How To Measure Mountain Bike Size

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Not only will a good mountain bike be easier and more pleasant to ride, but also faster and more efficient. Our detailed MTB bike sizing guide puts you on the right track to get the proper mountain bike, every first time. It is very important to know the right size bike that will fit you.

What exactly should be measured for the right bike size? (Inseam Measurement)

The two most important measurements to take when sizing a mountain bike are your height and inside leg circumference (leg inseam). Remove your shoes and stand up straight against a wall, making a small pencil line on the wall level with the top of your head, and then measuring the height of the mark off the ground using a measuring tape is a simple method of determining your height. This is correct way to measure a mountain bike size.

To take a measurement of your inside leg, stand with your feet approximately shoulder width apart and your shoes off, then measure from the ground up into your groin area. You can also use a pencil to make a mark on the wall where your inside leg will be and measure it afterwards.

Your inside leg measurement is taken in order to ensure that the standover height of your road bike is sufficient. When riding a bicycle, the standover height is defined as the distance between the top tube and the ground. There should be at least 1 inch of clearance between the top tube and your inside leg, according to the manufacturer.

Bike Sizing Chart For Adult’s

Rider Height (in)Rider Height (cm)Frame SizeFrame Size (in)Frame Size (cm)
4′ 11″ – 5′ 3″148cm – 157cmXS13″ – 15″33cm – 38cm
5′ 3″ – 5′ 4″158cm – 168cmS15″ – 17″38cm – 43cm
5′ 8″ – 5′ 9″169cm – 179cmM17″ – 19″43cm – 48cm
5′ 10″ – 6′ 2″180cm – 185cmL19″ – 21″48cm – 53cm
6′ 3″ – 6′ 5″186cm – 194cmXL21″ – 23″53cm – 58cm
6′ 5″ +195cm +XXL24″ +59cm +
Just calculate by your own preference

Bike Sizing Chart For Kid’s

Bike WheelChild Height (in.)Approx. AgeInseam (inside leg length)
11-inch30-372-412-17
13-inch38-433-516-20
15-inch44-515-818-22
19-inch51-576-1021-25
23-inch58-628-1224-28
25-inch63+10+ 
Just calculate by your Kids preference

What you’ll need to measure a bike frame

  • A tape measure
  • A clinometer to measure angles 
  • A long spirit level 
  • A plumb line

However, some manufactures still size mountain bikes in inches. Moots, for example, combines the two!

To keep things constant, stick to metric units. If you must, multiply centimetres by 2.54 to get inches.

A maker’s website will usually have a geometry chart for all sizes of a current frameset. If your road bike is still new, make a copy of this since it will be more exact than your measurements and likely a useful reference.

Measuring Top Tube Length

They used to be horizontal. Many hybrid bike now feature a sloped top tube.

A new bike geometry table usually includes the true top-tube length. In many geo charts, the horizontal top tube length is referred to as the effective top tube length or virtual top tube.

It’s the horizontal space between the head tube and the seatpost. To accurately measure it, use a spirit level or the clinometer app.

Many road bikes are sized by top tube length. Mountain bikes, on the other hand, have frames designated S, M, L, etc. Some road bikes, like Merida’s, employ the same system: S/M, M/L, and L.

Ridley’s size S frames have a top tube of 54cm, which is similar to several brands’ size medium frames.

Notably, not all brands measure virtual top tube length the same way.

Colnago, for example, measures the horizontal distance from the head tube to a vertical projected up from the top of the seat tube, therefore its values are smaller than others. A 54cm top tube is a 50s Colnago.

Measuring Seat Tube Length

Seat tube length is the distance between the bottom bracket and the seat tube.

In some cases, like the Trek Madone, the seat tube extends well over the top tube junction, while others employ a seatmast, making it difficult to compare dimensions.

Plus, mountain bikes often have a kink in the seat tube, so you don’t want to follow the longer tube’s route.

If you’re unsure, align your straight edge with the bottom bracket centre and the top of the seat tube.

Measuring reach and stack

We’ve observed that comparing frame top tube and seat tube lengths can be tricky. Most road bike manufactures now show reach and stack values for uniformity.

Less prone to frame design, these gauge the distance between two critical contact points: the bottom bracket and the head tube.

The reach is the distance between the two. You’ll need your spirit level to measure it.

Fix a plumb line to the level’s end. The glob at the end of your string should be reasonably symmetrical, and your string should hang straight down, else your measurement may be incorrect.

Align the level’s top edge with the head tube’s top centreline. Then move the level until the plumb line touches the bottom bracket spindle. Your reach is the distance between the top of the plumb line and the head tube.

Another method is to lean your bike fit against a wall and measure the distance between the top of the head tube and the bottom bracket. However, you must ensure that your measurement is horizontal.

Between the bottom bracket and the head tube lies the stack. Following the plumb line, you should be able to measure your stack once your reach is recorded.

Alternatively, measure the height of your bottom bracket from the ground and deduct it.

The plumb line approach requires two hands and is worth repeating to maintain consistency.

Measuring wheelbase

The wheelbase of your frame is the distance between the axles. It determines a frame’s ride quality and varies with frame size.

It’s simple to measure, but you must point the fork straight ahead to get an accurate reading.

Like reach and stack, it’s worth measuring numerous times to ensure the same result. To improve accuracy, measure the wheel size on both sides of the road bike fit and average it. This will correct for a crooked fork.

Measuring Chainstay Length

Chainstay length is one of two components that make up your wheel size and affects your frame’s handling. A frame with shorter chainstays feels more dynamic than one with longer stays.

The chainstay inseam length is the straight line distance between the bottom bracket axle and the rear dropout.

Front Center Measuring

The front center is also part of the wheelbase. Axle to front dropout is the same as chainstay inseam length.

It impacts handling and front wheel toe overlap. BMC, for example, shows it on its geometry charts.

This is because neither the chainstay inseam length nor the front center are measured horizontally.

Measuring Seat tube and head tube angles

For better handling, more upright seat and head tube angles are preferred. Use your clinometer app here.

If you have a straight seat tube, you can use the clinometer app on your smartphone to determine the seat tube angle. To get an accurate reading, your mountain bike fit must be vertical and on a flat surface.

If your seat tube is kinked, use a straight edge to trace the line between the bottom bracket shell and the top of the seat tube.

Most modern motorcycles have tapered head tubes, thus the front of the head tube is not parallel to the centreline.

Hold your phone at an angle to the centreline, or use a straight edge to align the top and bottom of the head tube.

If your fork legs are straight with no angle at the crown, the leg angle will be the same as the head tube angle. Again, the mountain bike frame must be vertical.

To measure head tube angle, align the clinometer’s steerer extension with the head tube.

Measuring BBR drop

The bottom bracket drop is the distance between the wheel axles and the crank axle centreline

To find it, subtract the rear axle’s height from the bottom bracket’s height. It’s another crucial metric on hybrid bike manufactures’ geo charts.

Measuring the Bottom Bracket

Finally, the bottom bracket height is measured from the ground to the shell’s center. So that’s easy to measure, but keep your gravel bike straight for an accurate reading.

Unlike BCD, it is little influenced by tyre pressure, so pump them to your typical running pressure.

Now you have all the measurements to size your frame. Keep your numbers secure; you don’t want to repeat the operation.

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