Bike Fit & Size Calculator
Saddle height starts from your inseam: 0.883 × inseam, measured from the center of the crank along the seat tube to the top of the saddle — 70.6 cm for an 80 cm (31.5 in) inseam. Then check that your knee still bends 25–35° at the bottom of the pedal stroke. Frame sizes aren’t standard between brands, and kids’ bikes should be sized by inseam, not age.
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Your measurements
Your saddle height
Center of crank → top of saddle
71.8cm
28.3 in · LeMond
Pedal axle → top of saddle
88.6cm
34.9 in · Hamley & Thomas
Start at 71.8 cm from the center of the crank, along the seat tube, to the top of the saddle
How this was calculated
Never apply 0.883 to a pedal-axle measurement or 1.09 to a crank-center one. Studies found the 109% rule missed the 25–35° knee range most of the time, which is why the knee check matters more than either formula.
Worked example
An 80 cm (31.5 in) inseam, 170 mm cranks, flat pedals
LeMond: 0.883 × 80 = 70.6 cm from the center of the crank to the top of the saddle. Hamley & Thomas: 1.09 × 80 = 87.2 cm from the pedal axle, with the crank pointing down in line with the seat tube. Subtract the 17.0 cm crank and it’s 70.2 cm from the crank center — 4 mm from LeMond’s answer.
Set the saddle at about 70.6 cm, ride, and check the knee angle at the bottom of the stroke. Research on the 109% rule found it put riders outside the 25–35° range more often than not, so the knee check is the real test; the formula gets you close.
Reference
Bike size charts
Saddle heights are calculated; everything else is the maker’s own published chart, as read on September 21, 2026.
Saddle height by inseam (flat pedals)
| Inseam | Crank center → saddle top (LeMond) | Pedal axle → saddle top (Hamley 109%) |
|---|---|---|
| 26 in (66 cm) | 58.3 cm (23.0 in) | 72.0 cm (28.3 in) |
| 28 in (71 cm) | 62.8 cm (24.7 in) | 77.5 cm (30.5 in) |
| 29 in (74 cm) | 65.0 cm (25.6 in) | 80.3 cm (31.6 in) |
| 30 in (76 cm) | 67.3 cm (26.5 in) | 83.1 cm (32.7 in) |
| 31 in (79 cm) | 69.5 cm (27.4 in) | 85.8 cm (33.8 in) |
| 32 in (81 cm) | 71.8 cm (28.3 in) | 88.6 cm (34.9 in) |
| 33 in (84 cm) | 74.0 cm (29.1 in) | 91.4 cm (36.0 in) |
| 34 in (86 cm) | 76.3 cm (30.0 in) | 94.1 cm (37.1 in) |
| 35 in (89 cm) | 78.5 cm (30.9 in) | 96.9 cm (38.2 in) |
| 36 in (91 cm) | 80.7 cm (31.8 in) | 99.7 cm (39.2 in) |
| 38 in (97 cm) | 85.2 cm (33.6 in) | 105.2 cm (41.4 in) |
Hybrid: Trek FX 2 Gen 4
| Size | Frame | Height | Inseam |
|---|---|---|---|
| XS | 40 cm | 4'10"–5'1" | 27–29 in |
| S | 45 cm | 5'1"–5'5" | 28–31 in |
| M | 50 cm | 5'5"–5'9" | 30–33 in |
| L | 55 cm | 5'9"–6'1" | 32–35 in |
| XL | 60 cm | 6'1"–6'6" | 34–37 in |
Mountain: Trek Marlin 5 Gen 3
| Size | Frame | Height | Inseam |
|---|---|---|---|
| XS | 14.5 in, 27.5 wheels | 4'9"–5'1" | 27–29 in |
| S | 15.5 in, 27.5 wheels | 5'1"–5'5" | 29–31 in |
| M | 17.5 in, 29 wheels | 5'5"–5'10" | 31–33 in |
| ML | 18.5 in, 29 wheels | 5'8"–5'11" | 32–34 in |
| L | 19.5 in, 29 wheels | 5'10"–6'2" | 33–35 in |
| XL | 21.5 in, 29 wheels | 6'2"–6'5" | 35–36 in |
