Ohm's law power calculator
A 12 V supply pushing 2 A delivers 24 W. Enter any two of voltage, current and resistance to get the power in watts and see which identity was used: V times I, I squared times R, or V squared over R. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Voltage
- 12
- Current
- 2
- Power
- 24 W
- Inputs used
- voltage, current
Electrical power in a resistive circuit is fully determined by Ohm's law once any two of voltage, current, and resistance are known. This Ohm's law power calculator accepts that pair and returns the power in watts using the matching identity: P equals V times I, P equals I squared times R, or P equals V squared over R.
What Ohm's law power means and which pair you need to send
Ohm's law relates voltage, current, and resistance on a simple resistive path: V equals I times R. Electrical power is the product of voltage and current, so substituting Ohm's law immediately produces two more closed forms that people search for as ohms law power: P equals I squared times R, and P equals V squared divided by R. Those three identities describe the same physical quantity written from different measurements.
If a lab has a voltmeter and an ammeter, P equals V times I is the direct product. If a datasheet lists current and ohmic resistance, P equals I squared R is the dissipation in the resistor. If a supply voltage and a load resistance are known and current was never measured, P equals V squared over R recovers the same watts.
This calculator exists for that search intent: given any two of voltage, current, and resistance, return power in watts, name the identity that was used, and echo the pair you supplied. It does not solve the full four-quantity Ohm's law wheel for an unknown voltage when power is already known, and it does not convert milliamps or kilohms for you. Send SI-consistent volts, amperes, and ohms so the result is a true SI watt.
How voltage, current, and resistance are read and when zero is an error
Missing objects, empty strings, non-numeric text, NaN, and Infinity fail before any wattage is returned, and failed validation is not charged. Numeric strings that parse to finite numbers are accepted so forms and CSV-style pipelines can pass values without an extra cast. Negative resistance is rejected because ohmic R in these identities is non-negative in this tool.
When voltage and current are both present, power is their product and no divisor is required, even if resistance is also supplied. When current and resistance are both present without voltage, power is I squared times R. Magnitudes above the documented ceiling are rejected.
Where Ohm's law power checks fit, what they cost, and what they leave out
Use the browser tool when you want an immediate watt check from a pair of meter readings, a resistor colour-code value plus a supply voltage, or a homework line that already states two of V, I, and R. Choose units consistently — if voltage is in volts, current in amperes, and resistance in ohms, power is true SI watts. Results are fully deterministic: the same pair always yields the same watts.
There is no language model, no parts catalogue, and no randomness, so the capacity is safe to pin in continuous-integration checks and reproducible lesson materials. Pair the result with a separate energy or cost calculator when you need kilowatt-hours or a bill; this tool does not integrate over time, apply power factor, or convert milliamps. Keep requests small, pin the rounded power field in tests, and you get reproducible Ohm's-law watts for homework, design notebooks, and edge workers without standing up a circuit-simulator stack of your own.
What you can do with it
Homework and lab Ohm's-law watt checks
Compute power from any two of V, I, and R before grading resistor-power worksheets or DC lab write-ups that already state a pair of measurements.
Quick load checks from two meter readings
Estimate dissipation on a DC rail when only a voltmeter and a known resistance, or an ammeter and a resistor value, are available.
FAQ
Which formula is used for each pair?
Voltage and current → P = V · I. Current and resistance → P = I² · R. Voltage and resistance → P = V² / R. If all three arrive, power is still V · I because that product needs no divisor.
What units should I send?
Use a consistent SI set: volts, amperes, and ohms. Then power is in watts (W). The tool does not convert prefixes such as milliamps or kilohms for you.
What happens if I enter all three values?
When voltage and current are both present, power is simply their product and resistance is not needed, even if you typed it. With 12 V and 2 A you get 24 W whatever resistance you entered.
Why is a resistance of zero rejected?
Only when it has to be divided by: with voltage and resistance alone, power is voltage squared over resistance, and zero would give infinity. The calculator reports the problem instead of returning an endless wattage.
Can I get the power from a single measurement?
No. One value alone does not determine power, so at least two of voltage, current and resistance are needed. With only a voltage, for example, the load could draw any current.
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/phys/ohms-law-power \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"voltage":12,"V":1}'const res = await fetch("https://api.kit.forhosting.com/phys/ohms-law-power", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"voltage": 12,
"V": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/phys/ohms-law-power",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"voltage": 12,
"V": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/ohms-law-power", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"voltage":12,"V":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"voltage":12,"V":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/ohms-law-power", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"voltage": 12,
"V": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "phys.ohms_law_power",
"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_value | 1000000000000000 |
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
When the formula needs a divisor and that divisor is zero, the request is rejected with invalid_input instead of returning Infinity. There is no network call, no sampling, and no date-dependent shortcut — only closed-form watts from the two measurements you send, at $0.002 per successful API request.
What Ohm's law power means and which pair you need to send
When a formula needs a divisor and that divisor is zero, the engine refuses the request rather than emitting Infinity or NaN into a worksheet.
How voltage, current, and resistance are read and when zero is an error
Send a JSON body with any two of voltage, current, and resistance. None of the three fields is required on its own, but fewer than two present values is invalid_input because a single measurement does not determine power. Aliases are recognized: voltage also accepts V, volts, v, and potential; current also accepts I, amps, amperes, amp, and i; resistance also accepts R, ohms, ohm, and r. When voltage and resistance are both present without current, power is V squared over R — and if resistance is zero that path is division by zero, which this calculator reports as invalid_input instead of Infinity. Results are rounded to twelve decimal places so the same pair always yields the same JSON across JavaScript engines. There is no phasor path, no reactive power, and no Monte Carlo sampling: the engine either evaluates a closed form or names the field that made the request illegal.
Where Ohm's law power checks fit, what they cost, and what they leave out
Each successful request is one billable item at $0.002; failed validation does not consume balance. Wire the endpoint into lab worksheets, contest answer checkers, or preprocessing steps where you already prefer explicit domain errors — such as refusing a zero resistance in P equals V squared over R — over opaque widgets that silently print Infinity.
Integration use cases
Deterministic fixtures for power oracles
Generate reproducible wattages for unit tests and teaching demos that assert P=VI, P=I²R, or P=V²/R without re-deriving the identities in every language.
Technical questions
How do I compute Ohm's law power from two of V, I, and R?
Send any two of voltage (volts), current (amperes), and resistance (ohms) to phys.ohms_law_power. The response returns power in watts, the formula used, a short method tag, and the inputs_used list.
What happens if resistance is zero?
When power would be computed as V² / R, zero resistance is invalid_input (division by zero) and is not billed. A current-and-resistance pair with R = 0 yields P = 0 because I² · R needs no divisor.
What does the Ohm's law power calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.