What Size Bike Do You Need?
Inseam sets the frame size. Height fills in when you have not measured it yet.
Your Size
| Letter size | Frame (cm) | Rider height |
|---|
Saddle height is measured from the center of the bottom bracket to the top of the saddle along the seat tube (LeMond method, inseam x 0.883). Frame sizes are traditional seat tube lengths; confirm with the manufacturer's chart for the model you want.
How It Works
Legs size the frame. Height alone gets you close; inseam gets you there.
The Formulas
Road, gravel, cyclocross: frame (cm) = inseam (cm) x 0.70 Hybrid, city, e-bike: frame (cm) = inseam (cm) x 0.685 Mountain bike: frame (in) = inseam (in) x 0.59 Inseam estimate (no measurement): inseam = height x 0.46 Saddle height (cm) = inseam (cm) x 0.883The frame multipliers are the shop-floor rules of thumb that bike fitters have used for decades, and the ones most online size calculators quote: 0.70 for drop-bar road bikes, a little less for upright hybrids and e-bikes, and 0.59 for mountain bikes, whose frames are sized in inches and sit lower for standover clearance. The result is a traditional seat tube length, center of the bottom bracket to the top of the seat tube.
When you leave the inseam blank, the calculator estimates it as 46% of your height, which is the middle of the normal range. Two riders of the same height can differ by 2 inches of inseam, which moves the frame size by a full size, so measure before you order a bike you cannot test ride.
From Centimeters to Letters
Most brands now sell by letter size, and each brand draws its own lines. The chart in the results uses common boundaries: on a road bike, 54 to 56 cm is usually an M and 56 to 58 cm an L; on a mountain bike, 17 to 18 inches is an M and 19 to 20 inches an L. A result on a boundary means "try both," and the brand's chart for that model is the final word.
A Worked Example
A rider of 5 feet 10 inches with a measured 32.5 inch inseam (82.6 cm) gets a 57.8 cm road frame, a 56.5 cm hybrid, or a 19.2 inch mountain frame, with a saddle height of 72.9 cm. On most road charts that is a 56 cm or 58 cm bike, L on a borderline, and the choice comes down to torso length and how much reach the rider wants. On a mountain bike it sits on the line between M and L, so the test ride decides.
Why Fit Matters Beyond Comfort
A frame a size too big puts your weight too far forward and lengthens every reach for the brake; a size too small cramps the knees and loads the hands. Both make a bike harder to control at the moment you need it most. Get the size right, then protect the bike: a new bike is covered against theft, crash damage, and transit loss at stated value on a road bike, mountain bike, or e-bike policy from the day it arrives.
Frequently Asked Questions
- How do I measure my inseam for bike sizing?
- Stand against a wall in socks with your feet about 6 inches apart, put a hardcover book between your legs with its spine pressed firmly up into your crotch, and measure from the top of the book to the floor. Measure twice. The number is your cycling inseam, which runs a little longer than a trouser inseam, and it is the measurement that sets the frame size and the saddle height.
- What should I do if I am between two frame sizes?
- Pick the smaller frame if you want a nimbler bike or have a shorter torso and arms, and the larger frame if you have a long torso or want a stretched, stable position. The smaller frame is the easier one to adjust, because a longer stem and more seatpost extend it, while a frame that is too big cannot be shrunk. A test ride on both settles most cases.
- Why do manufacturers' size charts disagree with the calculator?
- The calculator gives a traditional seat tube length, and brands measure frames differently: some by seat tube, some by top tube, many by letter size only. Geometry also differs between a race road bike and an endurance one of the same nominal size. Use the result to land in the right size range, then check the brand's own chart and, if you can, the stack and reach numbers for the model you want.
- How do I set my saddle height from the calculator's number?
- Measure from the center of the bottom bracket, along the seat tube, to the top of the saddle, and set that distance to the saddle height shown. The figure uses the LeMond method, inseam multiplied by 0.883, which puts most riders within a centimeter of a good position. Fine-tune from there: a slight bend in the knee at the bottom of the stroke, hips steady, no reaching for the pedal.
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