LED resistor calculator
A 5 V supply, a 2 V LED and 0.02 A need a 150 Ω resistor dissipating 0.06 W, so a 0.125 W part is suggested. Enter the two voltages and the current to get the resistance and a power rating with double margin. Free in your browser.
- Free
- Runs in your browser
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- Your data stays on your device
Run — free
Worked example
- Supply voltage
- 5
- Forward voltage
- 2
- Forward current
- 0.02
- Voltage drop
- 3
- Resistance ohms
- 150
- Power watts
- 0.06
- Recommended power watts
- 0.125
An LED needs a series resistor whenever the supply voltage is higher than the diode forward voltage, otherwise current runs away and the LED fails. Ohm’s law applied to the leftover headroom gives the resistance: R equals supply voltage minus forward voltage, divided by the desired forward current, while the resistor itself dissipates power equal to that same voltage drop times the current.
What supply voltage, forward voltage and current mean for one LED
Supply voltage is the DC rail that feeds the LED branch, typically five volts from USB, three point three volts from a microcontroller pin through a buffer, nine or twelve volts from a wall adapter, or twelve to twenty-four volts on industrial panels.
Forward voltage is the voltage that appears across the LED when it is conducting at the chosen operating current; red LEDs often sit near one point eight to two point two volts, green and yellow a little higher, blue and white closer to three volts, while infrared emitters sit lower—always take the figure from the datasheet at the current you intend to run, not a generic colour rule of thumb.
Forward current is the steady current you want through the LED in amperes; many indicator LEDs are happy at ten to twenty milliamps, so you send zero point zero one to zero point zero two, not the milliamp integer. Missing fields, non-positive voltages or current, and non-finite values all fail with invalid input and are not charged.
Formulas, power rating, headroom errors and reading the response
The engine subtracts forward voltage from supply voltage to obtain the resistor voltage drop, divides that drop by forward current to obtain resistance in ohms, and multiplies the drop by current to obtain continuous power in watts. The formula field restates R equals open parenthesis Vs minus Vf close parenthesis over If, and P equals If times open parenthesis Vs minus Vf close parenthesis, so UI copy can show the identity students write by hand.
Recommended power watts is the smallest common commercial rating at or above twice the calculated dissipation, covering one sixteenth, one eighth, one quarter, one half, one, two, three, five, ten, twenty-five and fifty watt steps, with a deterministic ceiling above the catalog. Power margin is always two. When the two voltages are equal, resistance would be zero and the LED would not be current-limited, so that case is also invalid input.
Zero or negative current and supply are rejected the same way. Successful responses echo the three inputs after stable rounding, plus voltage drop, resistance ohms, power watts, recommended power watts, power margin, formula and method led series resistor so downstream code can branch without sniffing shapes. Use resistance ohms to pick the nearest E-series part on your own; this product returns the exact ohmic need, not a preferred-value table.
What you can do with it
Breadboard and Arduino indicators
Makers enter a five-volt rail, a two-volt red LED and twenty milliamps to get the series ohms and a safe quarter-watt or eighth-watt part before wiring a status light.
Panel and appliance pilot lamps
Industrial designers size dropping resistors for twelve or twenty-four volt panels from datasheet Vf and If so indicators neither burn out nor run dim.
FAQ
What happens if the LED forward voltage exceeds the supply?
A series resistor cannot create headroom when Vf is greater than Vs; equal voltages are also rejected because R would be zero.
Is the LED resistor calculator free?
The browser tool runs free on your device.
Should I enter 20 mA or 0.02?
Enter amperes, so 20 mA is 0.02. Many indicator LEDs are happy at 10 to 20 mA, which means typing 0.01 to 0.02 rather than the milliamp whole number.
Which forward voltage should I use for my LED?
Take it from the datasheet at the current you intend to run. As a rough guide, red LEDs often sit near 1.8 to 2.2 V, green and yellow a little higher, blue and white closer to 3 V and infrared lower, but a colour rule of thumb is no substitute.
What if the supply voltage equals the LED forward voltage?
The calculation is refused. With no voltage left over, the resistance would be zero and nothing would limit the current through the LED, so it could fail. The supply has to be higher than the forward voltage.
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/led-resistor \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"supply_voltage":5,"forward_voltage":2,"forward_current":0.02}'const res = await fetch("https://api.kit.forhosting.com/elec/led-resistor", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"supply_voltage": 5,
"forward_voltage": 2,
"forward_current": 0.02
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/led-resistor",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"supply_voltage": 5,
"forward_voltage": 2,
"forward_current": 0.02
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/led-resistor", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"supply_voltage":5,"forward_voltage":2,"forward_current":0.02}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"supply_voltage":5,"forward_voltage":2,"forward_current":0.02}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/led-resistor", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"supply_voltage": 5,
"forward_voltage": 2,
"forward_current": 0.02
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.led_resistor",
"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 computes both values deterministically from a JSON body, rejects cases where forward voltage exceeds or equals the supply, and recommends a commercial wattage with a standard two-times safety margin so hobbyists, firmware teams and classroom tools get a complete bill of materials number rather than a bare ohm figure.
What supply voltage, forward voltage and current mean for one LED
Send all three fields as finite JSON numbers or numeric strings; aliases such as Vs, Vf and If are accepted when you prefer textbook symbols. Magnitudes must stay within published finite bounds so Worker isolates remain stable. Identical inputs always produce identical JSON because the path is pure arithmetic with no network, no random seed and no wall clock.
Formulas, power rating, headroom errors and reading the response
When forward voltage exceeds supply, the request fails with invalid input and a message that names both voltages, because a series resistor cannot drop a negative headroom.
API usage, pricing, panels, education and practical limits
Call the free in-browser LED resistor calculator for breadboard work, Arduino pin protection, panel indicators and lab exercises without opening a spreadsheet. Authors shipping SDKs can map resistance ohms and recommended power watts directly into BOM lines that match what technicians stock in drawers. Send a JSON body with supply voltage, forward voltage and forward current in volts and amperes. Use the prepaid API when a parts configurator, IoT manufacturing tool, electronics homework oracle or CAD plugin 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 datasheets list milliamps or millivolts, and with power calculators when you next need branch watts for a multi-LED board. This product does not model PWM average current, constant-current drivers, multi-string ballast networks or thermal derating beyond the fixed two-times wattage margin; for pulsed or high-power emitters use a dedicated driver topology.
Integration use cases
Classroom and homework oracles
Electronics courses pin deterministic JSON for R equals (Vs minus Vf) over If drills, including intentional Vf-exceeds-supply cases that must fail with invalid_input.
Technical questions
How do I calculate the series resistor for an LED?
Send supply_voltage (Vs), forward_voltage (Vf) and forward_current (If in amperes). The response returns resistance_ohms as (Vs − Vf) / If and power_watts as If · (Vs − Vf).
Should I enter current in milliamps or amps?
Enter amperes. For a typical 20 mA indicator LED use forward_current: 0.02. Aliases If, current, led_current, i_f and amps are accepted.
How is the recommended power rating chosen?
Power dissipation is If times the voltage drop. recommended_power_watts is the smallest common commercial wattage at or above twice that dissipation (power_margin is always 2).