E-bike rider heights (maker-published)
| E-bike | Size | Rider height |
|---|---|---|
| Lectric XP4 750 (folding) | One size | 4'10"–6'3" |
| Lectric XPedition2 (cargo) | One size | 4'11"–6'5" |
| Rad RadRunner | One size | 4'11"–6'2" |
| Rad RadWagon 5 | One size | 4'11"–6'3" |
| Aventon Abound SR | One size | 4'11"–6'3" |
| Aventon Level 3 | Regular | 5'3"–5'10" |
| Aventon Level 3 | Large | 5'10"–6'4" |
| Velotric Discover 3 | Regular | 5'2"–5'11" |
| Velotric Discover 3 | Large | 5'9"–6'7" |
| Super73 MZFT | One size | 4'10"–5'10" |
Kids’ bike size charts compared (inseam, in)
| Wheel | REI inseam · height · age | Trek inseam · height · age | woom inseam · height · age |
|---|---|---|---|
| 12 in | 14–17 · 2'10"–3'4" · 2–3 | 15–18 · 3'0"–3'3" · 2–3 | — |
| 14 in | 16–20 · 3'1"–3'7" · 3–4 | — | 16–19.1 · 3'1"–3'7" · 3–4.5 |
| 16 in | 18–22 · 3'7"–4'0" · 4–6 | 16–22 · 3'5"–4'0" · 4–6 | 19.1–25.2 · 3'5"–3'11" · 4–6 |
| 20 in | 22–25 · 4'0"–4'5" · 5–8 | 19–25 · 3'9"–4'6" · 5–8 | 22.1–27.9 · 3'9"–4'3" · 6–8 |
| 24 in | 24–28 · 4'5"–4'9" · 7–11 | 23–28 · 4'1"–4'11" · 8–11 | 26.4–32.3 · 4'1"–4'9" · 7–11 |
| 26 in | 28+ · 4'9"+ · 10+ | 25+ · 4'8"+ · 10–12 | 29.5–37.4 · 4'7"–5'5" · 10–14 |
Guardian’s seat-height rule: inseam plus 0 in for a first pedal bike, up to 2 in for a confident balance-bike graduate, and 2–3 in for a second bike.
How to use it
In 4 steps
- 1Measure your inseam with the book method: shoes on, back to a wall, a hardcover book pulled firmly up between your legs, spine up. Measure floor to spine twice.
- 2Saddle height: enter the inseam and your crank length (stamped on the crank arm). Measure along the seat tube from the center of the crank bolt to the top of the saddle.
- 3Frame size: enter your height and the kind of bike, then compare with the maker's own chart — sizes differ between brands.
- 4Kids: measure the inseam the same way and look for a bike whose lowest saddle height matches the number shown.
Why the formulas disagree — and what actually decides it
The two classic saddle-height formulas measure from different points. Greg LeMond’s 0.883 × inseam runs from the center of the crank to the top of the saddle. Hamley and Thomas’s 109% runs from the pedal axle with the crank pointing straight down the seat tube. Mixing them up — 0.883 from the pedal, or 109% from the crank — puts the saddle several centimeters off.
Both are starting points. The measure research supports is the knee angle: about 25–35° of bend at the bottom of the stroke, with a review by Bini and colleagues suggesting 25–30° to reduce the risk of knee injury. Studies testing the 109% rule found it landed outside that range most of the time, so set the saddle from the formula, then fine-tune by feel and by the knee.
On an e-bike the fit matters as much as on any bike — the motor helps, but a saddle that’s too low still loads the knee on every stroke. Many direct-to-consumer e-bikes come in one size covering riders from under 5 feet to over 6 feet; the saddle and bars do the fitting, which is why a proper setup visit is worth doing.
