Bike Size Calculator

Bike size is the frame size that lets you stand over the bike, reach the bars and pedal with a slight bend in your knee. For adults it is a number (such as 54 cm) or a letter (S, M, L). For kids it is the wheel size. This calculator works it out from your inseam first, and from your height if you have no inseam.

What Bike Size Measures

A frame size is traditionally the length of the seat tube, which runs from the pedals up to the saddle⁶. It tells you roughly how tall the frame is, not how long it is, so two bikes of the same size can still fit differently. Your inseam (inside leg) matters more than your height, because it sets how much room you have over the top tube¹.

The Formula

The calculator uses Ergon's published inseam multipliers⁴:

Road and gravel: frame size (cm) = inseam (cm) x 0.665

Hybrid or city: frame size (cm) = inseam (cm) x 0.66

Mountain: frame size (in) = inseam (cm) x 0.226

Standover target = inseam minus 1 or 2 in (2.5 or 5 cm), by bike type

Ergon calls these guidelines that vary with frame design⁴, and Gazelle uses 0.68 for its city bikes⁵. The standover clearances come from REI¹ ².

How This Calculator Computes Your Result

Choose a bike type, then enter your inseam, your height, or both, in cm or in feet and inches. Adults get three results:

• Size on a typical size chart, from Trek's published charts for that bike type³. If inseam and height point to different sizes, the inseam size comes first.

• Seat tube length by formula, your inseam times the multiplier above, rounded to the nearest cm (half inch for mountain bikes).

• Standover target, the highest standover height to look for on a brand's geometry chart.

For kids, you also choose a riding stage, and get likely wheel sizes, a seat height target and a standover target. The formula, seat height and standover results need an inseam.

What Your Result Means

These are Trek's road sizes, which Trek also uses for its gravel bikes³.

Mountain and hybrid bikes use letter sizes³:

The rows overlap because arm and leg length also decide fit³. For example, an 81 cm inseam fits a 54 or 56 road frame, and a height of 178 cm fits a 56, so the calculator shows 56 with 54 as a second option. The formula gives 81 x 0.665 = 54 cm. A one size gap like this is normal, because brands measure frames in different ways.

How to Measure Correctly

Inseam. You need a hardback book, a wall, a pencil and a tape measure.

• Stand with your back to the wall in socks or your riding shoes, and use the same choice each time. Ergon and Gazelle measure without shoes⁴ ⁵, REI in bike shoes¹.

• Put your feet about 20 cm (8 in) apart⁵.

• Pull the book up firmly between your legs, spine up, as if sitting on a saddle¹.

• Mark the wall at the top of the spine and measure from the floor to the mark. Repeat once or twice¹.

Standover. Straddle the bike and lift it until the top tube touches you. REI suggests about 1 in (2.5 cm) of gap under the tires on a road bike with a level top tube, and 2 in (5 cm) or more if it slopes¹. Mountain bikes need at least 2 in, and aggressive riders often want 3 to 5 in². Step-through and cruiser frames do not need this check¹.

Why Brands Measure Frames Differently

Some makers measure the seat tube from the crank axle (the bottom bracket) to the top of the tube, others to the center of the top tube, and sloping frames often get a "virtual" size⁶. Sheldon Brown called seat tube sizes "nearly meaningless unless you know how they are measured"⁶. Many modern bikes are sold as S, M or L and fitted by stack and reach instead. Stack is the height from the crank axle to the top of the head tube, and reach is the horizontal distance between them⁷. These set how high and how far away the handlebars are, so check your result against the brand's own chart.

Kids Bike Sizes

Kids bikes are sized by wheel. Trek's chart goes by height³, and Two Wheeling Tots goes by inseam, which it says matters more than age or height⁸.

The seat height to aim for is about equal to the inseam for a new rider, up to 3 in above it with training wheels, and 2 to 4 in above it for a confident rider⁸. Pick a bike whose lowest seat setting is close to, but not above, that number⁸.

What to Do Between Sizes

Think about your reach. Riders with long arms and torso for their height often prefer the larger frame, and riders who like to sit upright often prefer the smaller one. For mountain bikes, REI advises leaning smaller, because a smaller frame is easier to adjust². A different stem length can fine tune the reach⁶. Then set the saddle so your leg reaches about 80 to 90 percent of full extension¹, and test ride both sizes if you can.