I²C Pull-Up & Rise-Time Utility

I2C Pull-Up & Rise Time Calculator

Use this I2C Pull-Up & Rise Time Calculator to estimate the valid pull-up resistor range from bus capacitance, supply voltage, low-level sink capability and I2C speed mode. Check the actual RC rise time, pull-up current and whether your selected resistor meets the chosen timing limit.

✓ Pull-Up Resistance ✓ Rise Time ✓ Bus Capacitance ✓ Sink Current ✓ I2C Speed Modes

Calculate I2C Pull-Up Range

● Ready

Bus Supply Voltage Voltage connected to the SDA and SCL pull-up resistors.

Estimated Bus Capacitance Total SDA/SCL capacitance including devices, PCB traces, cables and connectors.

Capacitance Unit

I2C Speed Mode Preset maximum rise-time values are used for planning.

Custom Maximum Rise Time

Custom Rise-Time Unit

Maximum LOW Output Voltage VOL value used for the pull-up minimum resistance calculation.

Allowed LOW Sink Current Current the active device is allowed to sink while maintaining the selected VOL.

Sink Current Unit

Selected Pull-Up Resistor Optional resistor value to validate against the calculated range.

Resistance Unit

Important: The calculated resistance window is a first-order electrical check. Real I2C buses also depend on device input capacitance, wiring, level shifters, pin sink capability, voltage thresholds, ringing and device-specific timing. Use the weakest pull-up that still gives sufficient rise-time margin while remaining above the required minimum resistance.

I2C Pull-Up Result Calculated

Recommended Electrical Resistance Window –

Minimum Rp –

Maximum Rp –

Selected Rp –

Selected Status –

Estimated Rise Time –

Rise-Time Limit –

LOW Sink Current –

Pull-Up Current at 0 V –

Bus RC Constant –

Bus Capacitance –

Supply Voltage –

Rise Margin –

Selected Pull-Up Position

Minimum Maximum

Calculation Breakdown

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What Is an I2C Pull-Up & Rise Time Calculator?

An I2C Pull-Up & Rise Time Calculator helps choose pull-up resistor values for the open-drain SDA and SCL lines used by an I2C bus. Unlike a push-pull output, an I2C device normally drives the line LOW but relies on a resistor to return the signal HIGH.

The resistor must therefore satisfy two opposing requirements. A resistor that is too large produces a slow RC rise time, while a resistor that is too small forces devices to sink excessive current whenever the bus is LOW.

This calculator estimates both limits and checks a selected resistor against the resulting electrical window.

I2C Pull-Up Resistor Maximum Formula

For an RC-charged I2C line, the commonly used 30%-to-70% rise-time approximation is:

tr ≈ 0.8473 × Rp × Cbus

Solving for the maximum resistor:

Rp(max) = tr(max) / (0.8473 × Cbus)

A larger resistor produces a longer rise time for the same bus capacitance.

I2C Pull-Up Resistor Minimum Formula

The pull-up cannot be arbitrarily small because a device pulling the line LOW must sink the resistor current.

Rp(min) = (VDD – VOL(max)) / IOL(max)

For a 3.3 V system using VOL = 0.4 V and 3 mA sink current:

Rp(min) = (3.3 – 0.4) / 0.003 ≈ 966.7 Ω

I2C Rise Time Formula

Once a pull-up resistor is selected, its approximate rise time can be estimated directly:

tr = 0.8473 × Rp × Cbus

For a 2.2 kΩ resistor and 100 pF bus:

tr = 0.8473 × 2200 × 100 pF ≈ 186.4 ns

Example: 3.3 V, 100 pF, Fast-Mode I2C

Assume:

VDD = 3.3 V VOL(max) = 0.4 V IOL(max) = 3 mA Bus Capacitance = 100 pF Rise-Time Limit = 300 ns Selected Rp = 2.2 kΩ

Minimum pull-up:

(3.3 – 0.4) / 0.003 ≈ 966.7 Ω

Maximum pull-up:

300 ns / (0.8473 × 100 pF) ≈ 3.54 kΩ

A 2.2 kΩ resistor lies inside that electrical range.

Why I2C Needs Pull-Up Resistors

I2C SDA and SCL are normally open-drain or open-collector signals. Devices can actively pull the line LOW, but they do not actively drive it HIGH in ordinary I2C operation.

The external pull-up resistor restores the HIGH level when every connected device releases the line. This architecture allows multiple devices to share the same wires without push-pull output contention.

Bus Capacitance and I2C Rise Time

The pull-up resistor charges the distributed capacitance of the I2C bus. Higher capacitance means the voltage rises more slowly for a given resistor.

Device Inputs

Every connected I2C device contributes some input and package capacitance.

PCB Traces

Longer or wider routing can contribute additional distributed capacitance.

Cables

Off-board cabling can add substantial capacitance compared with short PCB traces.

Level Shifters

Bidirectional translators and protection components can alter the effective bus loading.

Typical I2C Rise-Time Limits

The maximum permitted rise time depends on the selected I2C operating mode. This calculator includes commonly used planning presets.

I2C Mode	Nominal Bus Speed	Rise-Time Preset
Standard-mode	Up to 100 kbit/s	1000 ns
Fast-mode	Up to 400 kbit/s	300 ns
Fast-mode Plus	Up to 1 Mbit/s	120 ns

Use the custom mode when your device or bus specification requires a different rise-time limit.

