Pull Up Resistor Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

I2C speed: The bus mode you plan to use. Faster modes allow less rise time, so they usually need a lower pull-up resistance.

Pull-up voltage (V): The voltage connected to the pull-up resistors. This sets the high signal level and affects low-signal current.

Bus capacitance (pF): The total capacitance on one I2C signal line from chips, traces, wires, and connectors. More capacitance makes the line rise more slowly.

Allowed low voltage (V): The highest voltage that still counts as a low signal when a device pulls the line down.

Weakest sink current (mA): The lowest current limit of any device that must pull the I2C line low.

Existing pull-ups on this line (ohms): Pull-up resistors that are already connected to the same signal line, such as resistors on sensor breakout boards.

Final value with your resistor: The effective resistance after the checked resistor is placed in parallel with any existing pull-ups.

Rise time: The estimated time for the signal to rise from 30% to 70% of the pull-up voltage.

Mistake: Entering the chip supply voltage instead of the actual Pull-up voltage (V) .
Fix: Use the voltage connected to the pull-up resistors, such as 3.3 V or 5 V.

Mistake: Leaving breakout-board resistors out of Existing pull-ups on this line (ohms) .
Fix: List every pull-up already connected to the same SDA or SCL line, separated by commas.

Mistake: Using a guessed Bus capacitance (pF) that is far too low for long wires or many modules.
Fix: Raise the estimate when the bus has long leads, connectors, several devices, or a large board trace.

Mistake: Treating Resistor you may use (ohms) as the final value when existing pull-ups are also present.
Fix: Read Final value with your resistor , because parallel resistors make the final resistance lower.

Mistake: Leaving Weakest sink current (mA) blank when a device data sheet allows less than the I2C default.
Fix: Enter the lowest sink-current rating from the connected devices.

Mistake: Ignoring a failing Your resistor check because one detail looks good.
Fix: A usable value must pass both Rise time with your resistor and Low-signal current with your resistor .

• The calculator is for passive resistor pull-ups on an I2C-style open-drain or open-collector bus.
• Results depend strongly on Bus capacitance (pF). If capacitance is only guessed, treat the answer as a design estimate and verify the real bus if timing is tight.
• The default Weakest sink current (mA) is 3 mA for Standard and Fast mode, and 20 mA for Fast-mode Plus. Enter a device-specific value if a data sheet is lower.
• The default Allowed low voltage (V) is 0.4 V. Some devices or voltage levels may use a different low-level limit.
• Adding another pull-up resistor can only lower the final resistance. It cannot fix a bus where Existing pull-ups by themselves are already too strong.
• The math does not model active pull-ups, rise-time accelerators, level shifters, bus buffers, leakage current, signal ringing, or layout problems.
• I2C normally needs matching pull-ups on both SDA and SCL, but the capacitance and existing resistors can differ between lines.

Safe range

The calculator first finds the allowed final pull-up resistance range. The smallest safe value is set by low-signal current. The largest safe value is set by rise time.

R_min = (V_DD - V_OL_max) / I_OL

R_max = t_r / (0.8473 * C_b)

V_DD is Pull-up voltage (V), V_OL_max is Allowed low voltage (V), I_OL is Weakest sink current (mA) converted to amps, and C_b is Bus capacitance (pF) converted to farads. The 0.8473 factor is the 30% to 70% rise-time part of an RC charge curve.

Checked resistor

If Existing pull-ups on this line (ohms) are entered, the calculator combines them with Resistor you may use (ohms) as parallel resistors.

R_parallel = 1 / (1/R_1 + 1/R_2 + ... + 1/R_n)

It then estimates the checked rise time and the current a device must sink while holding the line low.

t_r_checked = 0.8473 * R_effective * C_b

I_low = (V_DD - V_OL_max) / R_effective

The checked value passes only when the final effective resistance is at least R_min and at most R_max.

Recommendation

When a safe range exists, the exact target is the middle of the safe range on a ratio scale.

R_target = sqrt(R_min * R_max)

If there are no existing pull-ups, the recommended resistor is the target rounded to the selected common series when possible. If existing pull-ups are present and are higher than the target, the calculator solves for the extra resistor that would make the final parallel value land near the target.

R_add_exact = 1 / (1/R_target - 1/R_existing)

For E12 or E24 rounding, the calculator tests common resistor values and chooses the candidate whose final effective resistance is inside the safe range and closest to the target on a ratio scale.

best = smallest abs(ln(R_candidate_effective / R_target))

Mini example

For 3.3 V Fast-mode I2C, 100 pF bus capacitance, 0.4 V allowed low voltage, and the default 3 mA sink current, the safe range is about 967 ohms to 3541 ohms. The target is about 1851 ohms, so an E24 recommendation is 1800 ohms. A checked 4700 ohm resistor gives about 398 ns rise time and 0.617 mA low-signal current, so it is too weak for the 300 ns Fast-mode rise-time limit even though its current is safe.