diff --git a/builder/sizes_test.go b/builder/sizes_test.go
index 16760218c..c1d44f443 100644
--- a/builder/sizes_test.go
+++ b/builder/sizes_test.go
@@ -42,9 +42,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 3884, 280, 0, 2268},
-		{"microbit", "examples/serial", 2844, 360, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 7349, 1491, 116, 6912},
+		{"hifive1b", "examples/echo", 3896, 280, 0, 2268},
+		{"microbit", "examples/serial", 2860, 360, 8, 2272},
+		{"wioterminal", "examples/pininterrupt", 7361, 1491, 116, 6912},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the
diff --git a/src/runtime/gc_blocks.go b/src/runtime/gc_blocks.go
index 85f499490..408656b25 100644
--- a/src/runtime/gc_blocks.go
+++ b/src/runtime/gc_blocks.go
@@ -311,22 +311,31 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 		return unsafe.Pointer(&zeroSizedAlloc)
 	}
 
-	if preciseHeap {
-		size += align(unsafe.Sizeof(layout))
-	}
-
 	if interrupt.In() {
 		runtimePanicAt(returnAddress(0), "heap alloc in interrupt")
 	}
 
+	// Round the size up to a multiple of blocks.
+	rawSize := size
+	size += bytesPerBlock - 1
+	if preciseHeap {
+		// Add space for the layout.
+		size += align(unsafe.Sizeof(layout))
+	}
+	if size < rawSize {
+		// The size overflowed.
+		runtimePanicAt(returnAddress(0), "out of memory")
+	}
+	neededBlocks := size / bytesPerBlock
+	size = neededBlocks * bytesPerBlock
+
 	// Make sure there are no concurrent allocations. The heap is not currently
 	// designed for concurrent alloc/GC.
 	gcLock.Lock()
 
-	gcTotalAlloc += uint64(size)
+	// Update the total allocation counters.
+	gcTotalAlloc += uint64(rawSize)
 	gcMallocs++
-
-	neededBlocks := (size + (bytesPerBlock - 1)) / bytesPerBlock
 	gcTotalBlocks += uint64(neededBlocks)
 
 	// Continue looping until a run of free blocks has been found that fits the