Why 4.7 kΩ Is Common on I2C

A 4.7 kΩ pull-up is common because it often works well for moderate-voltage, low-to-medium-capacitance Standard-mode buses. It is not, however, a universal I2C resistor value.

At higher capacitance or faster bus speeds, 4.7 kΩ can be too weak and produce an excessively slow rising edge. On a lightly loaded low-speed bus, a smaller resistor may simply waste current without offering meaningful benefit.

Choose the resistor from the actual voltage, capacitance, speed and device sink capability rather than selecting 4.7 kΩ automatically.

I2C Pull-Up Current

When a device pulls the line LOW, current flows from the supply through the pull-up resistor into that device.

At a nonzero LOW voltage:

I = (VDD – VOL) / Rp

For 3.3 V, VOL = 0.4 V and Rp = 2.2 kΩ:

I = 2.9 / 2200 ≈ 1.318 mA

Why Smaller I2C Pull-Ups Rise Faster

The RC time constant is:

τ = Rp × Cbus

Reducing Rp lowers the RC time constant, so the bus voltage climbs toward VDD more rapidly after a device releases the line.

The tradeoff is increased LOW-state current. This is why I2C design requires both a lower and upper resistance limit rather than simply choosing the smallest resistor available.

I2C Pull-Up Resistor Range

A practical resistor should satisfy:

Rp(min) ≤ Rp(selected) ≤ Rp(max)

If the calculated minimum exceeds the maximum, the assumed combination of bus capacitance, required rise time and allowed sink current has no valid resistor solution under the simplified model.

In that situation you may need to reduce capacitance, use devices with higher permitted sink current, lower the bus speed or redesign the interconnect.

Separate Pull-Ups for SDA and SCL

Most I2C buses use one pull-up resistor on SDA and another on SCL. They are often the same value, but they do not have to be identical if the effective capacitance or timing requirements differ.

For critical designs, measure or estimate the capacitance of each line separately and validate both.

I2C Level Shifters and Pull-Ups

Bidirectional I2C level translators often require pull-up resistors on both voltage domains. The effective rise behavior then depends on the translator architecture and the capacitance present on each side.

Do not automatically treat both sides as one simple RC network unless the level-shifter datasheet supports that model.

Common I2C Pull-Up Mistakes

Always Choosing 4.7 kΩ

One resistor value cannot suit every bus capacitance, voltage and speed.

Ignoring Bus Capacitance

Capacitance directly controls RC rise time.

Using Very Small Pull-Ups

Strong pull-ups can exceed device sink-current capability.

Ignoring VOL

The minimum resistor should consider the device’s LOW-level voltage at the allowed sink current.

Using Frequency Alone

I2C specifications constrain rise time, not simply a resistor value for each clock frequency.

Ignoring Level Shifters

Translators can significantly change bus capacitance and rising-edge behavior.

I2C Pull-Up & Rise Time Calculator FAQs

How do I calculate an I2C pull-up resistor?

Calculate a minimum resistance from LOW-state sink current and a maximum resistance from the allowed rise time and bus capacitance. A practical pull-up should lie between those limits.

What is the I2C rise-time formula?

A commonly used first-order approximation is tr = 0.8473 × Rp × Cbus for the 30%-to-70% RC rise interval.

What is the maximum I2C pull-up resistor formula?

Rp(max) = tr(max) / (0.8473 × Cbus).

What is the minimum I2C pull-up resistor formula?

Rp(min) = (VDD – VOL(max)) / IOL(max).

Why does I2C require pull-up resistors?

I2C devices normally use open-drain signaling. Devices actively pull the line LOW and rely on external resistors to restore the HIGH state.

Is 4.7 kΩ always correct for I2C?

No. It is a common value, but suitability depends on bus voltage, capacitance, speed and device sink capability.

What happens if the I2C pull-up resistor is too large?

The RC charging time becomes longer and the rising edge may exceed the allowed rise-time specification.

What happens if the I2C pull-up resistor is too small?

LOW-state current increases and the device pulling the bus LOW may exceed its allowed sink current or VOL specification.

How does bus capacitance affect I2C?

Higher capacitance slows the rising edge and therefore requires a stronger pull-up for the same rise-time target.

What is the rise-time limit for 100 kHz I2C?

Standard-mode commonly uses a 1000 ns maximum rise-time limit.

What is the rise-time limit for 400 kHz I2C?

Fast-mode commonly uses a 300 ns maximum rise-time limit.

What is the rise-time limit for 1 MHz I2C?

Fast-mode Plus commonly uses a 120 ns maximum rise-time limit.

How do I estimate I2C pull-up current?

Use I = (VDD – VOL) / Rp while the line is being actively held LOW.

What is the RC time constant of an I2C line?

The first-order RC time constant is τ = Rp × Cbus.

Can SDA and SCL use different resistor values?

Yes. They are often equal for convenience, but each line can be designed independently when capacitance or timing differs.

Can I calculate I2C pull-ups accurately without knowing capacitance?

Not accurately. Bus capacitance is one of the main variables controlling the rising edge.

Does this calculator replace oscilloscope verification?

No. It provides a first-order design estimate. Measuring SDA and SCL on real hardware is recommended for timing-critical or heavily loaded buses.

Calculate I2C Pull-Up Resistance and Rise Time

Enter supply voltage, bus capacitance, I2C speed and sink-current limits to calculate the electrical pull-up resistor window and verify the rise time of your selected resistor.