Calculator Inputs
Use the form below to estimate timing limits, throughput, and pull-up suitability.
Formula Used
This calculator uses practical first-order timing relationships often applied during embedded hardware design. It is intended for planning and comparison, not final compliance certification.
1) Rise Time Estimate
tr ≈ 0.8473 × Rp × Cbus
2) Low-Level Sink Current
Isink = VDD / Rp
3) Timing Floor for Practical Period
Tmin ≈ tLOW(min) + tHIGH(min) + tr + tf + 2 × tpd
4) Practical Maximum Clock
fpractical = min(fmode, 1 / Tmin)
5) Recommended Working Clock
frecommended = fpractical / (1 + safety margin)
6) Transaction Time and Throughput
Bits ≈ 2 + 9 × (address bytes + data bytes)
Ttransaction = Bits / f + stretch delay
How to Use This Calculator
- Select the target I2C mode that matches your intended design range.
- Enter the bus voltage and effective pull-up resistor used on each line.
- Estimate total capacitance from devices, traces, cables, and connectors.
- Add expected fall time, long-route length, and propagation delay if relevant.
- Enter the address bytes, payload bytes, and any expected clock stretching.
- Set a design safety margin and optional desired clock target.
- Press Calculate I2C Speed to view results above the form.
- Review warnings, compare recommended and practical clocks, then validate on hardware with a scope.
Example Data Table
These examples show how changing pull-ups and capacitance affects practical bus speed and rise-time margin.
| Mode | VDD | Pull-Up | Capacitance | Data Bytes | Practical Clock | Rise Time | Status |
|---|---|---|---|---|---|---|---|
| Standard | 3.3 V | 4.7 kΩ | 100 pF | 8 | 100 kHz | 398 ns | Comfortable margin |
| Fast | 3.3 V | 2.2 kΩ | 120 pF | 16 | 400 kHz | 224 ns | Within mode limits |
| Fast Plus | 3.3 V | 1.0 kΩ | 80 pF | 32 | 1.00 MHz | 68 ns | Good for short buses |
| Fast | 5.0 V | 4.7 kΩ | 250 pF | 24 | 257 kHz | 996 ns | Pull-up should be lowered |
| High Speed | 3.3 V | 680 Ω | 35 pF | 8 | 3.40 MHz | 20 ns | Segment looks healthy |
Frequently Asked Questions
What does this I2C speed calculator estimate?
It estimates rise time, practical maximum clock, pull-up current, transaction time, and payload throughput from your voltage, pull-up resistor, bus capacitance, timing mode, and safety margin.
Why does bus capacitance matter so much?
I2C lines rise through pull-up resistors. More capacitance slows that rise. Slow edges reduce timing margin, limit maximum clock speed, and can cause setup, hold, or logic-threshold problems.
How is rise time calculated here?
The calculator uses the common first-order estimate tr ≈ 0.8473 × Rp × Cbus. It is a practical engineering approximation for open-drain RC charging behavior on I2C buses.
Why can a selected mode fail even below its headline speed?
A mode rating assumes timing limits are still met. Excess capacitance, weak pull-ups, long traces, slow fall times, and added propagation delay can reduce the achievable real clock.
What is a good pull-up resistor choice?
Choose a value low enough to meet rise-time requirements, but high enough to keep sink current within device limits. This tool shows both a rise-time-based maximum and a current-based minimum.
Does this replace oscilloscope validation?
No. It is a design-stage estimator. Final validation should include scope measurements of SDA and SCL rise time, fall time, noise margin, and clock stretching on the actual hardware.
What is payload efficiency in the results?
Payload efficiency compares useful data bits to total transferred clock bits. Start, stop, address, ACK, and protocol overhead reduce effective data throughput below raw clock speed.
Can I use this for cable-based I2C links?
Yes, as an estimate. Increase capacitance, cable length, and propagation delay inputs realistically. Long cables often need slower modes, stronger pull-ups, buffering, or a different interface.