RPM calculator
At 100 km/h a 0.632 m tire turns 839.424805 times a minute; multiply by the gear ratio to get engine speed. Enter speed, tire diameter and ratio for a logbook, a workshop check or a classroom problem. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Speed
- 100
- Tire diameter
- 0.632
- Gear ratio
- 1
- Circumference
- 1.985487 rpm
- Wheel rpm
- 839.424805
- Rpm
- 839.424805
This RPM calculator turns three everyday vehicle figures — road speed in kilometers per hour, tire diameter in meters, and overall gear ratio — into revolutions per minute at the wheel or the engine. Enter speed, tire diameter, and gear ratio; the engine first builds the rolling circumference as π times the tire diameter, converts the road speed into meters per minute, and divides that travel rate by the circumference to obtain wheel RPM. Multiplying by the gear ratio then yields engine RPM when the ratio is the overall drivetrain reduction, or the same wheel RPM when the ratio is one. The path is fully deterministic: no tachometer feed, no CAN bus, and no date-dependent shortcut. Small checks you may already know include one hundred kilometers per hour on a zero point six three two meter tire at ratio one yielding eight hundred thirty-nine point four two four eight zero five revolutions per minute.
What wheel and engine RPM mean and what comes back
Revolutions per minute answers a practical household and workshop question: given how fast a vehicle is moving, how large its tires are, and how the gearbox multiplies wheel turns into crankshaft turns, how fast is that wheel or that engine spinning. Dashboard tachometers, GPS speed apps, and mixed-unit converters are useful in other moments, but the everyday search for an RPM calculator is usually this three-number identity and nothing more. This page stays on that identity.
If you already know RPM and a path radius and need linear rim speed, use an RPM-to-linear-speed helper; if you are converting cadence and chainring teeth into bicycle road speed, use a bike-speed-from-gear helper; if you already have an input shaft speed and only need the output shaft of a gear pair, use a gear-output-speed helper. This landing is the generic speed-and-tire tachometer so logbooks, APIs, and the free browser demo share one stable contract.
How the three inputs are read and how RPM is computed
Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a message that names the field, so a bad cell does not become a wrong tachometer reading.
After validation the engine applies the school identity: circumference equals π times tire diameter; meters per minute equals speed times one thousand divided by sixty; wheel RPM equals that travel rate divided by the circumference; and the returned rpm equals wheel RPM times the gear ratio.
That is why one hundred kilometers per hour on a zero point six three two meter passenger tire at ratio one is eight hundred thirty-nine point four two four eight zero five revolutions per minute at the wheel, and why ninety kilometers per hour on a zero point six five meter tire at an overall ratio of three point seven three is seven hundred thirty-four point five six one two seven six wheel RPM and two thousand seven hundred thirty-nine point nine one three five five nine engine RPM after six-decimal rounding.
Speed may be zero when the tire is a real size: a parked vehicle reports zero RPM, which is a legitimate logbook entry. Tire diameter may not be zero or negative. The specified error path is the zero-diameter case, because division by a zero circumference is undefined and inventing infinity would hide a data-entry mistake.
Gear ratio must be positive; use one when you only want wheel RPM.
Where a simple RPM check belongs in daily life and software
Private drivers meet this calculation when a new tire size, a different final drive, or a highway cruising speed needs a tachometer estimate before the next road trip. Households use it to compare two cars on the same posted limit without opening a dealer scan tool.
Students and driving instructors use it to connect the textbook idea of circumference and revolutions with a concrete number from a weekend journey, including the classic one hundred kilometers per hour and zero point six three two meter example that yields eight hundred thirty-nine point four two four eight zero five wheel RPM. Gearbox and hobby-build products call the contract from scripts when a user logs speed and tire diameter and the app must show engine RPM without shipping a full vehicle-dynamics stack.
Keep requests inside the published magnitude ceiling, pin the six-decimal RPM in tests, and you get a reproducible revolution figure for logbooks, LMS plugins, and edge workers without standing up a drivetrain model of your own.
What you can do with it
Tachometer check after a tire change
Enter highway speed, the new overall tire diameter, and the overall gear ratio to estimate engine RPM before the next trip.
FAQ
How is RPM calculated from speed and tire diameter?
The returned rpm is that wheel rate times the gear ratio.
What happens if tire diameter is zero?
A zero-size tire has no circumference, so the calculator will not invent infinity or a silent NaN.
When should gear ratio be 1?
Use 1 to obtain wheel RPM. Use the overall drivetrain ratio (transmission times final drive) when you want engine RPM at that road speed.
Which units should I enter?
A typical passenger tire around 205/55R16 is about 0.632 m overall diameter.
What does the RPM calculator cost?
It is free in your browser on this page.
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/life/rpm \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"speed":100,"tire_diameter":0.632,"gear_ratio":1}'const res = await fetch("https://api.kit.forhosting.com/life/rpm", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"speed": 100,
"tire_diameter": 0.632,
"gear_ratio": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/life/rpm",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"speed": 100,
"tire_diameter": 0.632,
"gear_ratio": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/life/rpm", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"speed":100,"tire_diameter":0.632,"gear_ratio":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"speed":100,"tire_diameter":0.632,"gear_ratio":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/life/rpm", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"speed": 100,
"tire_diameter": 0.632,
"gear_ratio": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "life.rpm",
"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
max_abs | 1000000000 |
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
Zero tire diameter is rejected with invalid_input because a wheel with no diameter has no circumference and cannot define a revolution rate.
What wheel and engine RPM mean and what comes back
There is no live vehicle network and no date stamp: the engine either builds a finite non-negative revolution rate inside the supported numeric range or rejects the input with invalid_input. Rejected cases include a missing speed, tire_diameter, or gear_ratio field, a non-finite token, a magnitude above the documented ceiling, a negative road speed, a non-positive gear ratio, and any non-positive tire diameter, because a zero-size tire does not define a circumference.
How the three inputs are read and how RPM is computed
The preferred fields are speed for kilometers per hour, tire_diameter for meters of overall tire diameter, and gear_ratio for engine revolutions per wheel revolution, but aliases such as v, velocity, kmh, kph, speed_kmh, diameter, d, tyre_diameter, wheel_diameter, ratio, gr, gear, and overall_ratio are accepted so spreadsheets, query strings, and typed forms can reuse the same contract without renaming columns. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline.
Where a simple RPM check belongs in daily life and software
Education tools use the single-rate response when the audience already works in kilometers per hour and meters and does not need a miles-and-inches companion on the same payload. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the tier-A catalog and lets you move from a dashboard demo to an automated pipeline without changing the mathematical result.
Integration use cases
Homework and driving-school checks
Confirm the identity (speed × 1000 / 60) / (π × diameter) × gear ratio, including the required invalid_input when tire diameter is zero.
Deterministic fixtures for vehicle apps
Generate a reproducible rpm field for gearing tools, CI assertions, and metric-only dashboards that already store km/h and meters.