Inches to mm converter
1 inch is exactly 25.4 mm and 12 inches is 304.8 mm: turn bolt lengths, hole diameters, sheet thickness or model dimensions into millimetres for ISO drawings, slicers and caliper checks. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Inches
- 12
- Result
- 304.8
- Mm
- 304.8
- Factor
- 25.4
- Factor label
- 1 in = 25.4 mm (exact)
Converting inches to millimeters is a daily task when mechanical drawings, fastener catalogs, 3D print models, and school science sheets cross the imperial–metric boundary. This calculator multiplies your length by the exact international factor of 25.4 millimeters per inch—the same definition used in ISO standards, CNC toolpaths, and education since the 1959 yard and pound agreement fixed one inch at 25.4 mm.
Why 25.4 is an exact number
One inch is defined as exactly 25.4 millimetres. That is not a measured average but the international inch fixed by the 1959 yard and pound agreement, which replaced the older national inch standards. It is the factor written into ISO standards, CNC toolpaths and design software defaults, and the one taught in schools. Converting takes one multiplication: the length in inches times 25.4 gives millimetres. One inch is 25.4 mm on the nose and a foot of twelve inches is 304.8 mm. The page also shows the working on one line, so the step is easy to check.
Drawings, fasteners and 3D prints
Engineers and machinists convert bolt lengths, hole diameters and sheet thickness from inches into millimetres for ISO drawings and metric stock orders. Makers turn imperial design dimensions into millimetres for slicers, caliper checks and parts documented in both units. Students use it to check answers built on the exact 25.4. Two things are worth knowing: negative values are accepted for signed offsets, so make sure a minus sign is intended, and anything that is not a number, such as a word or a dimension with letters attached, gets a clear message rather than an invented length.
How the exact inches-to-millimeters factor works
The international inch is defined so that one inch equals exactly 25.4 millimeters. That definition replaced older national inch standards and is the factor taught in schools and hard-coded in standards bodies, CAD defaults, and packaging regulations worldwide. To convert, take your length in inches and multiply by 25.4; the product is the length in millimeters.
For example, one inch equals 25.4 millimeters on the nose, and twelve inches—one foot—equals 304.8 millimeters. Negative values are allowed when you model signed deltas or offsets; zero converts to zero.
What you can do with it
Mechanical drawings and fasteners
Engineers and machinists convert bolt lengths, hole diameters, and sheet thickness from inches to millimeters for ISO drawings and metric stock orders.
3D printing and maker projects
Makers convert imperial design dimensions into millimeters for slicers, caliper checks, and dual-unit part documentation without opening a spreadsheet.
Homework and classroom checks
Students and teachers verify inches-to-mm answers with the exact 25.4 factor, see a readable formula line, and get a clear error when a non-numeric value is submitted.
FAQ
What happens if I send non-numeric input?
Missing fields, NaN, Infinity, and non-numeric strings are rejected and not charged.
Is the inches to mm converter free?
The browser tool runs free on your device.
How many millimetres is 12 inches?
12 inches, one foot, is exactly 304.8 millimetres. The result comes with the working written out, 12 in × 25.4, which you can paste next to a drawing dimension or a homework answer.
Are negative inch values allowed?
Yes. A negative length is accepted when it stands for a signed offset or delta, and the millimetre result keeps that sign. A value of zero simply returns zero millimetres.
Is there an option for the old survey inch?
No. Every conversion uses the international inch of exactly 25.4 mm agreed in 1959, the same factor used in ISO standards and CNC toolpaths. There is no historical variant and no approximate table.
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/conv/inches-to-mm \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"inches":12}'const res = await fetch("https://api.kit.forhosting.com/conv/inches-to-mm", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"inches": 12
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/inches-to-mm",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"inches": 12
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/inches-to-mm", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"inches":12}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"inches":12}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/inches-to-mm", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"inches": 12
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.inches_to_mm",
"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.
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
Results are deterministic and instant: the free browser tool runs entirely on your device, and the prepaid API returns the same JSON contract for apps, CAD helpers, and automation at $0.002 per successful item. Invalid or missing input fails clearly with invalid_input and is not charged, so you can validate forms without burning balance.
How the exact inches-to-millimeters factor works
This capability accepts a single finite numeric field named inches (aliases value, in, inch, and length) as a JSON number or a plain numeric string, including scientific notation when you need very large or very small lengths. Non-numeric strings, empty bodies, NaN, and Infinity raise invalid_input with a message that names the field, so client validation can surface the same error the API returns. There is no network call, no approximate lookup table, and no historical survey-inch option: every successful response uses the same exact factor and the method label exact_international_inch so downstream code can trust the contract.
Reading the response fields and using the formula string
The formula field restates the arithmetic as a single readable line—for example twelve in times 25.4 equals 304.8 mm—so user interfaces can show the working without rebuilding the expression. On success the JSON echoes the parsed inches value, returns mm and the synonymous result field with the converted length, and includes factor set to 25.4 plus a short factor_label that states the exact identity one inch equals 25.4 millimeters. Units are also named explicitly via from_unit in and to_unit mm for multi-converter dashboards that switch labels dynamically. Use mm or result when you only need the number for a database column; use formula when tutoring software, homework helpers, or shop-floor labels should display the calculation; use factor when you cache the constant for offline batch jobs that must match this endpoint bit-for-bit. Missing inches, pure text such as twelve inches spelled out without a number, and infinite magnitudes all fail with invalid_input and never consume prepaid balance after the engine rejects the request.
API usage, pricing, workflows, and practical limits
Send a JSON body with the required inches field, or any documented alias, and call either the free in-browser inches-to-mm converter for one-off checks or the prepaid KIT API when product catalogs, e-commerce size charts, logistics middleware, classroom tools, or mobile apps need a stable machine-readable contract. Successful conversions bill as one item at $0.002; validation failures are free. Pair this endpoint with your own reverse conversion or multi-unit length tools when a pipeline must round-trip imperial and metric labels, but keep this landing focused on the high-volume search intent inches to mm. Magnitudes above one quadrillion inches are rejected as out of range so Worker isolates stay safe.
Technical questions
How do I convert inches to millimeters?
Send JSON with inches set to a finite number (for example inches: 1). The response returns mm and result equal to inches × 25.4, plus a formula string.
What conversion factor does this calculator use?
The exact international factor: 1 inch = 25.4 millimeters. There is no approximate or survey-inch mode.
Can I use aliases instead of the field name inches?
Yes. value, in, inch, and length are accepted aliases for the same required length field.