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Bike Frame Size Calculator

Q: What if the inseam size and the height chart disagree?

Leg length determines standover clearance and saddle height, so start from the inseam size; a longer torso or arms may justify the next size up for reach, which a test ride settles.

Q: Why is the mountain bike size in inches?

Mountain bike frames are traditionally labelled in inches (or S/M/L); the cm value here is the same length converted, not a road-bike size.

Q: Are the results valid for compact and sloping frames?

Only as a starting point: many modern frames use a virtual seat-tube length or letter sizes, so match the estimate to the brand's geometry chart (effective top-tube length, reach and stack).

Estimates bicycle frame size from the rider's inseam with the common multiplier rules (road/gravel: inseam × 0.67 cm centre-to-top; hybrid/city: inseam × 0.685 cm; mountain: inseam × 0.226 inches), maps it to an XS–XL label, gives a standover-height target and cross-checks the result against a typical rider-height chart.

When to use

A rider knows their inseam (and optionally height) and wants a starting frame size or S/M/L label for a road, gravel, hybrid or mountain bike.

Do not use when: You need a precise fit (saddle height, reach, stack) or a brand's own geometry chart, which differ between manufacturers, for children's bikes (sized by wheel diameter), or for gearing questions (use bike-gear-ratio).

Formula

road/gravel: frame_cm = inseam_cm × 0.67; hybrid: frame_cm = inseam_cm × 0.685; mountain: frame_in = inseam_cm × 0.226; frame_in = frame_cm / 2.54 (and vice versa); standover_height_cm = inseam_cm − 3

The multipliers are the widely used rule-of-thumb estimates of seat-tube length (centre of bottom bracket to top of seat tube) for classic geometry; brands with compact or sloping frames label sizes differently, so treat the result as a starting point and confirm with the maker's chart and a test ride. The height chart is a typical guide and varies by brand.

Inputs

Parameter	Type	Unit	Required	Description
`inseam_cm`	number	cm	yes	Inside leg length: floor to crotch, standing in socks with feet 15–20 cm apart and a book pressed firmly up into the crotch. Range: ≥ 50, ≤ 120
`bike_type`	enum: road \| gravel \| hybrid \| mountain		default road	Bicycle type; selects the multiplier and the unit the industry uses for that type.
`height_cm`	number	cm	no	Optional rider height, used only to cross-check the inseam result against a typical manufacturer height chart. Range: ≥ 120, ≤ 230

Outputs

Output	Type	Unit	Description
`frame_size_cm`	number	cm	Seat-tube length in cm (road/gravel/hybrid: the primary size; mountain: the inch size × 2.54).
`frame_size_in`	number	in	Seat-tube length in inches (mountain: the primary size; road/gravel/hybrid: cm / 2.54).
`size_label`	string		XS/S/M/L/XL from the frame size: road cm < 50 XS, 50–53 S, 54–56 M, 57–59 L, ≥ 60 XL; mountain inches < 15 XS, 15–16 S, 17–18 M, 19–20 L, ≥ 21 XL.
`standover_height_cm`	number	cm	inseam_cm − 3: a top-tube standover height leaving about 3 cm of clearance (road); mountain bikes should have 5 cm or more.
`height_chart_size`	string		Size suggested by a typical manufacturer height chart for height_cm (only when height_cm is given).
`method_text`	string		The multiplier rule applied.

Example

Inseam 80 cm, road bike, height 175 cm: {"inseam_cm":80,"bike_type":"road","height_cm":175} → {"frame_size_cm":53.6,"frame_size_in":21.1,"size_label":"S","standover_height_cm":77,"height_chart_size":"M (54–56 cm)"}

Inseam 80 cm, mountain bike: {"inseam_cm":80,"bike_type":"mountain"} → {"frame_size_in":18.1,"frame_size_cm":45.9,"size_label":"M","standover_height_cm":77}

GET https://tttkmbb.com/api/v1/calculate/bike-frame-size?inseam_cm=80&bike_type=road&height_cm=175

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/bike-frame-size (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/bike-frame-size · Markdown: https://tttkmbb.com/fitness/bike-frame-size.md · JSON definition: https://tttkmbb.com/fitness/bike-frame-size.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="bike-frame-size"
OpenAPI operationId: estimate_bike_frame_size
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

REI Expert Advice – Bike Fit Basics (frame size, standover height, reach) (reference)
Wikipedia – Bicycle frame (frame sizing) (reference)

FAQ

What if the inseam size and the height chart disagree?

Leg length determines standover clearance and saddle height, so start from the inseam size; a longer torso or arms may justify the next size up for reach, which a test ride settles.

Why is the mountain bike size in inches?

Mountain bike frames are traditionally labelled in inches (or S/M/L); the cm value here is the same length converted, not a road-bike size.

Are the results valid for compact and sloping frames?

Only as a starting point: many modern frames use a virtual seat-tube length or letter sizes, so match the estimate to the brand's geometry chart (effective top-tube length, reach and stack).

Related calculators

Bike Gear Ratio Calculator — Gearing calculations for the same bike.
Cadence to Speed Calculator — Speed from cadence and gearing.