I²C pull-up resistor calculator

Find the permissible I²C pull-up resistance range from capacitance, rise time and device sink-current limits. Check standard resistor values.

Calculate the effective passive pull-up resistance for one SDA or SCL line. Enter the guaranteed IOL current and corresponding VOL,max from the weakest driver’s datasheet.

Pull-up supply VCC (V)

Guaranteed low-level voltage VOL,max (V)

Guaranteed sink current IOL at VOL,max (mA)

Total capacitance of one line Cb (pF)Add the capacitance of every pin, PCB trace, connector and cable on this SDA or SCL line. Use pin capacitances from datasheets and a worst-case estimate for the wiring.

Rise-time limit preset

Maximum 30–70% rise time tr (ns)

Resistor tolerance (±%)

A preset changes only the rise-time limit. Also check IOL and VOL,max in every device’s datasheet, especially for Fast-mode Plus and low supply voltages.

Minimum actual resistance0.966667 kΩDriver current limit

Maximum actual resistance1.77033 kΩRise-time limit

Nominal resistance range including tolerance: 1.01754–1.68603 kΩ.

E24 values meeting both limits across resistor tolerance
Nominal (kΩ)	Slowest rise (ns)	Current at VOL,max, Rmin (mA)
1.1	195.726	2.77512
1.2	213.52	2.54386
1.3	231.313	2.34818
1.5	266.9	2.03509
1.6	284.693	1.90789

Method and assumptions

Rp,min = (VCC − VOL,max) / IOL

Rp,max = tr / (0.8473 × Cb)

The equations use volts, amperes, seconds and farads. With fractional tolerance t, the nominal resistance must be at least Rp,min / (1 − t) and at most Rp,max / (1 + t).

TI comparison example: 3.3 V, 0.4 V VOL,max, 3 mA IOL, 200 pF and 300 ns gives approximately 967 Ω–1.77 kΩ before tolerance.

Combine parallel pull-ups into one effective resistance and include board, cable and all pin capacitances. This model assumes a passive RC rise from 30% to 70%; it does not model active pull-ups, buffers, leakage or nonlinear level translators.

Supply and capacitance tolerances are not included. Use worst-case inputs, verify pin voltage ratings and low-level compatibility, and measure rise time and VOL on the actual board. The table evaluates current at VOL,max; actual current can be higher when VOL is lower.

Source, presets and comparison example: Texas Instruments — I2C Bus Pullup Resistor Calculation (SLVA689).

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