What Size?

Bicycles come in multiple different frame sizes, wheel sizes, and frame styles. 
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Manufacturers have made buying mountain bikes which are the correct size relatively straight forward. Most manufacturers provide a sizing and geometry chart for each specific model. Sizing and geometry charts are sufficient for the vast majority of riders to pick the right size bike for them.

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The two key frame measurements on the geometry chart are seat tube length and top tube length.

Seat tube length — This can affect how much stand-over clearance there is between you and the bike. This is important for new riders as you may well need to stop and get your feet on firm ground quickly. However, some bikes — such as those with longer suspension travel — can have zero standover clearance, and still be the correct size.
Top tube length — This determines the comfort, fit and handling of your bike to a large extent. Ideally, you should aim not to feel either cramped or stretched.

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    These charts are a great tool to determine a good fit.

     
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    Kids Bikes:
     
     Child height (Inch)
    Child height (cm)
    Wheel size
    2’9”-3’1″
    85-90 cm cm 10/12 inches
    3’1”-3’3″ 90-100 cm 12/14 inches
    3’3”-3’7″ 100-110 cm 14/16 inches
    3'7"-3'8" 110-115 cm 16/18 inches
    3’8”-4’0″ 115-120 cm 18/20 inches
    4'0”-4’5″ 120-135 cm 20/24 inches
    4'5"-4'9" 135-145 cm 24/26 inches
    5 inches+ 145 cm + 26 inches
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    Mountain Bikes:
    Adult height (Inch)
    Frame Size (Inches) Frame size
    4'10-5'2" 13 - 14" X-Small
    5'2-5'6" 15 - 16" Small
    5'6-5'10" 17 - 18" Medium
    5'10-6'1" 19 - 20" Large
    6'1-6'4" 21 - 22" X-Large
    6'4-6'6" 22 - 24" XX-Large
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    Road Bikes:
    Adult height (Inch)
    Frame Size (Inches) Frame size
    4'10-5'2" 47-48 cm XX-Small
    5'2-5'6" 49 - 50 cm X-Small
    5'3-5'6" 51 - 53 cm Small
    5'6-5'9" 54 - 55 cm Medium
    5'9-6'0" 56 - 58 cm Large
    6'0-6'3" 58 - 60 cm X-Large
    6'3-6'6" 61 - 63 cm XX-Large