Enter Message and Network Details
Example Data Table
| Protocol | Identifier | Nominal Rate | Data Rate | Payload | Message Rate | Stuffing | Purpose |
|---|---|---|---|---|---|---|---|
| Classical CAN | Standard | 500 kbit/s | Not used | 8 bytes | 100 frames/s | 15% | Periodic sensor frame |
| CAN FD | Extended | 500 kbit/s | 2,000 kbit/s | 32 bytes | 50 frames/s | 15% | Diagnostic data group |
Formula Used
The calculator estimates frame transmission time first. It then multiplies time by message frequency. The result is the message share of available bus time.
Classical CAN frame bits ≈ base frame bits + (payload bytes × 8) + estimated stuff bits + 3 intermission bits
Classical CAN load (%) = frames per second × (estimated frame bits ÷ nominal bit rate) × 100
CAN FD with switching frame time ≈ (nominal phase bits ÷ nominal rate) + (data phase bits ÷ data rate)
Combined load (%) = calculated message load + known background load
For Classical CAN, the model includes standard or extended framing, payload bits, an adjustable stuffing allowance, and three intermission bits. For CAN FD, it separates nominal timing from the faster data and CRC phase when switching is enabled. Bit stuffing depends on the actual transmitted pattern. Treat the output as a design estimate, not a timing certification.
How to Use This Calculator
- Choose Classical CAN or CAN FD.
- Select the identifier length used by the message.
- Enter the nominal network bit rate.
- For CAN FD, enter the data rate and choose switching.
- Enter payload bytes and the message rate.
- Add a conservative stuffing estimate and known background traffic.
- Set the maximum load your design should normally reach.
- Calculate and review both combined load and remaining headroom.
Planning CAN Capacity
CAN networks carry many messages through one shared channel. Each frame consumes transmission time. Bus load shows how much of that channel is already reserved. A low number gives messages more room to wait, retry, and recover. A high number increases delay and reduces design flexibility.
Frame Overhead Matters
Frame size is more than payload size. Arbitration, control fields, cyclic redundancy checking, acknowledgements, end bits, and intermission use capacity. Standard identifiers use fewer protocol bits than extended identifiers. Classical frames accept up to eight data bytes. CAN FD can carry larger payloads and may use a faster data phase.
Frequency Changes the Result
Message frequency is equally important. A small frame sent every millisecond can use more capacity than a large frame sent every second. Convert each message rate into frames per second. Multiply that rate by estimated transmission time. Then combine every message contribution to find the total planned occupancy.
Allow for Stuffing
Bit stuffing creates additional timing variation. A CAN transmitter inserts synchronization bits when needed. The exact number depends on identifier, payload, and CRC bit patterns. This calculator uses an adjustable average stuffing allowance. Use a measured value when a trace is available. Use a conservative value during early planning.
Check More Than Capacity
The calculated percentage is a capacity estimate. It does not prove that every deadline will be met. CAN arbitration gives higher-priority identifiers earlier access. A lower-priority frame can still wait when several urgent frames arrive together. Consider worst-case response time when safety, motion, or control timing matters.
Make Better Decisions
Start with the nominal bit rate used by the network. Select the frame format and identifier length. Enter the payload bytes and message frequency. For CAN FD, enter the data-phase rate and state whether bit-rate switching is enabled. Add known background traffic. Set a practical planning ceiling, such as seventy percent.
Review the result after each change. The message load shows one stream's share. Combined load includes the background estimate. Headroom compares total traffic with the selected target. A result over one hundred percent cannot be scheduled without reducing traffic, increasing available bandwidth, or changing the network design.
Use the calculator for early sizing and change reviews. Confirm the final design with logged traffic, controller timing, error behavior, and deadline analysis. Keep margin for diagnostics, firmware updates, fault recovery, and future devices. Capacity planning improves performance smoothly before integration begins.
Frequently Asked Questions
1. What is CAN bus load?
CAN bus load is the percentage of available transmission time used by traffic. It considers frame duration and message frequency. It is a practical capacity measure for shared CAN networks.
2. Does payload size alone determine bus load?
No. Identifier format, framing overhead, bit stuffing, bit rate, and transmission frequency also affect load. A short message sent very often can consume substantial bus time.
3. Why does this calculator use a stuffing allowance?
Stuff bits depend on actual transmitted bit patterns. A fixed allowance supports early planning when exact identifiers, payload values, and CRC patterns are not yet available.
4. What is a reasonable planning ceiling?
Many designs reserve margin below full utilization. The best ceiling depends on timing requirements, retries, priorities, diagnostics, and expected future expansion. Use a stricter target for time-critical traffic.
5. Does a result below 100% guarantee deadlines?
No. A bus can have available average capacity while low-priority messages still wait too long. Review priority assignments, burst traffic, and worst-case response time separately.
6. What changes with extended identifiers?
Extended identifiers use 29 identifier bits rather than 11. They add frame overhead and generally increase transmission time. The calculator includes this difference in its estimate.
7. When should I select CAN FD?
Select CAN FD when the message uses CAN FD framing, larger DLC payloads, or a faster data phase. Ensure every connected controller and network component supports the selected mode.
8. What does bit-rate switching do?
Bit-rate switching lets a CAN FD frame use a faster data and CRC phase. The arbitration and closing frame portions remain at the nominal rate.
9. Should background load include error frames?
Use measured background traffic when possible. During risk assessment, include practical allowance for retransmissions, diagnostic bursts, and error recovery. Do not treat normal error activity as free capacity.
10. Can I calculate an entire network with this page?
Calculate each message stream, then add results using background load or a separate total. For detailed scheduling, use a complete message list with priorities, periods, deadlines, and burst patterns.
11. What is the best next step after sizing?
Plan CAN capacity early for safer, smoother system growth.