ForHosting KIT · Engineering & construction

Power from current and resistance

Two amperes through 10 Ω dissipate 40 watts of heat. Enter the current and the resistance to get the power the resistor turns into heat, plus the voltage across it, without working out the voltage first. Free in your browser.

Free
Runs in your browser
No sign-up
Your data stays on your device

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

Worked example

If you enter

Current: 2
Resistance: 10

You get

Power: 40 W
Voltage: 20

Electrical power dissipated in a resistor is the heat that resistor must shed. When you already know the current through a branch and the resistance of the part, Joule’s law gives that heat directly as P equals I squared times R, without first computing voltage. Designers use P equals I squared R to size resistor wattage ratings, check whether a sense resistor will overheat at load current, and estimate thermal rise before a layout is finished. Students meet the same formula in every introductory circuit course when they compare P equals VI, P equals I squared R, and P equals V squared over R.

What P = I²R means and when to use it

Joule heating states that the instantaneous power converted to heat in a passive resistance is the product of the square of the current and the resistance itself. Squaring the current removes any sign dependence: a current of two amperes or negative two amperes through ten ohms both dissipate forty watts. That property is why lab manuals and datasheets quote power from root-mean-square current for AC and from DC current for steady bias.

The identity is algebraically equivalent to P equals V times I once Ohm’s law V equals I R is substituted, and to P equals V squared over R when voltage is the known quantity instead of current. Choose P equals I squared R whenever the branch current and the ohm value are the measurements or design targets you already have—current-sense shunts, LED series resistors after you fixed the target milliamps, heater elements specified in ohms and drive amps, and fuse or wire thermal checks.

Do not use a negative resistance in this tool: ideal passive ohms are non-negative, and a negative R would reverse the physical meaning of dissipation. Zero resistance with finite current still yields zero power in the formula, which is consistent with a perfect short that does not drop voltage and therefore does not dissipate energy in that ideal element. Keep units SI-consistent: amperes and ohms produce watts.

If your current is in milliamps or resistance in kilohms, convert before calling so the returned power stays in watts without silent unit mistakes.

Where I²R power checks appear in real work

Resistor selection is the most common industrial use: after you fix operating current and choose a preferred E-series ohm value, you compute P equals I squared R and pick a package whose rated wattage exceeds that dissipation by a safety margin, often two times or more for continuous duty in still air. Current-sense designs place a low-ohm shunt in series with a load; the same formula estimates heat in the shunt at full-scale amps so the board layout can add copper pour or derate ambient.

Heater and soft-start resistor banks are specified by cold resistance and expected current; manufacturing test scripts call this calculator to assert that programmed set-points stay inside thermal limits. Electronics teachers and LMS authors need a deterministic oracle that grades student answers for P equals I squared R without importing a full SPICE engine. Field technicians troubleshooting overheated droppers on legacy equipment measure current and read the color code, then confirm that the part was undersized relative to computed watts.

Content pages that rank for power from current and resistance calculator benefit from a focused landing that does only I and R to P, separate from general electrical power tools that also accept voltage pairs.

What you can do with it

Resistor wattage sizing

From design current and chosen ohms, compute P=I²R and select a package rated above the dissipation with a safety margin.

Current-sense shunt heat check

Estimate heat in a low-ohm shunt at full-scale amps so layout and derating stay within thermal limits.

FAQ

What formula does this calculator use?

P = I² · R. Power in watts equals the square of current in amperes times resistance in ohms. Voltage is also returned as V = I · R.

Can current be negative?

Yes. Power depends on I squared, so a negative current produces the same non-negative dissipation as the positive magnitude.

What if resistance is negative or missing?

Missing current or resistance also fails validation and is not billed.

How is this different from a general P=VI calculator?

This page is dedicated to power from current and resistance only (P=I²R). General tools that accept any two of V, I, and R serve a broader search intent.

What if my current is in milliamps?

Convert it to amperes first, and kilohms to ohms. Amperes and ohms give a result in watts; mixing units produces a wrong figure that is easy to miss.

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

POSThttps://api.kit.forhosting.com/elec/power-i2r

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/elec/power-i2r \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"current":2,"resistance":10}'

const res = await fetch("https://api.kit.forhosting.com/elec/power-i2r", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "current": 2,
    "resistance": 10
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/elec/power-i2r",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "current": 2,
    "resistance": 10
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/power-i2r", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"current":2,"resistance":10}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"current":2,"resistance":10}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/power-i2r", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "current": 2,
  "resistance": 10
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "elec.power_i2r",
  "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

Per request$0.002

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

Input contract, response fields, and validation

Resistance must be greater than or equal to zero; a negative R is rejected with a clear message. On success the body echoes the cleaned current and resistance, reports power in watts under power with unit set to W, includes voltage as I times R for convenience, and documents the identity with formula P equals I squared times R and method P equals I squared R. Classic checks: two amperes through ten ohms yields forty watts and twenty volts; half an ampere through one hundred ohms yields twenty-five watts and fifty volts; zero current at any finite R yields zero power. Round-trips in automated tests should pin the numeric fields because floating-point is rounded to twelve decimal places for stable goldens across runtimes. POST a JSON object with two required fields: current, the branch current I in amperes, and resistance, the ohmic value R in ohms. Both accept JSON numbers or numeric strings; aliases I, i, amps, amperes, and amp map to current, and R, r, ohms, and ohm map to resistance. Missing fields, empty strings, non-numeric text, NaN, and Infinity fail with invalid_input and are not billed. Magnitude is capped so runaway scientific-notation typos cannot monopolize a Worker. Pricing for a successful API item is the published base of $0.002; validation failures do not consume prepaid balance.

Integration use cases

Lab and LMS grading

Grade student answers for P=I²R with a deterministic API oracle that matches the free browser widget byte-for-byte.

Technical questions

What does the power from current and resistance calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not charged.

Read the full KIT documentation →

Run — free