I2C Pull-up Resistor Calculator

I2C bus issues that only show up at certain clock speeds or bus lengths are almost always a pull-up resistor problem. This calculator finds the valid pull-up range straight from the I2C-bus specification's sink-current and rise-time limits, for your actual bus capacitance and target clock speed, rather than defaulting to whatever value ended up in the last design.

I2C Pull-up Resistor

Bus pull-up range from the I2C-bus specification's sink-current and rise-time limits

Show your work

Bus speed mode

Vdd

Total bus capacitance

Sum of every device + trace on the busVOL max

Sink current (IOL)

Rp min (sink-current limit)

967 Ω

Rp max (rise-time limit)

3.54 kΩ

Suggested E24 value

1.80 kΩ

Rise time at suggested value

153 ns

Two pull-ups are needed — one on SDA, one on SCL — each sized from this same range. Most designs use matched values on both lines.