Bike Frame Size Calculator
Recommends a road-bike frame size in centimeters from your inseam length using the standard 0.67 multiplier. Useful for initial bike sizing, narrowing down a frame size search, and comparing brand-specific size charts.
Last updated: May 2026
Compare with similar
About this calculator
The formula is the classic Lemond-derived inseam multiplier: Recommended Frame Size (cm) = round(Inseam × 0.67). Variables: Inseam is your barefoot inseam in cm, measured by standing with feet 15 cm apart and placing a hardback book firmly against your crotch (top edge represents seat post height — measure from floor to book's top edge). The 0.67 multiplier is for road-bike geometry; mountain bikes typically use 0.66 and hybrids 0.65, but this formula hardcodes 0.67. The Bike Type field is informational; the formula does not use its multiplier. Edge cases: the formula returns the seat-tube length (center-to-top or center-to-center, depending on manufacturer convention) — modern frames often have shorter, more compact geometry where the labeled 'size' (S/M/L or 52/54/56) refers to top-tube length rather than seat-tube length, so the result must be cross-referenced with brand-specific size charts. Body proportions vary — a long torso with short legs needs a larger frame than the inseam alone suggests; vice versa for short torso, long legs. The result is a starting point for frame search, not a final purchase decision. Professional bike fitting (Retül, Trek Precision Fit) typically refines the size based on torso length, arm length, flexibility, and riding style — and is essential for high-mileage or competitive use. For confidence: combine this formula's result with the manufacturer's size chart and a test ride if possible.
How to use
Example 1 — Road bike. Inseam 80 cm, road factor 0.67. round(80 × 0.67) = round(53.6) = 54 cm — a 54 cm ('Medium') frame. Example 2 — Mountain bike. Inseam 80 cm, MTB factor 0.66. round(80 × 0.66) = round(52.8) = 53 cm. Cross-check the brand's stack/reach chart to confirm.
Frequently asked questions
How do I measure my inseam accurately?
Stand barefoot against a wall with your feet 15 cm (6 inches) apart. Place a hardback book firmly against your crotch with the spine flat against the wall, applying upward pressure to simulate seated weight. Have a friend measure from the floor to the top edge of the book (or mark the wall with a pencil and measure later). Repeat 2-3 times and use the average. This measurement is your 'cycling inseam' and is typically 5-10 cm longer than your trouser inseam (which measures crotch to ankle, not crotch to floor). Accurate to within ~5 mm; greater precision doesn't change the frame size recommendation by more than one frame size.
Does this formula work for all bike types?
It's designed for road bikes with traditional drop-bar geometry (multiplier 0.67). Mountain bikes use 0.66; hybrid or comfort bikes use 0.65; gravel and cyclocross bikes use ~0.66-0.67. The differences are small (1-2 cm) but matter at the size-transition boundaries. For modern bikes with sloping top tubes (most current production), the 'frame size' label increasingly refers to virtual top-tube length or reach measurement, not the seat-tube length the formula returns. Always cross-reference with the manufacturer's published size chart and stack/reach numbers. The formula is best treated as a 'narrow down to 1-2 candidate sizes' tool, not a definitive prescription. For aero/TT bikes the geometry is so aggressive that inseam-only sizing is unreliable.
What if I'm between two sizes?
Sizing decisions when the formula falls between standard sizes (e.g., 55.5 cm when brands offer 54 and 56) come down to riding style and proportions. For aggressive performance road riding, generally pick the smaller frame and use a longer stem and lower stack — this gives a more aerodynamic, racing-style fit. For comfort riding, longer rides, or upright posture, pick the larger frame and use a shorter stem — this gives more reach without bending the rider too far over. Riders with a longer torso typically size up; riders with shorter torsos size down. Many brands now offer 1 cm increments (53, 54, 55, 56) for the most common sizes, reducing the in-between problem. A bike fitter or experienced bike-shop technician can resolve borderline cases far better than the formula alone.
Why doesn't this formula factor in my height or arm length?
Frame size is dominated by leg length (inseam) because saddle-to-pedal distance is the primary fit constraint — it must allow nearly full leg extension at the bottom of the pedal stroke. Height is correlated with inseam but the correlation isn't tight enough for direct prediction (inseam ranges 75-90 cm for adults 170-180 cm tall, depending on body proportions). Arm length and torso length determine reach (saddle-to-handlebar distance), which is adjusted via stem length and saddle position — not frame size. Two riders with identical inseam and very different torso proportions may end up on the same frame size but with stems differing by 20-30 mm. The formula gets you 80% of the way there; bike fit refines the remaining 20%.
When should I not use this calculator?
Skip it for kid sizes — children's bikes are sized by wheel diameter (12, 16, 20, 24 inches), not inseam multiplier. Do not use it for triathlon/TT bikes where geometry is so aggressive and aero-focused that inseam-based sizing is unreliable; professional TT fit is essential. Skip it for adaptive cycling, e-bikes with non-standard geometry, or any bike with unusual frame design (Brompton folder, Moulton small wheel, recumbent, tandem captain). For high-value purchases (over $2,000), invest in a professional fit before buying — a poorly-sized frame costs more to live with than the fit cost. The result of this calculator is one input among many; don't treat it as a sole criterion. Brand-specific size charts and dealer test rides are essential complements.