Bike fit methodology: formulas and sources
Updated
This page collects every formula and coefficient FitInspector’s tools use, and where each one comes from. Where a number is our own engineering judgment rather than a published source, we say so.
The results are a starting point, not a replacement for a professional bike fit. Change your position in small steps and check how it feels.
Saddle height
The bike fit calculator starts from the LeMond/Hinault formula:
saddle height (mm) = inseam (mm) × 0.883Discipline offsets
| Discipline | Offset |
|---|---|
| Road | 0 mm |
| Gravel | −2 mm |
| XC mountain bike | −5 mm |
| Touring | −5 mm |
| Cyclocross | −6 mm |
| Time trial / triathlon | +8 mm |
Crank length correction
The base formula assumes 170 mm cranks. If you enter your crank length, the saddle height changes by the difference, millimetre for millimetre, and the result is rounded to the nearest millimetre:
correction (mm) = 170 − crank length (mm)Kids’ bikes
kids’ saddle height (mm) = inseam (mm) × 0.883 × 0.85Knee angle at the bottom of the pedal stroke
The position check measures knee flexion (0° = a straight leg) with the crank at 6 o’clock and compares it with the range that matches how you captured your position:
| Capture | Target | Source |
|---|---|---|
| Photo, pedal held at the bottom (static) | 25–35° | Holmes, Pruitt & Whalen (1994), as cited |
| Video or several photos while pedalling (dynamic) | 30–40° | Ferrer-Roca et al. (2012) |
Road position: handlebar drop and saddle to hoods
For road bikes the calculator adds two of Dan Empfield’s published norms. D = saddle height, H = rider height.
handlebar drop = D × 0.11saddle nose to hoods = 0.72 × H − 0.78 × DTime trial and triathlon starting position
From Dan Empfield’s static tri-fit system. D = saddle height, H = rider height.
- Seat angle: 79.5° (bottom bracket to saddle rail centre).
- Cockpit length, saddle nose to the end of the extensions: 0.72 × H − 0.67 × D.
- Saddle nose to armrest: 0.58 × cockpit length.
- Armrest drop (cm): 0.005 × D² + 0.0075 × seat angle × D − 0.7775 × D, with D in cm; most riders fall within ±1.5 cm.
Saddle setback (approximate)
The bike fit calculator also estimates the saddle setback: from the centre of the bottom bracket to the centre of the saddle (not to the saddle nose), measured horizontally — the point where the seat-tube line meets the top of the saddle:
setback (mm) ≈ saddle height (mm) × cos(seat-tube angle)| Discipline | Angle |
|---|---|
| Road | 73.5° |
| Gravel | 73.5° |
| Cyclocross | 73.5° |
| Touring | 73° |
| XC mountain bike | 75° |
| Time trial / triathlon | 79.5° |
Handlebar height in spacers and stem angle
When the position check has a wheel in the photo to set the scale and your torso angle is outside its range, it estimates how far to move the bars (torso length measured on the photo × the change in the sine of the torso angle) and converts that height into parts:
spacers ≈ height change ÷ sin(head-tube angle)stem height change = L × (sin(α′) − sin(α)), where α = stem angle + (90° − head-tube angle)Example: raising the bars by 20 mm is about 20 mm of spacers, or flipping the stem from −6° to +6°, which gives about 20 mm.
Frame size charts
The size guide and the calculator’s frame size use general industry size ranges by height and inseam from The Bike Shoppe’s bike size guide, not a specific brand’s chart.
- Road: road chart. Gravel: same sizing as road in the source chart.
- Cyclocross: road chart — cyclocross frames are commonly sized like road frames.
- XC mountain bike: mountain bike chart. Kids: sized by wheel size. The size guide also shows a hybrid chart.
- TT/triathlon and touring: no reliable chart, so no size is shown.
Tire pressure
The tire pressure calculator adds your weight and the bike’s weight (8 kg if left empty) and splits the total 45% front, 55% rear. For each wheel:
pressure (bar) = wheel load (kg) × K ÷ tire width (mm)| Surface | Clincher | Tubeless | Tubular |
|---|---|---|---|
| Road | 3.7 | 3.1 | 3.9 |
| Gravel | 2.5 | 2.1 | 2.5 |
| MTB | 2.2 | 1.9 | 2.2 |
What the model doesn’t cover
- Differences in leg length between your left and right side.
- Hamstring and hip flexibility, shoe and cleat stack height, and saddle shape — each can move your best saddle height by a few millimetres.
- Pain or injury. The tools give fit guidance, not a medical diagnosis; if something hurts, see a professional.
Who builds FitInspector and how we approach accuracy: about FitInspector.
Sources
- LeMond G, Gordis K. Greg LeMond’s Complete Book of Bicycling. New York: Putnam; 1987.
- Holmes JC, Pruitt AL, Whalen NJ. Lower extremity overuse in bicycling. Clin Sports Med. 1994;13(1):187–205. (25–35° range as cited in later literature; full text not verified.) Link
- Ferrer-Roca V, Roig A, Galilea P, García-López J. Influence of saddle height on lower limb kinematics in well-trained cyclists: static vs. dynamic evaluation in bike fitting. J Strength Cond Res. 2012;26(11):3025–3029. Link
- Empfield D. Coordinate Predictors Versus Body Angles in Road Bike Fit. Slowtwitch; 2012-03-13.
- Empfield D. Road Bike Fit. Slowtwitch; 2012-02-09.
- Empfield D. Devising a static tri-fit system. Slowtwitch; 2009-06-29.
- The Bike Shoppe, bike size guide (thebikeshoppe.com) — general size ranges used in the size charts.
- Crank length correction: practice described by myvelofit.com, bikeradar.com, bikedynamics.co.uk and applemanbicycles.com.