Heap Memory Calculator

This tool calculates available heap size, peak dynamic memory usage (watermark), worst-case fragmentation overhead, and recommended minimum heap allocation for RTOS-based embedded systems. It models malloc/free behavior under constrained memory, factoring in metadata, alignment, and fragmentation penalties. Outputs help verify heap sufficiency before deployment.

Use it during system architecture planning, RTOS porting, or memory-constrained firmware optimization—especially before finalizing linker scripts or heap initialization. It’s critical when adding new tasks, dynamic buffers, or third-party libraries that rely on heap allocation (e.g., lwIP, FreeRTOS queues with dynamic storage).

Fragmentation (%) estimates worst-case internal + external fragmentation due to block sizing, alignment padding, and allocation/deallocation patterns. The calculator applies this percentage to total usable heap to reserve additional space—ensuring contiguous blocks of your Max Block Size remain allocatable even after repeated malloc/free cycles.

Max Block Size should reflect your largest single dynamic allocation (e.g., a network packet buffer or FFT array). Number of Blocks is the concurrent count of such allocations expected at peak load—not total lifetime allocations. For safety, include worst-case scenarios like error-handling paths or burst traffic handling.

Static Allocation covers global/`static` variables and `.bss/.data` sections—memory reserved at link time. Task Stacks are *separate* from heap; they’re pre-allocated per-task RAM (often in static RAM or dedicated stack sections) and must be subtracted from total RAM before computing heap headroom. Confusing them causes dangerous overestimation of available heap.

For Cortex-M3/M4 with common RTOS heaps (e.g., FreeRTOS heap_4), 10–25% fragmentation is typical under moderate allocation churn; >30% suggests redesigning allocation strategy. Max Block Size usually falls between 1–64 KB—exceeding 1/8 of total heap often triggers fragmentation spikes or allocation failures.

The tool models worst-case theoretical usage, not average runtime behavior. If your app works but the calculator flags insufficiency, you may be relying on undefined behavior (e.g., heap overflow, unaligned access) or have unaccounted allocations (e.g., driver internal buffers, C++ `new`, or RTOS object creation). Always validate with runtime heap watermarking (e.g., `xPortGetFreeHeapSize()`).

Unlike basic “total RAM minus static” estimators, this tool explicitly models fragmentation impact, block-level allocation overhead, and concurrency constraints. It accounts for heap metadata (e.g., 8-byte headers per block), alignment padding (typically 8-byte), and provides actionable guidance—not just raw totals—to prevent subtle runtime failures in long-running systems.

Yes—but adjust assumptions: omit RTOS-specific overhead (e.g., task stack separation), and manually account for any custom allocator metadata or alignment rules. Ensure your `malloc` implementation’s header size and alignment match the calculator’s defaults (8-byte header + 8-byte alignment), or adjust inputs accordingly.

Under-sizing risks heap exhaustion, leading to silent `malloc` failures, system hangs, or corruption—especially under transient load. Over-sizing wastes precious RAM, potentially starving other critical sections (e.g., DMA buffers or cache). The recommendation balances safety margin (for fragmentation & growth) against resource efficiency for deterministic real-time operation.