How is the minimum resistor value calculated?

R_min = Vcc / I_max, where I_max is the maximum current the driving device is rated to sink (pull-up) or source (pull-down) when it actively pulls the line. Going below R_min risks exceeding that current rating.

How is the maximum resistor value calculated?

For a pull-up, R_max = (Vcc - V_threshold) / I_leak, using the input high threshold. For a pull-down, R_max = V_threshold / I_leak, using the input low threshold. Going above R_max risks the released line drifting past the logic threshold due to leakage current.

What input leakage current should I use?

Check the receiving device datasheet for input leakage current (often 1uA or less for a single CMOS input). If several devices share the same line, add up their leakage currents since they all draw through the same resistor.

Why do smaller resistors like 4.7k show up so often for I2C?

A few kilo-ohms sits comfortably inside the valid range for typical 3.3V-5V logic with a few milliamps of sink current, while still keeping the bus rise time reasonable and power dissipation low, so it became a common default even though the exact optimum depends on your bus capacitance.

What is the difference between pull-up and pull-down?

A pull-up resistor connects the line to the supply so it rests high and is actively pulled low. A pull-down resistor connects the line to ground so it rests low and is actively pulled high. Pick whichever matches your default idle state.

Does this account for the resistor's power dissipation?

Yes. The calculator shows the power dissipated in the resistor when the line is actively pulled (worst case), using P = Vcc^2 / R, so you can check it against the resistor's power rating.

Pull-Up Resistor Calculator

Enter supply voltage, max sink current, and the logic threshold to get a valid pull resistor range plus a recommended value.

Circuit Parameters

Supply Voltage Vcc (V)

Max Sink/Source Current I_max (mA)

Max current the driving device (GPIO, open-drain output, switch) can safely sink or source.

Input High Threshold V_IH (V)

Input Leakage Current (uA)

Sum leakage current across all inputs sharing this line.

Target Pull Current (mA, optional)

Used to compute a single recommended resistor value.

Results

Minimum R

Recommended R

Maximum R

Power at Recommended R (worst case) --

Standard Preset Values

Green = inside the valid range. The value closest to the recommended R is marked Best.

Summary

This calculator finds the resistor range for a pull-up or pull-down on a digital logic input. The lower bound keeps the current draw within the driving device's sink or source rating when the line is actively pulled. The upper bound keeps the released line above (or below) the logic threshold despite input leakage current. It also flags which standard E12 resistor values fall inside that range.

How it works

Choose pull-up (resistor to supply) or pull-down (resistor to ground).
Enter the supply voltage and the max current the driving device can sink or source.
Enter the logic threshold voltage (or pick a TTL/CMOS preset) and the expected input leakage current.
Optionally enter a target current to get a single recommended resistor value.
Read the valid resistor range and check which standard preset values fall inside it.

Use cases

Size a pull-up for an open-drain output like I2C SDA/SCL.
Choose a pull-up for a button or switch wired to a microcontroller GPIO pin.
Pick a pull-down so an unconnected input reads a defined logic low at power-up.
Check whether a common 4.7k or 10k resistor is actually valid for a given supply and current budget.
Estimate resistor power dissipation before committing to a value in a design.
Verify a pull resistor design still meets the logic threshold with multiple fan-out inputs (higher leakage).

Frequently Asked Questions

Last updated: 2026-09-09 · Reviewed by Nham Vu