Ohm's law current calculator
A 12 V battery across a 4 Ω heater drives 3 A. Enter the voltage across a load and its resistance to get the direct current in amperes, with the sign kept for reversed polarity. Free in your browser.
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Worked example
- Voltage
- 12
- Resistance
- 4
- Current
- 3
- Current unit
- A
Ohm's law relates voltage, current, and resistance in a simple conductor: the current through the element equals the voltage across it divided by its resistance. This calculator solves that identity for current and returns I equals V over R in amperes when you supply voltage in volts and resistance in ohms. Resistance must be a finite non-zero number; a zero-ohm short would make the division undefined, and a non-finite resistance is not a physical ohmic load.
What Ohm's law current computes and which quantities to enter
Georg Ohm observed that the current through a metallic conductor is proportional to the potential difference across it when temperature is held fixed. Written for current, that statement is I equals V divided by R: voltage in volts, resistance in ohms, current in amperes. This landing answers one physics question only — given those two quantities, what is the current in amperes.
It does not solve for voltage or resistance, does not include reactive impedance, and does not add a power result P equals V I. Enter voltage as the drop across the same two terminals that define the resistance. A twelve-volt battery across a four-ohm heater yields three amperes; five volts across two hundred twenty ohms yields about 0.0227 amperes.
Negative voltage is allowed and simply reverses the sign of the current, matching signed-source convention in circuit analysis.
Where an Ohm's law current calculator earns its keep
Secondary-school physics and first-year electronics courses constantly assign Ohm's law problems that give voltage and resistance and ask for current in amperes, often for a lamp, a heater, a series resistor, or a bench supply driving a known load. Exam-prep generators need a deterministic oracle that marks wrong the case of a zero-ohm resistance instead of silently returning infinity that a buggy grader might treat as success.
Laboratory notebooks and light DC estimates use the same SI rearrangement as a baseline before anyone reaches for AC impedance or a non-linear I-V curve. Failed validation does not consume balance. Keep voltage in volts and resistance in ohms end to end, convert millivolts or kilohms before calling, and pin the ampere value in tests so homework checks, LMS plugins, and edge workers stay reproducible without standing up a circuit simulator of your own.
Power dissipation, voltage-from-current, and AC reactance remain outside this landing, so treat the number as the classic DC I equals V over R current in amperes. Round-trip checks are easy: multiply the returned current by resistance and confirm it equals the supplied voltage within the same twelve-decimal rounding used for goldens.
What you can do with it
DC load and heater estimates
Estimate current drawn by lamps, heaters, and resistive loads from a known supply voltage without embedding a full circuit simulator.
LMS and answer-key generators
Produce deterministic ampere keys for Ohm's law items that reject a short-circuit resistance instead of returning Infinity.
FAQ
How do I calculate current from voltage and resistance with Ohm's law?
Send finite numbers voltage (volts) and resistance (ohms, non-zero).
What happens if resistance is zero?
Ohm's law I = V/R is undefined at zero resistance, so a short is not billed as a successful ampere reading.
What units does the Ohm's law current calculator use?
Voltage is volts, resistance is ohms, and current is amperes. Convert millivolts, kilohms, or milliamps before calling; the solver does not rescale prefixes.
Can voltage be negative?
Yes. A negative voltage reverses the sign of the returned current, matching signed-source polarity in circuit analysis. Resistance still must be a non-zero finite number.
Does it work for AC circuits with reactance?
No. It applies the classic DC relation for a resistive load and does not include reactive impedance or a non-linear current curve. Treat the result as a baseline before moving on to AC impedance.
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-current \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"voltage":12,"resistance":4}'const res = await fetch("https://api.kit.forhosting.com/phys/ohms-law-current", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"voltage": 12,
"resistance": 4
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/phys/ohms-law-current",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"voltage": 12,
"resistance": 4
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/ohms-law-current", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"voltage":12,"resistance":4}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"voltage":12,"resistance":4}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/ohms-law-current", 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,
"resistance": 4
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "phys.ohms_law_current",
"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
The path is pure arithmetic with no network, no clock, and no randomness, so homework checks, lab notebooks, and API clients always receive the same JSON.
What Ohm's law current computes and which quantities to enter
Resistance of exactly zero is rejected because a short circuit would make I undefined in this DC ohmic model; the API returns invalid_input rather than Infinity so clients never treat an overflow as a successful ampere reading. The arithmetic is a single division of two finite numbers, fully deterministic, with no network, no random seed, and no wall clock, so identical inputs always serialize to the same JSON.
Input contract, response shape, and error behaviour
Resistance must be a non-zero finite number: a zero or negative-zero resistance fails with a message that Ohm's law I equals V over R is undefined at zero resistance, matching the product promise that the one-over-R singularity never masquerades as a finite ampere reading. Non-finite resistance (NaN or Infinity) is likewise rejected because it is not a physical ohmic load. Classic textbook checks include twelve volts across four ohms, which lands at exactly three amperes, and five volts across two hundred twenty ohms, which lands at five divided by two hundred twenty amperes. POST a JSON body with two required finite numbers: voltage, the potential difference V in volts; and resistance, the ohmic resistance R in ohms. Each field may arrive as a JSON number or as a numeric string; aliases V, v and volts for voltage, and R, r and ohms for resistance, are accepted so notebooks and lab exports map without renaming columns. Missing fields, empty strings, and non-finite values fail with invalid_input naming the field. On success the body echoes cleaned voltage and resistance, reports current in amperes from V over R, and documents formula I equals V over R with method ohms_law_current. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Integration use cases
Homework and exam checks
Enter voltage in volts and resistance in ohms, compare I in amperes with hand work, and get a clear invalid_input when resistance is zero or not finite.
Technical questions
What does the Ohm's law current calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.