I²C Pull-up Resistor Calculator

Estimate the maximum I²C pull-up resistor from bus capacitance, PCB trace geometry, and the selected rise-time mode.

Max pull-up resistor–

Total bus capacitance–

Trace capacitance–

Estimated impedance–

Rise-time limit–

Sizing the I²C pull-up

Rp(max) = t_r / (0.8473 × C_bus)

The maximum pull-up is set by the bus rise-time limit for the selected I²C speed and the total bus capacitance — trace, device pins, connectors, and any extra. When you don't know the trace capacitance, it is estimated from microstrip or stripline geometry.

This is the largest pull-up that still meets the rise-time limit. Your final resistor must also be small enough for noise margin and large enough that every device can sink the low-level current.