diff --git a/src/gc.h b/src/gc.h
index 1328ef88c2d7c..72899b721a102 100644
--- a/src/gc.h
+++ b/src/gc.h
@@ -33,8 +33,8 @@
 extern "C" {
 #endif
 
-#define GC_PAGE_LG2 14 // log2(size of a page)
-#define GC_PAGE_SZ (1 << GC_PAGE_LG2) // 16k
+#define GC_PAGE_LG2 16 // log2(size of a page)
+#define GC_PAGE_SZ (1 << GC_PAGE_LG2) // 64k
 #define GC_PAGE_OFFSET (JL_HEAP_ALIGNMENT - (sizeof(jl_taggedvalue_t) % JL_HEAP_ALIGNMENT))
 
 #define jl_malloc_tag ((void*)0xdeadaa01)
@@ -230,19 +230,23 @@ STATIC_INLINE jl_gc_pagemeta_t *pop_lf_page_metadata_back(jl_gc_global_page_pool
     }
 }
 
+// Region values are
+// index shift = ptr_size - shift of the page counts before
+// i.e 64 - 18 = 46 ...
+// after that AND with the actual page count
 #ifdef _P64
 #define REGION0_PG_COUNT (1 << 16)
-#define REGION1_PG_COUNT (1 << 16)
+#define REGION1_PG_COUNT (1 << 14)
 #define REGION2_PG_COUNT (1 << 18)
-#define REGION0_INDEX(p) (((uintptr_t)(p) >> 14) & 0xFFFF) // shift by GC_PAGE_LG2
-#define REGION1_INDEX(p) (((uintptr_t)(p) >> 30) & 0xFFFF)
+#define REGION0_INDEX(p) (((uintptr_t)(p) >> GC_PAGE_LG2) & 0xFFFF) // shift by GC_PAGE_LG2
+#define REGION1_INDEX(p) (((uintptr_t)(p) >> 32) & 0x3FFF)
 #define REGION_INDEX(p)  (((uintptr_t)(p) >> 46) & 0x3FFFF)
 #else
 #define REGION0_PG_COUNT (1 << 8)
-#define REGION1_PG_COUNT (1 << 10)
+#define REGION1_PG_COUNT (1 << 8)
 #define REGION2_PG_COUNT (1 << 0)
-#define REGION0_INDEX(p) (((uintptr_t)(p) >> 14) & 0xFF) // shift by GC_PAGE_LG2
-#define REGION1_INDEX(p) (((uintptr_t)(p) >> 22) & 0x3FF)
+#define REGION0_INDEX(p) (((uintptr_t)(p) >> GC_PAGE_LG2) & 0xFF) // shift by GC_PAGE_LG2
+#define REGION1_INDEX(p) (((uintptr_t)(p) >> 24) & 0xFF) // 32 - pgcount1 shift
 #define REGION_INDEX(p)  (0)
 #endif
 
diff --git a/src/julia_internal.h b/src/julia_internal.h
index cf65521770681..9cba1c8f0e3cf 100644
--- a/src/julia_internal.h
+++ b/src/julia_internal.h
@@ -345,7 +345,7 @@ void *jl_gc_perm_alloc(size_t sz, int zero,
 void gc_sweep_sysimg(void);
 
 
-// pools are 16376 bytes large (GC_POOL_SZ - GC_PAGE_OFFSET)
+// pools are 65528 bytes large (GC_POOL_SZ - GC_PAGE_OFFSET)
 static const int jl_gc_sizeclasses[] = {
 #ifdef _P64
     8,
@@ -364,23 +364,22 @@ static const int jl_gc_sizeclasses[] = {
     144, 160, 176, 192, 208, 224, 240, 256,
 
     // the following tables are computed for maximum packing efficiency via the formula:
-    // pg = 2^14
+    // pg = 2^16 -8
     // sz = (div.(pg-8, rng).÷16)*16; hcat(sz, (pg-8).÷sz, pg .- (pg-8).÷sz.*sz)'
 
-    // rng = 60:-4:32 (8 pools)
+    // rng = 240:-16:128 (8 pools)
     272, 288, 304, 336, 368, 400, 448, 496,
-//   60,  56,  53,  48,  44,  40,  36,  33, /pool
-//   64, 256, 272, 256, 192, 384, 256,  16, bytes lost
-
-    // rng = 30:-2:16 (8 pools)
+    // 240,  227,  215,  195,  178,  163,  146,  132, /pool
+    // 248,  152,  168,  8,  24,  328,  120,  56, bytes lost
+    // rng = 120:-8:64 (8 pools)
     544, 576, 624, 672, 736, 816, 896, 1008,
-//   30,  28,  26,  24,  22,  20,  18,  16, /pool
-//   64, 256, 160, 256, 192,  64, 256, 256, bytes lost
+    // 120,  113,  105,  97,  89,  80,  73,  65, /pool
+    // 248,  440,  8,  344,  24,  248,  120,  8, bytes lost
 
-    // rng = 15:-1:8 (8 pools)
+    // rng = 60:-4:32 (8 pools)
     1088, 1168, 1248, 1360, 1488, 1632, 1808, 2032
-//    15,   14,   13,   12,   11,   10,    9,    8, /pool
-//    64,   32,  160,   64,   16,   64,  112,  128, bytes lost
+    // 60,  56,  52,  48,  44,  40,  36,  32, /pool
+    // 248,  120,  632,  248,  56,  248,  440,  504, bytes lost
 };
 static_assert(sizeof(jl_gc_sizeclasses) / sizeof(jl_gc_sizeclasses[0]) == JL_GC_N_POOLS, "");