Conductor Resistance Calculator
A 25 m conductor with a 2.5 mm² cross-section and a resistivity of 1.68 × 10⁻⁸ ohm-metres has a resistance of 0.168 ohms. Enter length, area and resistivity in SI units and read the result in ohms. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Length
- 25
- Area
- 0.0000025
- Resistivity
- 0.0000000168
- Resistance ohms
- 0.168
The conductor resistance calculator computes the direct-current resistance of a uniform conductor from three physical inputs: its length, its cross-sectional area, and the electrical resistivity of its material. It applies the standard relationship R = ρL/A and reports resistance in ohms. Use SI units—meters, square meters, and ohm-meters—to avoid hidden conversion errors. The calculation is deterministic, runs without external lookups, and clearly rejects missing, negative, non-finite, or zero-area inputs instead of returning a misleading result.
Understand the resistance equation and its assumptions
A uniform conductor has direct-current resistance R = ρL/A, where ρ is material resistivity in ohm-meters, L is conductor length in meters, and A is cross-sectional area in square meters. The units reduce directly to ohms. Length appears in the numerator because charge travels through more material as the path gets longer, so doubling length doubles resistance when everything else stays fixed.
Area appears in the denominator because a wider conductive path provides more room for current, so doubling area halves resistance. Resistivity describes the material itself: copper, aluminum, steel, and other conductors produce different resistance even when their dimensions match. This calculator assumes one homogeneous material, constant cross-section, steady direct current, and a supplied resistivity that already represents the intended operating condition.
It does not infer temperature, alloy composition, strand packing, contact resistance, or alternating-current effects. Those factors can matter in real installations, but separating them from the base geometric calculation makes the result transparent and easy to verify. The returned formula and normalized SI inputs show exactly what was calculated, which is useful when checking engineering notes, classroom work, or automated design calculations.
Prepare length, area, and resistivity correctly
Enter length in meters, cross-sectional area in square meters, and resistivity in ohm-meters. Unit consistency is the most important practical detail. If area is given in square millimeters, divide it by one million before submitting it; for example, 2.5 mm² is 0.0000025 m².
Convert centimeters, feet, or other length units to meters as well. Obtain resistivity from an appropriate engineering reference or material datasheet, paying attention to temperature and alloy grade because resistivity is not a universal constant for a broad material label. All three values must be finite.
Length and resistivity may be zero for mathematical boundary cases, but neither may be negative. Area must be strictly greater than zero because division by zero is undefined and a negative physical cross-section has no meaning in this model. Results are rounded deterministically to twelve significant digits, providing useful precision while avoiding irrelevant floating-point noise.
Interpret the result and know when to use a richer model
You can test the result with proportional reasoning: doubling length should double resistance, doubling area should halve it, and replacing the material with one having twice the resistivity should double it. For a two-conductor DC loop, calculate the full current-path length or add the resistance of the outgoing and return conductors rather than using only the one-way distance. For several segments with different materials or areas, calculate each segment separately and add their series resistances.
This base result can support voltage-drop and power-loss calculations, but those require current as another input and are outside this capability. Use a more detailed model when conductor temperature changes significantly, when contacts and joints dominate, when cable construction changes the effective metallic area, or when frequency-dependent skin and proximity effects matter. The calculator intentionally does not contain a material database, because an embedded generic value could conceal differences between pure metals, alloys, manufacturing conditions, and reference temperatures.
Supplying resistivity explicitly keeps the assumption visible and lets the same deterministic calculation work for common conductors, specialist alloys, laboratory samples, and hypothetical teaching examples.
What you can do with it
Size a DC conductor
Compare resistance across candidate lengths, areas, and materials before a detailed voltage-drop review.
Check engineering calculations
Verify a hand calculation with normalized SI inputs and a deterministic result in ohms.
FAQ
What formula does the calculator use?
It uses R = ρL/A for a uniform conductor, with the result expressed in ohms.
Which units should I enter?
Use meters for length, square meters for area, and ohm-meters for resistivity.
Does the calculator adjust for temperature?
No. Supply the resistivity value appropriate to the material and operating temperature you want to model.
How do I convert square millimetres to square metres?
Divide the figure by one million. For example, 2.5 mm² becomes 0.0000025 m², which is the area to enter if you want the resistance in ohms to come out right.
What happens to resistance if I double the length or the area?
Doubling the length doubles the resistance, while doubling the cross-sectional area halves it. This proportional check is a quick way to confirm that a result makes sense before you rely on it.
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/eng/conductor-resistance \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"length":25,"area":0.0000025,"resistivity":1.68e-8}'const res = await fetch("https://api.kit.forhosting.com/eng/conductor-resistance", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"length": 25,
"area": 0.0000025,
"resistivity": 1.68e-8
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/eng/conductor-resistance",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"length": 25,
"area": 0.0000025,
"resistivity": 1.68e-8
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/eng/conductor-resistance", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"length":25,"area":0.0000025,"resistivity":1.68e-8}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"length":25,"area":0.0000025,"resistivity":1.68e-8}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/eng/conductor-resistance", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"length": 25,
"area": 0.0000025,
"resistivity": 1.68e-8
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "eng.conductor_resistance",
"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
Prepare length, area, and resistivity correctly
The calculator accepts the descriptive field names length, area, and resistivity, plus documented SI-oriented aliases. If the computed resistance exceeds the supported numeric range, the request is rejected rather than returning infinity or an unstable value. One successful API calculation costs $0.002; browser execution can use the same pure calculation path.
Interpret the result and know when to use a richer model
The response reports resistance_ohms, the equation name, and a normalized inputs object. Treat resistance_ohms as the resistance of the specified uniform conductor segment under the conditions represented by the resistivity value you supplied.
Integration use cases
Build repeatable test fixtures
Generate stable resistance values for electrical design software and educational exercises.
Technical questions
Can the cross-sectional area be zero?
No. Zero area makes the equation undefined, so it returns an invalid_input error.
How much does an API calculation cost?
Each successful API request costs $0.002.