Watts, volts & amps calculator
120 V at 2 A is 240 W: give any two of power, voltage and current and the calculator works out the third from power equals voltage times current, and shows which one it solved. For DC and resistive AC. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Voltage V in volts
- 120
- Current I in amperes
- 2
- Watts
- 240
- Solved
- watts
Electrical power, voltage and current are linked by the simple identity P equals V times I. Given any two of those quantities you can recover the third: multiply volts by amps to get watts, divide watts by volts to get amps, or divide watts by amps to get volts. Use it for DC circuits and for pure resistive AC work when values are RMS.
What watts, volts and amps mean and which two to send
Watts measure electrical power, the rate at which energy is transferred. Volts measure electric potential difference, the pressure that drives charge through a circuit. Amps measure current, the flow of charge per unit time.
For direct current and for alternating current expressed in root-mean-square values on a pure resistive load, the three quantities obey P equals V times I with no phase factor. Providing none, only one, or all three of the quantities fails with invalid input because the product would either be underdetermined or over-constrained.
Formulas, solved field, rounding and division by zero
When volts and amps are known, the engine multiplies them to obtain watts and labels solved as watts with formula P equals V times I. When watts and volts are known, it divides power by voltage to obtain amps, labels solved as amps and uses formula I equals P over V. When watts and amps are known, it divides power by current to obtain volts, labels solved as volts and uses formula V equals P over I.
Zero power with a non-zero divisor is valid and yields zero for the unknown. Negative values are accepted as finite signed quantities for modeling reverse power or polarity conventions; interpret signs according to your circuit reference frame. Missing input objects, non-finite numbers and out-of-range magnitudes also fail with invalid input and are not charged.
What you can do with it
Breaker and branch-circuit checks
Electricians and DIY planners enter known voltage and load current to recover watts, or known watts and voltage to recover amps, before choosing wire size and breaker rating.
Appliance and nameplate verification
Support teams and product pages convert any two published ratings on a device label into the third quantity for comparison tables and energy estimates.
FAQ
Does this work for AC circuits?
Yes for DC and for pure resistive AC when you supply RMS volts and amps. It does not apply a power-factor correction or three-phase multipliers; those are separate intents.
Is the watts volts amps calculator free?
The browser tool runs free on your device.
Can I send only one value or all three?
No. You must provide exactly two quantities and omit the unknown.
What happens if I divide by zero volts or zero amps?
The calculator stops and tells you which value is zero instead of returning infinity. Asking for current with zero voltage, or for voltage with zero current, has no finite answer, while zero power with a non-zero divisor simply gives zero.
How does this help when choosing a breaker?
Enter the circuit voltage and the load current to get the power, or the power and voltage to get the current. Knowing the current a load draws is the starting point for picking wire size and breaker rating.
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/elec/watts-volts-amps \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"watts":240,"P":1}'const res = await fetch("https://api.kit.forhosting.com/elec/watts-volts-amps", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"watts": 240,
"P": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/watts-volts-amps",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"watts": 240,
"P": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/watts-volts-amps", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"watts":240,"P":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"watts":240,"P":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/watts-volts-amps", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"watts": 240,
"P": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.watts_volts_amps",
"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
This capability performs that solve deterministically from a JSON body, rejects incomplete or over-specified inputs, and raises a clear invalid input error on division by zero so calculators, panel tools and educational apps never return infinity or NaN.
What watts, volts and amps mean and which two to send
Send exactly two of the three fields watts, volts and amps as finite JSON numbers or numeric strings; leave the unknown field out entirely so the engine knows which quantity to solve. Aliases such as P, power and W for watts, V, voltage and potential for volts, and I, current, amperes and A for amps are accepted when you prefer textbook symbols. Magnitudes must stay within the published finite bounds so Worker isolates stay stable. Identical inputs always produce identical JSON because the path is pure arithmetic with no network, no random seed and no wall clock.
Formulas, solved field, rounding and division by zero
The method field is always p_equals_v_times_i so downstream code can branch without sniffing shapes. Division by zero is never allowed: if you ask for amps while volts is zero, or for volts while amps is zero, the request fails with invalid input and a message that names the zero divisor.
API usage, pricing, panels, education and practical limits
Call the free in-browser watts volts amps calculator for quick checks on breakers, appliance nameplates, LED drivers and lab exercises without installing a spreadsheet. Authors shipping SDKs can map the solved and formula fields directly into UI copy that highlights which quantity was computed and which identity was used, matching what electricians and students write by hand on a workbench. Send a JSON body with exactly two of watts, volts and amps. Use the prepaid API when a home-energy dashboard, electrical estimating tool, IoT firmware test harness or classroom grading pipeline needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with unit converters when inputs arrive in kilowatts or milliamps, and with Ohm-law style tools when resistance is the next unknown. This product does not model power factor, three-phase line-to-line factors or complex impedance; for those intents use dedicated AC power calculators.
Integration use cases
Classroom and lab grading oracles
Physics and electronics courses pin deterministic JSON fixtures for P=V·I drills, including intentional division-by-zero cases that must fail with invalid_input.
Technical questions
How do I solve for watts, volts or amps?
Send exactly two of the fields watts, volts and amps (for example volts: 120 and amps: 2). The response returns all three quantities, the name of the solved field, and the formula used.
What happens on division by zero?
If volts is zero when solving for amps, or amps is zero when solving for volts, the request fails with invalid_input and is not charged. The error message names the zero divisor.