{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"bike-frame-size"},"result":{"entity_type":"calculator","id":"bike-frame-size","calculator_id":"bike-frame-size","canonical_url":"https://tttkmbb.com/fitness/bike-frame-size","name":"Bike Frame Size Calculator","title":"Bike Frame Size Calculator – Road, Gravel, Hybrid and Mountain Bike Size from Inseam","category":"fitness","category_name":"Sports & Fitness","tool_name":"estimate_bike_frame_size","featured_mcp_tool":false,"description":"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.","use_when":"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).","inputs":[{"name":"inseam_cm","label":"Inseam","type":"number","unit":"cm","required":true,"min":50,"max":120,"description":"Inside leg length: floor to crotch, standing in socks with feet 15–20 cm apart and a book pressed firmly up into the crotch.","example":80},{"name":"bike_type","label":"Bike type","type":"enum","required":false,"default":"road","values":[{"value":"road","label":"Road bike (frame in cm, inseam × 0.67)"},{"value":"gravel","label":"Gravel / cyclocross (as road, inseam × 0.67)","aliases":["cyclocross","cx"]},{"value":"hybrid","label":"Hybrid / city / trekking (frame in cm, inseam × 0.685)","aliases":["city","trekking","commuter","fitness"]},{"value":"mountain","label":"Mountain bike (frame in inches, inseam × 0.226)","aliases":["mtb","hardtail","full suspension"]}],"description":"Bicycle type; selects the multiplier and the unit the industry uses for that type.","example":"road"},{"name":"height_cm","label":"Height","type":"number","unit":"cm","required":false,"min":120,"max":230,"description":"Optional rider height, used only to cross-check the inseam result against a typical manufacturer height chart.","example":175}],"outputs":[{"name":"frame_size_cm","label":"Frame size","type":"number","unit":"cm","decimals":1,"description":"Seat-tube length in cm (road/gravel/hybrid: the primary size; mountain: the inch size × 2.54)."},{"name":"frame_size_in","label":"Frame size","type":"number","unit":"in","decimals":1,"description":"Seat-tube length in inches (mountain: the primary size; road/gravel/hybrid: cm / 2.54)."},{"name":"size_label","label":"Size label","type":"string","decimals":4,"description":"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."},{"name":"standover_height_cm","label":"Standover height target","type":"number","unit":"cm","decimals":1,"description":"inseam_cm − 3: a top-tube standover height leaving about 3 cm of clearance (road); mountain bikes should have 5 cm or more."},{"name":"height_chart_size","label":"Height-chart size","type":"string","decimals":4,"description":"Size suggested by a typical manufacturer height chart for height_cm (only when height_cm is given)."},{"name":"method_text","label":"Method","type":"string","decimals":4,"description":"The multiplier rule applied."}],"input_schema":{"type":"object","properties":{"inseam_cm":{"description":"Inside leg length: floor to crotch, standing in socks with feet 15–20 cm apart and a book pressed firmly up into the crotch. Unit: cm.","type":"number","minimum":50,"maximum":120,"examples":[80],"x-unit":"cm"},"bike_type":{"description":"Bicycle type; selects the multiplier and the unit the industry uses for that type.","type":"string","enum":["road","gravel","hybrid","mountain"],"default":"road","examples":["road"]},"height_cm":{"description":"Optional rider height, used only to cross-check the inseam result against a typical manufacturer height chart. Unit: cm.","type":"number","minimum":120,"maximum":230,"examples":[175],"x-unit":"cm"}},"additionalProperties":false,"required":["inseam_cm"]},"output_schema":{"type":"object","properties":{"frame_size_cm":{"description":"Seat-tube length in cm (road/gravel/hybrid: the primary size; mountain: the inch size × 2.54). Unit: cm.","type":"number","x-unit":"cm"},"frame_size_in":{"description":"Seat-tube length in inches (mountain: the primary size; road/gravel/hybrid: cm / 2.54). Unit: in.","type":"number","x-unit":"in"},"size_label":{"description":"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.","type":"string"},"standover_height_cm":{"description":"inseam_cm − 3: a top-tube standover height leaving about 3 cm of clearance (road); mountain bikes should have 5 cm or more. Unit: cm.","type":"number","x-unit":"cm"},"height_chart_size":{"description":"Size suggested by a typical manufacturer height chart for height_cm (only when height_cm is given).","type":"string"},"method_text":{"description":"The multiplier rule applied.","type":"string"}}},"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","method":"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.","sources":[{"name":"REI Expert Advice – Bike Fit Basics (frame size, standover height, reach)","url":"https://www.rei.com/learn/expert-advice/bike-fit.html","type":"reference","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Bicycle frame (frame sizing)","url":"https://en.wikipedia.org/wiki/Bicycle_frame","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"Inseam 80 cm, road bike, height 175 cm","inputs":{"inseam_cm":80,"bike_type":"road","height_cm":175},"expected":{"frame_size_cm":53.6,"frame_size_in":21.1,"size_label":"S","standover_height_cm":77,"height_chart_size":"M (54–56 cm)"},"url":"https://tttkmbb.com/api/v1/calculate/bike-frame-size?inseam_cm=80&bike_type=road&height_cm=175"},{"name":"Inseam 80 cm, mountain bike","inputs":{"inseam_cm":80,"bike_type":"mountain"},"expected":{"frame_size_in":18.1,"frame_size_cm":45.9,"size_label":"M","standover_height_cm":77},"url":"https://tttkmbb.com/api/v1/calculate/bike-frame-size?inseam_cm=80&bike_type=mountain"}],"faq":[{"q":"What if the inseam size and the height chart disagree?","a":"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?","a":"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?","a":"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)."}],"tags":["bike frame size","bike size calculator","inseam frame size","road bike size","mountain bike frame size","standover height"],"related":[{"calculator_id":"bike-gear-ratio","reason":"Gearing calculations for the same bike."},{"calculator_id":"cadence-to-speed","reason":"Speed from cadence and gearing."}],"links":{"html":"https://tttkmbb.com/fitness/bike-frame-size","markdown":"https://tttkmbb.com/fitness/bike-frame-size.md","json":"https://tttkmbb.com/fitness/bike-frame-size.json","api":"https://tttkmbb.com/api/v1/calculate/bike-frame-size","schema":"https://tttkmbb.com/api/v1/calculators/bike-frame-size","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:52:04Z","next_actions":[{"tool":"run_calculator","calculator_id":"bike-frame-size","reason":"Run Bike Frame Size Calculator with the inputs above."}],"links":{"markdown":"https://tttkmbb.com/fitness/bike-frame-size.md"}}