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Pull-Up Resistor Calculator

Enter your supply voltage, output low voltage and sink current to size a pull-up resistor.

Result

967 Ω
11.27 mW
1,000 Ω (next E12)

How it is calculated

The minimum resistor is set so that, when the output pulls the line low, the current stays within the sink rating: Rmin = (Vcc − VOL) / IOL, with voltages in volts and IOL entered in mA (the tool divides by 1000). The power is P = Vcc² / Rmin, shown in mW; it uses the full supply voltage, so it is a slight over-estimate. The recommended value is the next E12 standard value strictly above Rmin, even if Rmin already sits exactly on an E12 value. The sink current must be above 0 and Vcc above VOL.

Worked example

A 3.3 V line is driven by a chip whose output-low voltage is 0.4 V at a sink current of 3 mA. The voltage across the resistor is 3.3 − 0.4 = 2.9 V. Rmin = 2.9 V / 0.003 A = 967 Ω. The power is 3.3² / 966.7 = 11.27 mW. The next E12 value above 967 Ω is 1,000 Ω, which the tool recommends. Any resistor of at least this value keeps the sink current within 3 mA.

Try this example →

Common mistakes

  • Entering the sink current in amps (0.003) instead of mA (3). The result would be 1000 times too large.
  • Treating Rmin as the value to use. It is the lowest allowed; a larger resistor draws less current when the line is low, but together with the line capacitance it also slows the rising edge.
  • Using the wrong VOL. Take the figure from the data sheet at the sink current you enter, not 0 V, otherwise the resistor is a bit too small.
  • Rounding the recommended value down to a nearby standard resistor. A lower value than Rmin pushes the output above its rated sink current.
  • Taking the mW figure as the rating needed for continuous use. It is only drawn while the line is held low, so the average power is lower.

About this calculator

In digital logic, a pull-up resistor connects a signal line to the supply voltage so it reads a stable HIGH when nothing is actively driving it, and drops LOW when a device (like an open-drain output or a switch to ground) sinks current through it. This calculator finds the minimum resistor value that still guarantees a valid logic-low voltage given the supply voltage, the driving device's rated output-low voltage, and its maximum sink current — plus the power the resistor dissipates at that current.

Electronics and firmware engineers size pull-ups constantly when working with I2C buses, open-drain GPIO pins, push-button inputs, and reset lines: too large a resistor slows signal transitions and makes the line noise-prone, while too small a value wastes power and can exceed the sinking device's current rating. Getting the value right is a routine step in schematic design for microcontroller and embedded systems boards.

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Comments (1)

  • Editorial team

    For a button that pulls the line to ground, the resistor also sets how much current flows while it is held down: Vcc divided by R. At 3.3 V a 1 kΩ pull-up wastes 3.3 mA the whole time the button is pressed, a 10 kΩ one only 0.33 mA, which matters on a battery. Post the Vcc, VOL and sink current for your chip and the value you ended up using.

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