FAQ
Fit & size: questions
How high should my bike seat be?
Start at 0.883 × your inseam, measured from the center of the crank along the seat tube to the top of the saddle (the LeMond method), minus 3 mm with clipless pedals. Then ride and check that your knee still bends 25–35° when the pedal is at the bottom of the stroke.
What size bike do I need for my height?
It depends on the brand, because frame sizes aren't standard. On Trek's FX hybrid chart, 5'5"–5'9" is a medium and 5'9"–6'1" a large; on its Marlin mountain bike, 5'5"–5'10" is a medium. If you're between sizes on a mountain bike, REI suggests erring smaller.
Do e-bikes come in sizes?
Some do. Many direct-to-consumer e-bikes are one size with a wide range — the Rad RadRunner is listed for 4'11"–6'2" and the Lectric XP4 750 for 4'10"–6'3" — while commuter models like the Aventon Level 3 come in Regular (5'3"–5'10") and Large (5'10"–6'4").
What size bike does a 5-year-old need?
Measure the inseam rather than going by age. REI's chart puts 16-inch wheels at an 18–22 in inseam (typically ages 4–6) and 20-inch at 22–25 in (ages 5–8), but brands differ. Guardian's rule is to find a bike whose lowest seat height equals the inseam plus a little heel lift for a confident rider.
Can kids ride e-bikes in California?
State law bars anyone under 16 from operating a Class 3 e-bike (Vehicle Code §21213) and requires a helmet for every bicycle rider under 18, e-bikes included (§21212). Some cities add rules of their own.
Something not answered here? Just ask — I’d rather talk it through than have you guess.
Sources
Where these numbers come from
- [1]RoadBikeRider, “The LeMond formula” (2022) (opens in a new tab)0.883 × inseam, crank center to saddle top; −3 mm for clipless.
- [2]Cycling Math, saddle height methods (2013) (opens in a new tab)LeMond and Hamley & Thomas endpoints compared.
- [3]Hamley & Thomas 1967, J Physiol (opens in a new tab)109% of inseam, pedal axle to saddle.
- [4]Peveler, Pounders & Bishop 2007, JSCR (opens in a new tab)109% fell outside 25–35° knee flexion 63% of the time.
- [5]Peveler & Green 2011, JSCR (opens in a new tab)73% of the time; 25° more economical than 35°.
- [6]Bini, Hume & Croft 2011, Sports Medicine (opens in a new tab)Review: 25–30° knee flexion to reduce injury risk.
- [7]Holmes, Pruitt & Whalen 1994, Clin Sports Med (opens in a new tab)Knee-angle method (25–35° as cited in later literature).
- [8]REI Expert Advice — Bike fitting (opens in a new tab)Book-method inseam; standover guidance.
- [9]REI — Kids' bikes: how to choose (2025) (opens in a new tab)Wheel size by height, inseam and age.
- [10]Trek Kids Buyer's Guide (opens in a new tab)Wheel size by height, inseam and age.
- [11]woom sizing (opens in a new tab)Height and inseam ranges by model.
- [12]Guardian Bikes sizing guide (opens in a new tab)Seat height = inseam + heel lift; never size by age.
- [13]Trek FX 2 Gen 4 sizing (opens in a new tab)Hybrid frame sizes by height and inseam.
- [14]Trek Marlin 5 Gen 3 sizing (opens in a new tab)Mountain frame sizes by height and inseam.
- [15]Lectric XP4 750 (opens in a new tab)One size, 4'10"–6'3".
- [16]Aventon Level 3 (opens in a new tab)Regular 5'3"–5'10", Large 5'10"–6'4".
- [17]Rad Power Bikes RadRunner (opens in a new tab)One size, 4'11"–6'2".
- [18]Velotric Discover 3 (opens in a new tab)Regular 5'2"–5'11", Large 5'9"–6'7".
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