Part of Circuit and resistor calculators
Pull-Up Resistor Calculator
Size a pull-up resistor from voltage and sink-current limits, with dissipation and rating guidance.
Inputs
Size a pull-up from the allowed low-level voltage and sink current.
A lower resistance gives a stronger pull-up and faster edges, but increases sink current and power.
Results
Actual selection may also be limited by bus capacitance, rise time, leakage current, and the device's guaranteed sink capability.
Use a pull-up resistor to define a logic high when the driver only pulls low
Open-drain outputs, open-collector outputs, reset lines, interrupt lines, and some logic inputs need a pull-up to define the high state. The useful first check is whether the pulling device can meet the low-level voltage at the chosen sink current.
Open-drain outputs
Choose a value that the output can pull low without exceeding its current or VOL limit.
Reset and enable pins
Set a defined high state while limiting current when the signal is forced low.
Shared logic lines
Check the static value before separately checking capacitance and rise time.
Equations and static model
The calculator sizes the resistor from the low-state condition. It assumes the pull-down device is active and the line is held at the specified low-level voltage.
Pull-up resistance
Choose resistance from the allowed low-level voltage and sink-current limit.
Low-state resistor power
Check resistor dissipation when the output is actively pulled low.
RC rise-time context
A static pull-up calculation does not prove the edge is fast enough for the bus or input.
Vpullup - Pull-up voltage
Unit: volts (V)
Voltage applied to the pull-up resistor.
VOL - Low-level voltage
Unit: volts (V)
Maximum acceptable low-level voltage while the device is sinking current.
Isink - Sink current
Unit: amps (A)
Current the pulling device must sink while holding the line low.
Rpullup - Pull-up resistance
Unit: ohms (Ω)
Resistor value that sets the low-state current at the chosen limit.
Worked example
The example below is checked against the same pull-up calculation helper used by the calculator.
Design question: A 3.3 V open-drain signal must stay below 0.4 V while the device sinks 1 mA. What pull-up value is implied?
Inputs: Vpullup = 3.3 V, VOL = 0.4 V, Isink = 1 mA.
Voltage across resistor: 3.3 V - 0.4 V = 2.9 V.
Pull-up resistance: R = 2.9 V / 1 mA = 2.9 kΩ.
Low-state power: P = 2.9 V × 1 mA = 2.9 mW.
Next check: confirm the selected standard value still satisfies VOL, leakage, bus capacitance, rise time, and static current budget.
Pull-up value trade-offs
A pull-up resistor is a compromise. The static low-state calculation is necessary, but it does not cover every interface requirement.
Lower resistance
- Improves noise margin against leakage and bias currents.
- Can improve rise time on capacitive lines.
- Increases sink current and low-state dissipation.
Higher resistance
- Reduces static current when the line is low.
- Can make the node more sensitive to leakage and noise.
- Can make edges too slow when capacitance is significant.
Assumptions and limitations
Static low-state check
The result is based on the line being pulled low. It does not prove high-state rise time.
Leakage not included
Input leakage, powered-down devices, ESD structures, and temperature can shift the effective logic level.
Rise time is separate
Bus capacitance, trace length, cable capacitance, and protocol timing must be checked separately for interfaces such as I²C.
Related calculators and next checks
Follow the next check based on whether the concern is dissipation, basic circuit values, units, or interface behaviour.
Resistor power calculator
Check dissipation and wattage margin for the low-state pull-up resistor.
Ohm's Law calculator
Sanity-check voltage drop, current, resistance, and power.
Engineering conversion calculator
Convert mA, µA, nF, ns, and related engineering units.
Circuit and resistor hub
Follow related resistor and interface workflows.
Pull-up resistor trade-offs guide
Review static current, leakage, logic margin, and rise-time trade-offs.
FAQ
Is 10 kΩ always a good pull-up value?
No. 10 kΩ is common, but the correct value depends on sink-current limits, leakage, noise margin, bus capacitance, rise-time requirements, and static current budget.
Why not use the smallest possible pull-up resistor?
A smaller pull-up gives stronger and faster edges, but it increases low-state current, resistor dissipation, and stress on the open-drain or open-collector device.
Does this calculator check I2C or bus timing?
No. It checks the static low-state current and resistor power. Protocol timing still needs bus capacitance and rise-time calculations against the relevant interface limit.
Engineering reference
Equations, assumptions, and design guidance
Sizes a pull-up from the allowed low-level voltage and the current the pulling device can sink.
Equations and variables
R = (Vp - Vol) / IsinkP = (Vp - Vol) * Isink- Vp
- Pull-up voltage (V)
- Vol
- Allowed low-level voltage (V)
- Isink
- Sink current (A)
Assumptions and limitations
Assumptions
- The low-level driver behaves as a current sink within its guaranteed limit.
- The line is evaluated at DC low level.
Limitations
- Rise time, bus capacitance, leakage, parallel pull-ups, and protocol-specific limits must be checked separately.
Worked example and design use
3.3 V open-drain line
Inputs: Vp = 3.3 V, Vol = 0.4 V, Isink = 1 mA
Outputs: R = 2.9 kOhm, P = 2.9 mW
Design guidance
- Lower pull-up resistance improves edge speed but increases sink current.
- For I2C and similar buses, verify rise-time limits from bus capacitance.