diff --git a/ext/msgpack/packer.h b/ext/msgpack/packer.h
index 6bee4c0b..bf0510ef 100644
--- a/ext/msgpack/packer.h
+++ b/ext/msgpack/packer.h
@@ -255,6 +255,17 @@ static inline void msgpack_packer_write_u64(msgpack_packer_t* pk, uint64_t v)
 
 static inline void msgpack_packer_write_double(msgpack_packer_t* pk, double v)
 {
+  float fv = v;
+  if (fv == v) {
+    msgpack_buffer_ensure_writable(PACKER_BUFFER_(pk), 5);
+    union {
+        float f;
+        uint32_t u32;
+        char mem[4];
+    } castbuf = { fv };
+    castbuf.u32 = _msgpack_be_float(castbuf.u32);
+    msgpack_buffer_write_byte_and_data(PACKER_BUFFER_(pk), 0xca, castbuf.mem, 4);
+  } else {
     msgpack_buffer_ensure_writable(PACKER_BUFFER_(pk), 9);
     union {
         double d;
@@ -263,6 +274,7 @@ static inline void msgpack_packer_write_double(msgpack_packer_t* pk, double v)
     } castbuf = { v };
     castbuf.u64 = _msgpack_be_double(castbuf.u64);
     msgpack_buffer_write_byte_and_data(PACKER_BUFFER_(pk), 0xcb, castbuf.mem, 8);
+  }
 }
 
 static inline void msgpack_packer_write_raw_header(msgpack_packer_t* pk, unsigned int n)
diff --git a/spec/format_spec.rb b/spec/format_spec.rb
index fd7c2052..2e16a805 100644
--- a/spec/format_spec.rb
+++ b/spec/format_spec.rb
@@ -72,10 +72,10 @@
   end
 
   it "double" do
-    check 9, 1.0
+    check 5, 1.0                # fits in a float
     check 9, 0.1
     check 9, -0.1
-    check 9, -1.0
+    check 5, -1.0               # fits in a float
   end
 
   it "fixraw" do
@@ -154,7 +154,7 @@
   end
 
   it "1.0" do
-    match 1.0, "\xcb\x3f\xf0\x00\x00\x00\x00\x00\x00"
+    match 1.0, "\xca\x3f\x80\x00\x00"
   end
 
   it "[]" do