Bike size calculator

Your frame size comes mainly from your inseam: for a road or gravel bike, inseam (cm) × 0.665 gives the classic frame size in cm; for a mountain bike, inseam (cm) × 0.226 gives the size in inches. Someone 178 cm tall with an 84 cm inseam is usually a 56 road frame or a medium to large MTB.

Calculator

Always check the manufacturer's own size chart before buying. Sizing varies between brands: one brand's L can be another brand's M.

Bike

Height

Inseam

Stand barefoot against a wall, a book pressed up between your legs; measure floor to the top of the book.

Frame size (inseam method)

55.9cm

By height (178 cm): size L, usually 54 to 56 cm.

Height	Size	Frame
under 160 cm	XS	47–50 cm
160 cm to 167 cm	S	50–52 cm
167 cm to 175 cm	M	52–54 cm
175 cm to 183 cm	L	54–56 cm
183 cm to 190 cm	XL	56–58 cm
190 cm+	XXL	58–61 cm

Road: inseam × 0.665 (cm). MTB: inseam (cm) × 0.226 (inches). Sizes vary between brands: always compare stack and reach on the brand's geometry chart.

How the bike size calculator works

Frame size is mostly about your legs, not your height. Two riders of the same height can have very different inseams, and the one with longer legs usually needs a taller frame. This calculator uses both: your inseam for the frame size, and your height for the size label most brands use. It is one of the free LeCoach cycling calculators.

The inseam formulas are retailer rules of thumb:

Road and gravel: frame size (cm) = inseam (cm) × 0.665
Mountain bike: frame size (inches) = inseam (cm) × 0.226

Road frames are traditionally measured in centimetres, mountain bikes in inches, and most brands now also use S, M, L and XL.

Worked example. You are 178 cm tall with an 84 cm inseam. For a road or gravel bike, 84 × 0.665 = 55.9 cm, so you are looking at a 56 frame. By height, 178 cm is a size L, usually 54 to 56 cm. The two agree. For a mountain bike, 84 × 0.226 = 19.0 inches. By height you are a size M (17 to 18 inches), so you sit between M and L.

How to measure your inseam

Your inseam for bike sizing is not your trouser leg length. It is longer, because it goes all the way up to the pelvis.

Stand barefoot against a wall, feet slightly apart.
Press a hardback book up between your legs, spine up, as firmly as a saddle would sit.
Mark the top of the spine on the wall, keeping the book level.
Measure from the floor to the mark. Repeat and average.

Measured this way, your inseam is usually longer than the inside leg on your trousers. An error of 1 cm changes the road result by less than 7 mm, so you do not need perfection, but you do need the real number. The same measurement sets your saddle height in the saddle height calculator.

Frame size by inseam

Inseam	Road and gravel frame	Mountain bike frame
72 cm	47.9 cm	16.3 in
76 cm	50.5 cm	17.2 in
80 cm	53.2 cm	18.1 in
84 cm	55.9 cm	19.0 in
88 cm	58.5 cm	19.9 in
92 cm	61.2 cm	20.8 in

Want this worked out for you every day? LeCoach builds your training around your own numbers.

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Frame size by height

Use these as a cross-check. They describe an average body, and yours may not be average.

Height	Road and gravel size	Frame
Under 160 cm (5'3")	XS	47–50 cm
160–167 cm (5'3" to 5'6")	S	50–52 cm
167–175 cm (5'6" to 5'9")	M	52–54 cm
175–183 cm (5'9" to 6'0")	L	54–56 cm
183–190 cm (6'0" to 6'3")	XL	56–58 cm
190 cm and up	XXL	58–61 cm

Height	MTB size	Frame
Under 160 cm (5'3")	XS	13–14 in
160–170 cm (5'3" to 5'7")	S	15–16 in
170–180 cm (5'7" to 5'11")	M	17–18 in
180–190 cm (5'11" to 6'3")	L	19–20 in
190 cm and up	XL	21–22 in

If the inseam and height results disagree by more than one size, your legs are longer or shorter than average for your height. Trust the inseam result, then check stack and reach. A rider 178 cm tall with a 90 cm inseam gets 59.9 cm from the inseam method but L from the height chart: a sign to look at taller frames.

Check stack and reach

Frame sizes are not standard between brands. One brand's 56 can fit like another's 54. The two numbers that let you compare any frames are on the brand's geometry chart:

Stack: the vertical distance from the centre of the bottom bracket to the top of the head tube. More stack means a taller front end and a more upright position.
Reach: the horizontal distance between the same two points. More reach means a longer frame and a more stretched position.

If you already have a bike that fits well, compare its stack and reach with the new one. That is more reliable than any size label. Gravel bikes often have more stack and less reach than road bikes of the same size, for a more upright, stable position.

Common mistakes

Using your trouser inseam. It is too short, and gives a frame that is too small.
Trusting the size label across brands. Always compare stack and reach.
Sizing up to fit a bike you found on sale. A frame that is too big cannot be made smaller; a slightly small one can often be adjusted with stem and seatpost.
Ignoring flexibility. Stiff back and hamstrings? Choose more stack.
Sizing an MTB by wheel size. 29er frames come in all sizes.

When to think again about size

You are between sizes. Choose the smaller frame for a sportier, more agile ride, the larger for comfort and stability, and confirm with stack and reach.
Your riding changes. Racing tends to favour a lower, longer position than long-distance comfort riding.
Pain or numbness. If knees, back, neck or hands hurt on longer rides, check saddle height first, then consider a professional bike fit.

New to the sport? Our guides to cycling training for beginners and the beginner cycling training plan cover the first months on the bike, and the mountain bike training plan does the same off-road. Once your bike fits, LeCoach builds the training plan with every workout written, and suggests changes, with the reason, when your recovery or week calls for it.

Sources

Retailer frame sizing guidance: road frame = inseam × 0.665; MTB frame (inches) = inseam (cm) × 0.226.

Frequently asked questions

What size bike do I need for my height?

For road bikes, about 175 to 183 cm is usually a size L (54 to 56 cm), 167 to 175 cm a size M (52 to 54 cm) and 183 to 190 cm an XL (56 to 58 cm). Height is only a starting point: your inseam gives a more personal answer, and stack and reach decide the final fit.

How do I calculate my road bike frame size?

Multiply your inseam in centimetres by 0.665. An 84 cm inseam gives 55.9 cm, so a 56 frame. Measure your inseam barefoot with a book pressed up between your legs, not from your trousers. Then compare the brand's size chart and stack and reach figures, because sizes vary between brands.

How do I calculate my mountain bike frame size?

Multiply your inseam in centimetres by 0.226 to get the frame size in inches. An 84 cm inseam gives 19.0 inches. Most brands now use S, M, L and XL, so compare that number with their size chart. For a rider 178 cm tall, it sits between a size M and an L.

What if I am between two sizes?

Choose the smaller frame if you want a sportier, more agile bike and are flexible enough for a lower position. Choose the larger one for more stability and an upright position. Then compare stack and reach: the frame closest to a bike you already find comfortable is usually the right one.

Are gravel bike sizes the same as road bike sizes?

Gravel bikes use the same sizing approach, so the inseam × 0.665 formula works as a starting point. Many gravel frames have more stack and slightly less reach than road frames of the same size, for a more upright position on rough ground. Always check the geometry chart rather than assume.

What are stack and reach?

Stack is the vertical distance from the centre of the bottom bracket to the top of the head tube. Reach is the horizontal distance between those two points. Together they describe how tall and how long a frame is, and they let you compare frames from different brands far more reliably than size labels.