Bike size calculator
A rider of 175 cm with an 81 cm inseam fits a 54 cm road frame, size M, even though height alone says L: the inseam sets the frame, because standover clearance is what keeps you safe. Free in your browser.
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- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Height
- 175
- Inseam
- 81
- Style
- Road
- Height (cm)
- 175 cm
- Inseam cm
- 81
- Frame cm
- 54.3
- Frame inches
- 21.4
A bicycle frame that is too small feels twitchy at speed and cramped on the hoods; a frame that is too large is hard to stand over and slow to steer. Shops therefore size a bike from two body measurements, not from a clothing label. Standing height places the rider on a letter-size chart from XXS through XXL. Inseam — the inside-leg length from crotch to floor in bare feet — drives the numeric frame, because standover clearance is a crotch-to-top-tube distance. This bike size calculator applies those standard height tables and inseam formulas for road, mountain, and hybrid bikes. It returns a recommended frame in centimetres and inches plus a letter size.
How height tables and inseam formulas size a frame
Bicycle frame size is not a clothing size. Two riders with the same standing height can need different frames if their legs are longer or shorter, which is why shops record both height and inseam instead of guessing from a T-shirt label.
Standing height is mapped onto a letter-size chart that most manufacturers publish for adult bikes: riders under 152 cm typically land on XXS, 152 to 160 cm on XS, 160 to 168 cm on S, 168 to 175 cm on M, 175 to 183 cm on L, 183 to 191 cm on XL, and 191 cm and above on XXL. That letter is a starting band, not a purchase order.
The numeric frame comes from inseam, the inside-leg length measured from the crotch to the floor in thin socks or bare feet, because standover clearance is the gap between the top tube and the rider's crotch when they straddle the bike. Classic shop formulas multiply inseam in centimetres by 0.67 for a road frame, by 0.66 and then subtract 10 cm for a traditional mountain-bike seat-tube measurement, and by 0.685 for a hybrid or city frame.
The calculator then snaps that number onto a published catalog — centimetre sizes 47 through 64 for road, inch sizes 13 through 23 for mountain, and a compact even-centimetre ladder for hybrid — and returns both the letter and the snapped frame so a product page can show M and 55 cm together. When the inseam-derived letter and the height-derived letter disagree, the numeric inseam result is the one to buy against, because standover is a safety constraint and height is only a band.
Where a bike size calculator earns its keep
Shoppers buying a road, mountain, or hybrid bike online need a reproducible frame size before they commit to a geometry they cannot straddle in a store. A publisher ranking for bike size calculator is targeting that intent: people who already have a height and an inseam and want a letter plus a centimetre or inch frame, not a full bike-fit with saddle setback, cleat position, or stem length.
Rental fleets and school cycling programmes need the same table so a 175 cm rider with an 81 cm inseam is not handed an XL one day and a small the next. Gift buyers and parents who can measure the rider at home but cannot visit a shop can send both numbers and read a catalog size that matches what a floor sticker would have said. Failed validation does not consume balance.
What you can do with it
Buying a road or hybrid bike online
Send standing height and inseam to get a catalog centimetre and a letter size before you order a frame you cannot straddle in a shop.
Rental fleets and school cycling programmes
Assign the same road, mountain, or hybrid size to a rider every time from two body measurements, instead of guessing from clothing size.
Gift buyers and remote product finders
Parents or engineers who have height and inseam but not the rider in the aisle can pin a catalog size and reject impossible inseam-over-height pairs.
FAQ
How do I measure inseam for a bike size?
Stand barefoot with a book pulled up into the crotch, then measure from the top of the book to the floor. Send that length as inseam in the same unit as height (cm by default, or in).
Should I size a bicycle from height or from inseam?
Use both. Height picks a letter band (XS–XXL). Inseam drives the numeric frame and standover.
What is the difference between road, mountain, and hybrid sizing?
Mountain uses inseam × 0.66 minus 10 cm, snapped to 13–23 inch frames. Hybrid uses inseam × 0.685 on an even-centimetre city catalog.
Why is an inseam longer than height rejected?
Inside-leg length cannot exceed standing height on a real rider.
What size road bike do I need at 175 cm?
It depends on your inseam. At 175 cm with an 81 cm inseam, the road formula gives 54.3 cm, which snaps to a 54 cm frame, size M. Height alone would suggest L, but the inseam result is the one to buy against.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/sports/bike-size \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"height":175,"inseam":81}'const res = await fetch("https://api.kit.forhosting.com/sports/bike-size", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"height": 175,
"inseam": 81
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/sports/bike-size",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"height": 175,
"inseam": 81
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/sports/bike-size", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"height":175,"inseam":81}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"height":175,"inseam":81}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/sports/bike-size", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"height": 175,
"inseam": 81
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "sports.bike_size",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
min_height_cm | 120 |
max_height_cm | 230 |
min_inseam_cm | 50 |
max_inseam_cm | 120 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
When inseam exceeds standing height the request is rejected as anatomically impossible. Successful calls are billed at $0.002; invalid input is not charged.
Input contract, styles, and error behaviour
Both values share unit, which is cm by default and may be in when you measured in inches; the pair is converted to centimetres with 2.54 cm per inch before any table is applied. style selects the catalog and the inseam formula: road (default), mountain (mtb is accepted), or hybrid. Height must lie between 120 and 230 cm after conversion; inseam must lie between 50 and 120 cm. Example: height 175 cm and inseam 81 cm on a road bike yield a mid-50 cm catalog frame. POST a JSON body with two required finite numbers and two optional selectors. height is standing height; aliases height_cm and stature are accepted. inseam is inside-leg length; aliases inseam_cm, inside_leg, and crotch are accepted. Each numeric field may arrive as a JSON number or as a numeric string. Missing fields, empty strings, non-finite values, non-positive measurements, a unit other than cm or in, or a style outside the published enum fail with invalid_input naming the field and are not billed. The hard anatomical rule is that inseam cannot exceed height: a crotch-to-floor length longer than standing height cannot occur on a real rider, so the task is rejected rather than inventing a frame. On success the body echoes height_cm, inseam_cm, unit, and style; returns frame_cm and frame_inches from the inseam formula; snaps those to catalog_cm and catalog_inches; reports frame_size from the inseam catalog and height_size from the height table; includes standover_cm after a style-specific clearance (2 cm road, 5 cm mountain, 3 cm hybrid); and, when the two letters differ, a fit_note telling you to prefer the inseam-derived frame. Pricing for a successful item is a flat $0.002.
Where a bike size calculator earns its keep
Keep requests small, pin catalog_cm and frame_size in tests, and you get stable bicycle sizing for landing pages, shop kiosks, and edge workers without standing up a fitting studio of your own.
Technical questions
What does the bike size calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.