I2C Pull-up Resistor Calculator

Calculate optimal I2C pull-up resistor values based on bus capacitance and speed mode. Supports Standard, Fast, and Fast Mode Plus.

I2CPull-upResistorBusCommunicationEmbedded

Calculator

Quick Presets

Bus Capacitance

Total SDA/SCL line capacitance

Supply Voltage (Vcc)

I2C Speed Mode

       Vcc           Vcc
        │             │
       ┌┴┐           ┌┴┐
       │ │ Rp        │ │ Rp
       └┬┘           └┬┘
        │             │
   SDA ─┼─────────────┼─ SCL
        │             │
     ┌──┴──┐       ┌──┴──┐
     │     │       │     │
     │ IC1 │       │ IC2 │
     │     │       │     │
     └─────┘       └─────┘

How to Use This Calculator

This I2C pull-up resistor calculator helps you find the optimal pull-up resistor values for your I2C bus based on bus capacitance, supply voltage, and speed mode requirements.

Estimate Bus Capacitance — Sum of all device input capacitances plus trace/wire capacitance
Enter Supply Voltage — Typically 3.3V or 5V
Select Speed Mode — Standard (100kHz), Fast (400kHz), or Fast Mode Plus (1MHz)
Click Calculate — Get the valid resistance range and recommended values

I2C Pull-up Formulas

R_max = t_r / (0.8473 × C_bus)

R_min = (V_cc - V_ol) / I_ol

Where t_r = max rise time, C_bus = total capacitance

Understanding the Constraints

Maximum Resistance: Limited by rise time requirements. Higher capacitance needs lower resistance to meet timing specs
Minimum Resistance: Limited by output sink current (I_ol). Must not exceed the device's ability to pull the line low
The 0.8473 factor: Derived from RC rise time to reach 0.7×V_cc (logic HIGH threshold)

I2C Speed Modes

Mode	Max Frequency	Max Rise Time	Max Capacitance	Sink Current
Standard Mode	100 kHz	1000 ns	400 pF	3 mA
Fast Mode	400 kHz	300 ns	400 pF	3 mA
Fast Mode Plus	1 MHz	120 ns	550 pF	20 mA
High Speed	3.4 MHz	—	400 pF	Special

Note: High Speed mode requires active pull-ups (current sources) rather than resistive pull-ups.

Estimating Bus Capacitance

Sources of Capacitance

Device Input Capacitance: 3-10 pF per device (check datasheets)
PCB Trace: ~1-2 pF/cm for typical trace widths
Connector: 2-5 pF per connector
Wire/Cable: 30-100 pF/meter depending on cable type

Example Calculation

3 devices on a 10cm PCB:

3 devices × 10 pF = 30 pF
10 cm trace × 1.5 pF/cm = 15 pF
Total: ~45 pF (add 20% margin = 54 pF)

Frequently Asked Questions

Can I use different pull-up values for SDA and SCL?

Typically you use the same value for both lines. However, if one line has significantly different capacitance (e.g., longer trace), you could optimize each independently.

What if my calculated range is very small or invalid?

This usually means your bus capacitance is too high for the speed mode. Solutions: reduce the number of devices, shorten traces, use a bus buffer/extender, or drop to a slower speed mode.

Should I use the internal pull-ups on my microcontroller?

Internal pull-ups (typically 20-50kΩ) are usually too weak for proper I2C operation, especially at Fast Mode speeds or with any significant bus capacitance. Always use external pull-ups for reliable communication.

Where should I place the pull-up resistors?

Place pull-ups at the master device end of the bus, or as close to the master as practical. Use only one pair of pull-ups per bus, not one pair per device.

Verify Your Component Selections

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