I2C pull-up resistor calculator

I2C needs a pull-up resistor on SDA and one on SCL. Too small and the chips cannot pull the lines low; too big and the lines rise too slowly. Enter the speed, the voltage and the bus capacitance to get the range that works.

Results are arithmetic on published formulas, not measurements. Check them on your hardware. Free, no sign-up

What is a pull-up resistor?

I2C chips never drive SDA and SCL high. Their outputs are open-drain: they can only connect the line to ground, or let go of it. A pull-up resistor to the supply brings the line back up whenever no chip is holding it low. Every I2C bus needs one on SDA and one on SCL.

Why there is a minimum and a maximum

Minimum: when a chip pulls the line low, current flows through the pull-up into that chip. The specification only guarantees that a chip can sink 3 mA while keeping the line at 0.4 V or less (20 mA in Fast-mode Plus), so a smaller resistor would ask for more current than the chip can take.

Maximum: every wire and every chip pin adds capacitance, and the resistor has to charge it. The line must rise from 30 % to 70 % of VDD within the rise time the speed allows: 1000 ns at 100 kHz, 300 ns at 400 kHz, 120 ns at 1 MHz.

Estimating the bus capacitance

Each chip pin adds up to 10 pF (the limit in the specification) and wiring adds more: short breadboard wires a few tens of picofarads, a metre of cable often around 100 pF. For a small hobby setup, 50 to 200 pF is a reasonable guess. The specification allows at most 400 pF (550 pF in Fast-mode Plus).

Pull-ups that are already there

Many breakout boards (BME280 sensors, SSD1306 displays, MPU6050 modules) already carry pull-ups, often 4.7 kΩ or 10 kΩ. Several modules on one bus put their pull-ups in parallel: two 4.7 kΩ make about 2.35 kΩ. Check the result against the smallest allowed value.

The formula

Rp(min) = (VDD − VOL(max)) / IOL     VOL(max) = 0.4 V, IOL = 3 mA (20 mA in Fast-mode Plus)
Rp(max) = tr / (0.8473 × Cb)         tr = 1000 ns, 300 ns or 120 ns; Cb = bus capacitance
tr      = 0.8473 × Rp × Cb          0.8473 = ln(0.7 / 0.3): the time to rise from 30 % to 70 % of VDD

Source: NXP UM10204, I2C-bus specification and user manual, section 7.1 “Pull-up resistor sizing” and table 10 (rise time limits).

Worked example

An ESP32 at 3.3 V with a sensor and a display on short wires, about 100 pF, at 400 kHz: Rp(min) = (3.3 − 0.4) V / 3 mA = 967 Ω, Rp(max) = 300 ns / (0.8473 × 100 pF) = 3.54 kΩ. The calculator suggests 1.8 kΩ (rise time 153 ns).

A 10 kΩ pull-up would give a 847 ns rise time: too slow for 400 kHz, but fine at 100 kHz, where up to 11.8 kΩ is allowed.

Questions and answers

Is 4.7 kΩ always right for I2C?

It is a common choice for 100 kHz at 3.3 V or 5 V with short wires, and there it usually works. At 400 kHz, or with long wires and many devices, it can be too weak: check it against the largest allowed value for your bus.

Do I need pull-ups if my module already has them?

Usually not. Most breakout boards include pull-ups on SDA and SCL. Check the module’s schematic, or measure the resistance between SDA and VCC with the power off. With many modules on one bus, their pull-ups add up in parallel and can become too strong.

Can I use the microcontroller’s internal pull-ups?

Only for short, slow buses. Internal pull-ups are weak, typically 20 to 80 kΩ depending on the chip, so the lines rise slowly. They can do for a quick test at 100 kHz with one sensor on short wires; use external resistors for anything else.

What happens if the pull-up is wrong?

Too strong (too small) and a chip cannot pull the line low enough, so bits are misread. Too weak (too large) and the edges become slow ramps; at higher speeds the bits blur, devices stop answering or the bus hangs. Both can look like a “device not found” error.

Should I pull up to 3.3 V or 5 V?

To the supply of the chips on the bus. If a 3.3 V board such as an ESP32 or a Raspberry Pi Pico is on the bus, pull up to 3.3 V: pulling its pins up to 5 V can damage it. To mix 3.3 V and 5 V devices, use an I2C level shifter.

Learn the background

Free lessons of the BoardPilot course that explain the ideas behind this calculator:

UART, I2C and SPI Hardware basics for software developers

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