Bike Size Calculator
Height charts lie kindly; your inseam tells the truth. One measurement — the floor-to-crotch length you would take for a trouser — drives every classic frame-sizing formula in cycling. Enter it, and this calculator returns your road, mountain and hybrid frame sizes in centimetres and inches, plus the LeMond saddle height that starts every good bike fit and half the arguments in every good bike shop.
Why the Inseam Rules the Whole Fitting
Two riders of identical height can need frames two sizes apart, because height hides the leg-to-torso ratio that bicycle geometry actually fits. The inseam — the leg's working length — determines both the frame's seat tube (through the classic multipliers this calculator runs: about 0.67 of inseam for a road frame measured centre-to-top, 0.61 for a mountain frame that must leave standover room for the trail, 0.65 for the hybrid between) and the saddle height, where Greg LeMond's 0.883 factor (inseam × 0.883, bottom bracket centre to saddle top along the seat tube) remains the fitter's universal first setting. Height charts survive because they need no measuring and are right for the median body; the calculator exists for everyone the median excludes — the long-legged, the short-legged, and anyone between sizes at the shop, which is eventually everyone who has ever stood in a showroom straddling two frame sizes and wished for a number with an argument behind it.
Worked Example: An 84 cm Inseam
The defaults run a common measurement: 84 cm of inseam (about 33 inches). Road: 84 × 0.67 = 56.3 cm — a 56 cm frame, the M/L borderland in most brands' charts (the calculator's letter lands on L, and the note's between-size advice applies in full: try the 54 as well). Mountain: 84 × 0.61 = 51.2 cm (about 20 inches in the MTB dialect of inches). Hybrid: 54.6 cm. And the saddle: 84 × 0.883 = 74.2 cm from bottom bracket to saddle top — the number to set before the test ride, so the ride tests the frame rather than a random seatpost height. One tape measurement, four numbers, and a shop visit that starts at the correct end of the size run.
Stack and Reach: The Modern Footnote to Sizing
Contemporary fitters grumble, fairly, that seat-tube length — the number this page computes — describes a frame's size the way a book's page count describes its story: nominal sizes have drifted (a "56" from one brand measures like a "54" from another), sloping top tubes broke the tape's reference points, and the dimensions that decide fit are stack (vertical height of the head tube top above the bottom bracket) and reach (its horizontal distance). The calculator's honest role is triage: it places you in the correct size neighbourhood from one honest measurement, warns about standover and between-size strategy in its note, and leaves the final stack-and-reach comparison to the geometry tables of the two or three models you are actually choosing between — where a centimetre of reach is worth more than the nominal size is worth at all.
Kids, Growth and the Moving Target
Children break the calculator's premises usefully: their proportions change monthly, so the industry sizes by wheel and standover rather than by formula, and the governing parental error is buying "to grow into" — a frame whose standover and reach defeat the rider today teaches fear today. The inseam habit still pays: measure it at each hand-me-down and each growth spurt, insist on clearance over the top tube, and let the saddle's adjustment range absorb the season's growth. The adult formulas await at the other end of the growth curve, where the same book-against-the-wall measurement, taken once honestly, starts a lifetime of frames that fit — the calculator will be here, unchanged, because legs and geometry keep their ancient relationship.
Second-Hand Bikes and Borrowed Geometry
The used market is where sizing discipline pays its dividend: listings advertise a frame by its nominal number (or worse, by the seller's height) and the buyer's defence is this page's output plus a tape. Ask for the actual stack and reach from the model's geometry chart for its year, compare against the calculator's neighbourhood, and physically check standover before money moves — a bargain in the wrong size is the most expensive bike in the world per mile ridden. Rental and bike-share fleets trade on the same triage in reverse: their two or three sizes must cover the population's inseam range, and the calculator's multipliers are why a medium fleet bike fits the broad middle honestly while the extremes of the inseam distribution are sent, correctly, to real shops and real fits.
