How it's calculated
- Classic CAN frame: 47 bits plus 8 per data byte for an 11-bit ID (67 for a 29-bit ID). Worst-case bit stuffing adds one bit for every 4 bits from start of frame to the CRC: up to 135 bits for an 8-byte standard frame, 160 for extended.
- CAN FD: the arbitration phase and the end of the frame run at the nominal rate; with BRS, the control, data and CRC fields run at the data rate. Payloads round up to valid lengths (12, 16, 20, 24, 32, 48, 64 bytes). The CRC is 17 bits up to 16 bytes and 21 bits above, with fixed stuff bits.
- Bus load = time on the bus per second for all frames ÷ 1 second. A common design rule is to stay below 50–70% to leave room for event messages and retries.
An estimate for planning. Error frames, retransmissions, overload frames and the exact sample point are not included.
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Questions
What bus load is safe for CAN?
Many OEM guidelines keep average bus load under 50% for new designs and treat 70–80% as a hard limit, so that event-driven messages, diagnostics and error retries still fit. Use the worst-case figure for design reviews.
Why are there two figures?
Bit stuffing depends on the data: after five equal bits the controller adds an opposite bit. 'Without stuffing' is the best case; 'worst case' assumes the maximum number of stuff bits.
How much faster is CAN FD?
With BRS, the data field runs at the data bit rate (for example 2 Mbit/s instead of 500 kbit/s) and can carry up to 64 bytes, so the same signals need far less bus time.
Planning a CAN or CAN FD network?
Dervo engineers design and test vehicle networks, restbus simulations and HIL benches for OEMs and suppliers.