Choose practical pull-up or pull-down values for logic inputs, buses, and signal lines.
Enter your supply voltage and target pull current; we’ll suggest an ideal resistor and
the nearest E12-series value, then show the actual current.
Pull-Up / Pull-Down Calculator
Supply voltage, target pull current, and configuration (up or down). We’ll compute
the ideal resistor, the nearest E12 value, and the resulting current.
Quick presets
What this calculator does (and doesn’t do)
This helper uses Ohm’s Law to propose pull resistors that draw a chosen current
at your logic voltage:
Rideal = V ÷ I (with I in amps)
We then map Rideal to the nearest
E12 resistor value (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 × powers of ten).
Finally we compute the actual current with that real resistor.
Units used on this page:
Voltage in volts: V
Target pull current in milliamps: mA
Resistance in ohms: Ω
Scope.
This tool is aimed at logic-level digital electronics: microcontroller inputs,
open-drain/collector outputs, buses like I²C, and simple detection lines.
It does not model capacitance, cable effects, or noise sources; treat it as a
first pass, then refine with your real layout and timing requirements.
Design checklist for pull resistors
After you run the numbers, use this checklist as a quick sanity check.
Input leakage and bias.
Check the input’s leakage current and internal pull-ups. Your external pull
should be significantly stronger than leakage but not so strong that it
wastes current.
Rise time / edge speed.
On buses like I²C, the pull-up, line capacitance, and speed mode
determine the rise time. If your edges are slow, consider a slightly
lower resistor (higher current).
Absolute max ratings.
Confirm that the pull resistor never drives more current into or out of a pin
than its absolute maximum ratings allow, even under fault conditions.
Total current budget.
If many lines share the same pull voltage, sum the worst-case pull currents.
Make sure the regulator and power budget can handle it.
Noise margin.
Weak pulls plus long unshielded runs can make lines vulnerable to noise.
When in doubt, strengthen the pull or add filtering/termination.
Examples from Celtic Engineering Solutions
These examples are simplified versions of situations where pull resistors were key
to clean, reliable behavior.
Example A
3.3 V microcontroller input with external switch
A pushbutton connected a GPIO pin to ground when pressed. When released,
the line needed to float high cleanly.
• V = 3.3 V, target I ≈ 0.1 mA
• Rideal ≈ 33 kΩ, nearest E12 = 33 kΩ
• I ≈ 3.3 V ÷ 33 kΩ ≈ 0.1 mA
The current is small enough for battery use, and the line is still
well defined when the switch is open.
Example B
5 V open-collector signal from external device
An external sensor exposed an open-collector output that needed to be read
by a 5 V MCU.
• V = 5 V, target I ≈ 0.3 mA
• Rideal ≈ 16.7 kΩ → nearest E12 ≈ 15 kΩ or 18 kΩ
• Using 18 kΩ keeps current ≈ 0.28 mA and eases loading on the sensor.
The result was a crisp logic level with very little idle current.
Example C
I²C bus at 3.3 V with longer traces
A board used I²C at 3.3 V with multiple devices and a moderate bus length.
To keep rise times within spec, the pull-ups needed to be stronger than the
gentle MCU-input case.
• V = 3.3 V, target I ≈ 1 mA
• Rideal ≈ 3.3 kΩ, nearest E12 = 3.3 kΩ
• I ≈ 1 mA per line when low.
The bus met timing comfortably without overstressing devices.
Example D
12 V detection line feeding an optocoupler
A 12 V “present/not-present” signal needed to drive an optocoupler LED
through a pull-up resistor.
• V = 12 V, desired LED current ≈ 1 mA
• Rideal ≈ 12 kΩ, nearest E12 = 12 kΩ
• The optocoupler input current was well within spec, with modest dissipation
in the resistor.
The pull-up both limited current and provided a simple way to sense line state.
Unsure how strong your pull should be?
If your pull resistor is part of a noisy bus, a long cable run, or any safety-relevant
detection circuit, treat this helper as a first pass. Send a short note with your
voltage, timing, and layout constraints, and we’ll help you choose a safe, robust value.
