ForHosting KIT · Developer Utilities

Pull-up resistor current calculator

This pull-up resistor current calculator applies Ohm's law to a digital input or open-drain output that is pulled to a positive supply through a resistor.

● BetaFree · in your browser

Run — free How it works & API

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Enter the supply voltage and resistance to find the current that flows when the signal is driven low, the equivalent sink-current requirement in amperes and milliamperes, and the resistor's ideal power dissipation. The calculation assumes a perfect zero-volt logic low, making it a clear first-pass check for component selection, GPIO loading, and power budgeting.

Understand what the pull-up current means

A pull-up resistor connects a signal node to a positive supply so the node has a defined high state when no device actively drives it. When an open-drain, open-collector, or ordinary digital output asserts a low state, that device creates a path toward ground. Current then flows from the supply, through the pull-up resistor, and into the output pin. This calculator treats the low state as an ideal zero volts, so the full supply voltage appears across the resistor. It computes current with I = V / R. That same current is reported as the logic-low sink current because the output device must absorb it continuously while holding the node low. Results appear in both amperes and milliamperes to make comparisons with datasheet limits convenient. The calculation is most useful as an initial design check: verify that the chosen resistor does not demand more sink current than the output can safely handle, and remember that every asserted-low signal consumes this current from the supply.

Choose a resistor using current, speed, and power

A larger pull-up resistance reduces logic-low current and static power, while a smaller resistance produces a stronger pull-up. The tradeoff is not purely about saving energy. The resistor must charge the signal's total capacitance, including traces, connectors, device pins, and probes. A very large value may create an edge that rises too slowly for the bus frequency or the receiving input. A very small value may exceed the driver's guaranteed sink-current rating or prevent it from maintaining a valid low voltage. Use this result beside the output pin's absolute maximum and recommended operating specifications, not as a replacement for them. The reported resistor power uses P = V² / R, which is the ideal dissipation while low. Select a resistor with comfortable power derating and account for temperature. For a signal that is low only part of the time, instantaneous sink current remains unchanged, although average supply power falls with the low-state duty cycle.

Interpret the ideal assumption in a real circuit

Real output transistors do not usually pull a signal to exactly zero volts. A datasheet specifies a maximum low-level output voltage, often called VOL, at a particular sink current. In the real circuit, the resistor sees approximately the supply voltage minus VOL, so actual current is slightly lower than the ideal value shown here. Treat this calculator's result as a conservative sink-current estimate when VOL is small compared with the supply. Then confirm the design iteratively: compare the calculated current with the datasheet test condition, inspect the corresponding VOL guarantee, and verify that the receiving device still recognizes that voltage as low. Also consider parallel pull-ups. If several resistors connect the same line to supplies that may be treated together, their combined conductance can increase the sink current substantially. Finally, check startup conditions, level-shifter topology, voltage tolerance, and leakage. Ohm's law answers the current question, but safe digital interfacing still depends on the limits and thresholds of every connected component.

What you can do with it

Check a microcontroller GPIO

Confirm that a GPIO can safely sink the current demanded by a pull-up when firmware drives the line low.

Estimate open-drain bus loading

Compare candidate pull-up values for an interrupt, reset, I²C, or other wired signal before checking rise-time constraints.

Budget static power

Calculate resistor dissipation and the supply current consumed whenever an active-low control signal remains asserted.

FAQ

What does the calculation cost?

It runs free in the browser. API execution costs $0.002 per request.

Is pull-up current the same as sink current?

Under the ideal zero-volt low-state assumption, yes. All current through the pull-up must be sunk by the device holding the node low.

Which resistance unit should I enter?

Enter ohms. For a 4.7 kΩ pull-up, enter 4700; for a 10 kΩ pull-up, enter 10000.

Does current flow when the signal is high?

Ideally, no steady current flows through the pull-up when both ends are at the supply voltage. Real circuits can have leakage current.

Why can the measured current be lower?

A real output has a nonzero low voltage, so the resistor voltage is supply voltage minus VOL rather than the entire supply voltage.

Does this calculator select an I²C pull-up value?

It checks low-state current and power only. I²C selection also requires bus capacitance, rise-time, frequency, and device voltage limits.

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/pull-up-resistor

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/pull-up-resistor \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"supply_voltage":5,"resistance_ohms":10000}'

const res = await fetch("https://api.kit.forhosting.com/elec/pull-up-resistor", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "supply_voltage": 5,
    "resistance_ohms": 10000
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/elec/pull-up-resistor",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "supply_voltage": 5,
    "resistance_ohms": 10000
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/pull-up-resistor", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"supply_voltage":5,"resistance_ohms":10000}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"supply_voltage":5,"resistance_ohms":10000}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/pull-up-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,
  "resistance_ohms": 10000
}

Example response

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

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.

Read the full KIT documentation →