diff --git a/lib/benchmark.rb b/lib/benchmark.rb
index 9f43255..5072bdc 100644
--- a/lib/benchmark.rb
+++ b/lib/benchmark.rb
@@ -307,6 +307,10 @@ def measure(label = "") # :yield:
 
   #
   # Returns the elapsed real time used to execute the given block.
+  # The unit of time is seconds.
+  #
+  #       Benchmark.realtime { "a" * 1_000_000_000 }
+  #       #=> 0.5098029999935534
   #
   def realtime # :yield:
     r0 = Process.clock_gettime(Process::CLOCK_MONOTONIC)