Spring stiffness converter
1000 N/m is 5.710147154733 lbf/in. Convert a spring rate between N/m, N/mm, kN/m, lbf/in, lbf/ft and kg/mm or kgf/mm when a coil-spring catalogue, a suspension sheet and a design textbook use different units. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Numeric spring stiffness
- 1,000
- Source unit
- N/m
- Target unit
- lbf/in
- Result
- 5.7101471547
- From label
- newton per meter
- To label
- pound-force per inch
- Nm
- 1,000
A spring stiffness converter—also searched as a spring rate converter—turns a force-per-length quantity written in one unit into another without hunting datasheet footnotes or re-deriving SI factors by hand.
Supported units and how the conversion is computed
The converter accepts spring-rate families under stable canonical codes: N/m for the SI newton per meter, N/mm and N/cm for common metric machine-design notations, kN/m for structural and large-coil scales, lbf/in (also accepted as lb/in or pounds per inch) for customary US spring catalogs, lbf/ft for foot-based force-per-length, kg/mm and kgf/mm for automotive and suspension rates quoted as kilograms per millimeter, plus kgf/cm and kgf/m when technical kilogram-force is explicit.
Factors follow SI and conventional engineering standards: one N/mm is one thousand N/m, one kN/m is one thousand N/m, one pound-force is zero point four five three five nine two three seven kilograms times standard gravity nine point eight zero six six five meters per second squared divided by zero point zero two five four meters for the inch, and one kilogram-force per millimeter is nine point eight zero six six five divided by zero point zero zero one newtons per meter.
The kg/mm code uses the same factor as kgf/mm because spring catalogs almost always mean technical kilogram-force when they print kg/mm. Because the intermediate is always N/m, converting A to B and B to A is consistent up to the published rounding of twelve decimal places, and there is no pairwise matrix that can drift out of sync. Pure force units, pure length units, mass units, and torque units are intentionally out of scope so designers get predictable spring-rate-only results under one clear convention.
Typical spring rate tasks
Use the free in-browser spring stiffness converter when you need a quick N/m-to-lbf/in, kg/mm-to-N/mm, or kN/m-to-pounds-per-inch check while reading a coil-spring catalog, suspension setup sheet, machine-design textbook, FEA material card, or homework problem. Open the page, enter the number and units, and copy the result without installing a spreadsheet, a unit library, or a desktop calculator pack. Source inputs follow the same retention rules as other KIT parse items, and results are safe to cache because the algorithm is fully deterministic.
Authors shipping mobile apps or chatbots can map the result and formula fields directly into cards that show both the number and the conversion path engineers, technicians, and students expect on paper.
What you can do with it
Suspension and automotive spring catalogs
Convert kg/mm and N/mm rates from European and Japanese datasheets into lbf/in for US setup sheets without hand-written factors or mixed-unit spreadsheets.
Machine design and FEA pipelines
Normalize N/m, N/mm, and kN/m spring stiffness values when CAD, FEA, and vendor PDFs disagree on force-per-length notation.
FAQ
Which spring stiffness units does this converter support?
N/m, N/mm, N/cm, kN/m, lbf/in (lb/in), lbf/ft, kg/mm, kgf/mm, kgf/cm, and kgf/m.
How many N/m is one lbf/in (pound per inch)?
About 175.126835246476 N/m under the ISO definition that multiplies the avoirdupois pound mass by standard gravity 9.80665 m/s² and divides by 0.0254 m.
Is kg/mm the same as kgf/mm?
Yes for this converter. Spring catalogs that print kg/mm almost always mean technical kilogram-force per millimeter. Both codes use the factor 9806.65 N/m.
What happens if I send an unknown unit?
The message names the field and lists supported units. Validation failures are not charged.
Is the spring stiffness converter free?
The browser tool runs free on your device.
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/stiffness \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"value":100,"from":"N/m","to":"lbf/in"}'const res = await fetch("https://api.kit.forhosting.com/conv/stiffness", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"value": 100,
"from": "N/m",
"to": "lbf/in"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/stiffness",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"value": 100,
"from": "N/m",
"to": "lbf/in"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/stiffness", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"value":100,"from":"N/m","to":"lbf/in"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"value":100,"from":"N/m","to":"lbf/in"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/stiffness", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"value": 100,
"from": "N/m",
"to": "lbf/in"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.stiffness",
"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
The browser tool runs free on your device; the prepaid API charges $0.002 per successful item when pipelines need the same contract from servers and automation.
Supported units and how the conversion is computed
Friendly aliases such as newtons per meter, pound-force per inch, kilograms per millimeter, and kilonewton per meter are normalized case-insensitively so forms and APIs can pass either short codes or full names. Every request multiplies the input value by the exact newtons-per-meter factor for the source, then divides by the newtons-per-meter factor for the target.
Request shape, response fields, and unknown-unit errors
The error message names the bad field and lists the supported set so clients can correct forms without guessing. Negative values are allowed when you need signed rate components in specialized analysis pipelines; zero converts cleanly to zero in every target unit. Send a JSON body with three required fields: value as a finite number, from as the source unit string, and to as the target unit string. Aliases amount, quantity, k, and rate are accepted for value; unit_from and source for from; unit_to and target for to. On success the response echoes the canonical from and to codes, human-readable from_label and to_label strings, the converted result, the intermediate nm quantity in newtons per meter, the dimensionless factor from source to target, a short formula line suitable for UI display, the method tag stiffness_si, and the fixed list of units_supported. Missing value, non-finite numbers such as NaN or Infinity, missing unit fields, and any token that does not map to a supported unit fail with invalid_input and are not charged after the engine rejects the request. Identical inputs always yield identical JSON because the path is pure arithmetic with no randomness, no remote lookup, and no dependence on the wall clock, which makes the endpoint safe to pin in continuous-integration goldens and SDK contract tests. Micro-rate and mega-rate magnitudes share the same code path, so extreme pairs remain well-defined as long as IEEE double precision can represent the intermediate N/m product.
Browser use, API pricing, and everyday workflows
Switch to the prepaid API when a CAD plugin, suspension-tuning app, manufacturing MES, robotics compliance pipeline, physics homework checker, or spring-selection tool must convert rates at scale under a stable KIT contract at $0.002 per successful item; validation failures that raise invalid_input do not consume balance after rejection. Pair this endpoint with a Hooke's-law calculator when you also need force from deflection, or call it alone when search traffic is specifically for spring rate unit conversion—newtons per meter, pounds per inch, and kilograms per millimeter—rather than a general force or length switchboard.
Integration use cases
Homework and lab automation
Convert lbf/in and N/m for class problems and automated lab scripts with a prepaid, deterministic API and clear invalid_input on unknown units.