diff --git a/src/xz/args.c b/src/xz/args.c
index 141347e4..d63760ec 100644
--- a/src/xz/args.c
+++ b/src/xz/args.c
@@ -92,6 +92,12 @@ parse_block_list(const char *str_const)
 	free(opt_block_list);
 	opt_block_list = xmalloc((count + 1) * sizeof(block_list_entry));
 
+	// Clear the bitmask of filter chains in use.
+	block_list_chain_mask = 0;
+
+	// Reset the largest Block size found in --block-list.
+	block_list_largest = 0;
+
 	for (size_t i = 0; i < count; ++i) {
 		// Locate the next comma and replace it with \0.
 		char *p = strchr(str, ',');
@@ -128,7 +134,11 @@ parse_block_list(const char *str_const)
 
 			const uint32_t filter_num = (uint32_t)(str[0] - '0');
 			opt_block_list[i].filters_index = filter_num;
+			block_list_chain_mask |= 1U << filter_num;
 			str += 2;
+		} else {
+			// This Block uses the default filter chain.
+			block_list_chain_mask |= 1U << 0;
 		}
 
 		if (str[0] == '\0') {
@@ -152,6 +162,14 @@ parse_block_list(const char *str_const)
 
 				opt_block_list[i].size = UINT64_MAX;
 			}
+
+			// Remember the largest Block size in the list.
+			//
+			// NOTE: Do this after handling the special value 0
+			// because when 0 is used, we don't want to reduce
+			// the Block size of the multithreaded encoder.
+			if (block_list_largest < opt_block_list[i].size)
+				block_list_largest = opt_block_list[i].size;
 		}
 
 		// Be standards compliant: p + 1 is undefined behavior
diff --git a/src/xz/coder.c b/src/xz/coder.c
index 6743849d..dee7d20a 100644
--- a/src/xz/coder.c
+++ b/src/xz/coder.c
@@ -27,6 +27,8 @@ bool opt_auto_adjust = true;
 bool opt_single_stream = false;
 uint64_t opt_block_size = 0;
 block_list_entry *opt_block_list = NULL;
+uint64_t block_list_largest;
+uint32_t block_list_chain_mask;
 
 /// Stream used to communicate with liblzma
 static lzma_stream strm = LZMA_STREAM_INIT;
diff --git a/src/xz/coder.h b/src/xz/coder.h
index a7feeb9c..7eeb0ba6 100644
--- a/src/xz/coder.h
+++ b/src/xz/coder.h
@@ -65,6 +65,19 @@ extern uint64_t opt_block_size;
 /// List of block size and filter chain pointer pairs.
 extern block_list_entry *opt_block_list;
 
+/// Size of the largest Block that was specified in --block-list.
+/// This is used to limit the block_size option of multithreaded encoder.
+/// It's waste of memory to specify a too large block_size and reducing
+/// it might even allow using more threads in some cases.
+///
+/// NOTE: If the last entry in --block-list is the special value of 0
+/// (which gets converted to UINT64_MAX), it counts here as UINT64_MAX too.
+/// This way the multithreaded encoder's Block size won't be reduced.
+extern uint64_t block_list_largest;
+
+/// Bitmask indicating which filter chains we specified in --block-list.
+extern uint32_t block_list_chain_mask;
+
 /// Set the integrity check type used when compressing
 extern void coder_set_check(lzma_check check);