--- a/dom/media/MemoryBlockCache.cpp
+++ b/dom/media/MemoryBlockCache.cpp
@@ -134,20 +134,22 @@ enum MemoryBlockCacheTelemetryErrors
   MoveBlockSourceOverrun = 5,
   MoveBlockDestOverflow = 6,
   MoveBlockCannotGrow = 7,
 };
 
 static int32_t
 CalculateMaxBlocks(int64_t aContentLength)
 {
+  int64_t maxSize = int64_t(MediaPrefs::MediaMemoryCacheMaxSize()) * 1024;
+  MOZ_ASSERT(aContentLength <= maxSize);
+  MOZ_ASSERT(maxSize % MediaBlockCacheBase::BLOCK_SIZE == 0);
   // Note: It doesn't matter if calculations overflow, Init() would later fail.
   // We want at least enough blocks to contain the original content length.
-  const int32_t requiredBlocks =
-    int32_t((aContentLength - 1) / MediaBlockCacheBase::BLOCK_SIZE + 1);
+  const int32_t requiredBlocks = maxSize / MediaBlockCacheBase::BLOCK_SIZE;
   // Allow at least 1s of ultra HD (25Mbps).
   const int32_t workableBlocks =
     25 * 1024 * 1024 / 8 / MediaBlockCacheBase::BLOCK_SIZE;
   return std::max(requiredBlocks, workableBlocks);
 }
 
 MemoryBlockCache::MemoryBlockCache(int64_t aContentLength)
   // Buffer whole blocks.