When the Calculator and the Shop Disagree
It will happen: the formula says 56, the shop's fitter walks out with a 54 and a knowing look. Neither is automatically wrong — the calculator fits your legs to the seat tube; the fitter is fitting your whole posture to a specific model's stack, reach, and intended use, and a long-torsoed rider on a relaxed-geometry 54 may be better served than on an aggressive 56 whose numbers the formula blessed. The grown-up protocol is to hold the calculator's output as the prior and the fitter's as evidence: insist the test ride start within one size of the computed figure, have the LeMond saddle height set before judging either, and be suspicious of any recommendation two sizes away that arrives without a geometry chart to justify it. Formulas travel between shops unchanged; opinions should have to show their working, as this page shows its own.
Related Tools
More from the Sports Calculators hub: the Bike Pace Calculator for the riding itself, and the Cycling Wattage Calculator for what the fitted legs can produce.
Frequently Asked Questions
How do I measure my inseam for bike sizing, properly?
Stand barefoot, back to a wall, feet about 20 cm apart; trap a hardback book between your legs, spine up, pulled firmly to the crotch as a saddle would sit, its top edge level against the wall; mark the wall at the book's top and measure floor to mark. That figure — not your trouser inseam, which is measured to the ankle's hem and runs shorter in implication — is the input this calculator expects. Measure twice; a centimetre of inseam is two-thirds of a centimetre of frame, and the book must be honestly level for either figure to deserve its decimals.
What if I am between two frame sizes, exactly?
The calculator's note gives the fitter's rule: choose the smaller. A smaller frame can be fitted upward and longer (longer stem, seatpost exposed within its safe limit, bars raised on spacers) across a wide and comfortable range, while a too-large frame's reach shortens only until the stem becomes twitchily short and the standover becomes an injury report. The exception is the rider seeking an upright endurance position, who may prefer the taller head tube of the larger size — a preference to decide with the stack figures of the geometry section, not by reflex.
Do women's bikes use different sizing rules?
The formulas are anatomy, not gender: inseam drives the frame identically. Women's-specific models historically altered geometry (shorter reach for a given seat tube, narrower bars, different saddles) around proportions that vary within each sex as much as between them — the industry's trend is toward unisex frames with fit components (crank length, bar width, saddle) chosen per rider. The calculator's numbers apply to every rider; the fit appointment that follows is where bodies stop being averages in either direction.
What frame size should I choose for a child?
Children's bikes are sized by wheel diameter (12, 16, 20, 24 inch) because growth outruns geometry nuance, and the governing test is standover plus the ability to place the forefoot down at a stop. The inseam still arbitrates: it must exceed the bike's standover height with clearance, and the saddle at its lowest must let the leg nearly extend at the pedal's bottom. The adult multipliers in this calculator do not apply to children's proportions; use the wheel-size charts of the manufacturers, keyed — as here — to the same honest inseam measurement.
How does saddle height relate to frame size, exactly?
They are siblings, not substitutes: frame size (seat tube) decides the range of saddle heights the seatpost can safely offer; the LeMond figure (inseam × 0.883) decides where in that range your legs belong. A correctly sized frame puts the LeMond height comfortably mid-range; needing the post at its minimum-insertion limit, or slammed to the collar, is the frame confessing it is the wrong size regardless of what its sticker claims. This is why the calculator prints the saddle height beside the frames — the test ride can check both in one minute with a tape.
What about crank length, then?
The third leg of the inseam's empire: cranks are commonly offered 165–175 mm, and the traditional matching puts shorter cranks with shorter inseams (a rough rule near 20% of inseam, capped by convention and availability) — mostly a matter of hip angle at the top of the stroke and ground clearance on mountain bikes. It is a refinement, not a sizing gate: stock cranks on the calculator's recommended frame will suit the large majority of riders perfectly well, and crank experiments belong to the fit-studio stage of a rider's life, after the frame itself is right and the miles have earned their opinions.