Stack Size Calculator Formula
Understand the math behind the stack size calculator. Each variable explained with a worked example.
Formulas Used
Recommended Stack Size
recommended_stack = ceil(total_raw * (1 + safety_margin_pct / 100))Base Call Stack
base_usage = base_stackInterrupt Stack
isr_usage = isr_stackTotal Before Margin
total_before_margin = total_rawVariables
| Variable | Description | Default |
|---|---|---|
max_call_depth | Max Call Depth | 8 |
avg_frame_bytes | Avg Stack Frame Size(bytes) | 32 |
interrupt_levels | Interrupt Nesting Levels | 2 |
isr_frame_bytes | ISR Stack Frame Size(bytes) | 64 |
safety_margin_pct | Safety Margin(%) | 25 |
base_stack | Derived value= max_call_depth * avg_frame_bytes | calculated |
isr_stack | Derived value= interrupt_levels * isr_frame_bytes | calculated |
total_raw | Derived value= base_stack + isr_stack | calculated |
How It Works
Estimating Stack Size for Embedded Systems
Stack overflow is a common and dangerous bug in embedded systems. Proper sizing prevents crashes.
Formula
Base Stack = Max Call Depth x Avg Frame Size
ISR Stack = Interrupt Levels x ISR Frame Size
Recommended = (Base + ISR) x (1 + Safety Margin)
The safety margin accounts for worst-case alignment, compiler-generated temporaries, and unforeseen call paths. A 25% margin is typical for production systems.
Worked Example
8-deep call stack, 32-byte frames, 2 interrupt levels at 64 bytes each, 25% safety.
- 01Base stack: 8 x 32 = 256 bytes
- 02ISR stack: 2 x 64 = 128 bytes
- 03Total raw: 256 + 128 = 384 bytes
- 04With 25% margin: 384 x 1.25 = 480 bytes
Frequently Asked Questions
How do I measure actual stack usage?
Fill the stack with a known pattern (e.g., 0xDEADBEEF) and check after running to see how far the pattern was overwritten.
What happens on stack overflow?
The stack corrupts adjacent memory, causing unpredictable behavior, hard faults, or silent data corruption.
Should I use a separate stack for ISRs?
Some architectures (ARM Cortex-M) have a separate MSP for interrupts. If available, use it to isolate ISR stack from task stacks.
Ready to run the numbers?
Open Stack Size Calculator