a stack of 605 mm and a reach of 384 mm. The Roubaix is taller and shorter, so you sit more upright on it. A rider who is comfortable on one could pick a different size label on the other to get a similar position.

Which one matters more depends on your build. GearJunkie says a longer torso or longer arms suit a longer reach, while longer legs relative to your torso suit a taller stack. It adds that road and gravel bikes are more sensitive to reach, and mountain bikes less so because riders move around on them.

Two older terms still show up. Effective top tube is the horizontal distance between the head tube and the seat tube, which REI says indicates how long the bike feels when you are seated; reach indicates how long it feels standing. BikeRadar notes that road charts use effective top tube in centimetres, and that studying the geometry chart matters more for gravel and mountain bikes than for road bikes.

A worked example

Take a rider who is 5'9" (175 cm) with a 31 inch (79 cm) inseam.

• Road Bike Rider: 5'7" to 5'10" and a 29 to 31 inch inseam is a medium, a 54 to 56 cm road frame.
• Trek's road chart: 168–174 cm is a 54, and 174–180 cm is a 56. At 175 cm the rider sits in the 56 band. The 79 cm inseam falls in the overlap of the 52 (76–79) and the 54 (78–82).
• BikeRadar: 170–178 cm is a 54.

The sources agree on 54 to 56. The inseam leans small, so this rider would try a 54 first and ask about the reach, then sit on a 56 if the 54 feels short.

Cycling UK offers a different shortcut when a brand sells several sizes. The medium in a range is the middle size; for 52, 54, 56 and 58 cm that is 55 cm. Take half of the difference between your height and the average height of the range, and add it to or subtract it from the medium. Its example: a rider of 182 cm on a range with a 51 cm medium takes the 54 cm (51 plus half of 6).

Between two sizes

BikeRadar gives the common advice: take the smaller of the two. Saddle height, saddle setback and handlebar height can all be adjusted. Road Bike Rider explains why: a frame that is a bit small can be stretched with a longer stem, while a frame that is too large is hard to correct. REI agrees for mountain bikes: the right frame is the foundation, and a wrong size is hard to fix.

There are exceptions, and the sources give them.

• Long torso for your height: go larger so the reach is long enough (Cycling UK).
• Long legs for your height: go smaller and raise the seatpost (Cycling UK).
• Comfort-focused endurance riding often favours the larger size, a racing position the smaller one (Road Bike Rider).
• Gravel, where the geometry is more upright: riders between sizes often prefer the smaller frame (Road Bike Rider).

After you pick: saddle height and a test ride

Cycling UK's saddle check: put your heels on the pedals and pedal backwards until the cranks are vertical, and your leg should be just straight. Riding on the ball of your foot then leaves a slight knee bend. REI gives the same idea for mountain bikes as 80 to 90 percent of full leg extension at the bottom of the stroke. If you have to raise the post above its minimum insertion mark to get there, REI says a different frame size may be needed.

For reach on drop bars, Cycling UK has a posture test. Sit on the stationary bike, take your hands off the bars and hold them loosely behind your back. If you cannot keep your posture, the bar is probably too far or too low for you. Swapping stems moves the bar several centimetres fore and aft, and sliding the saddle adds a couple more.

All the sources end the same way: a chart gets you into the neighbourhood, and a test ride at a shop settles the size. REI recommends riding several bikes for immediate feedback. Related reading: how a bike helmet should fit and, for a younger rider, the kids bike size guide.

Related tools

Bike size chartRoad, gravel and mountain frame size from height and inseam Kids bike size chartWheel size from inseam, height and age Bike helmet size chartHelmet size from head circumference