diff --git a/impatient b/impatient
index 20ac413..e8d595e 100755
--- a/impatient
+++ b/impatient
@@ -6,16 +6,15 @@ import subprocess
 import sys
 import time
 
-WINDOW_SIZE = 100
-
 SUFFIXES = 'KMGT'
 
 parser = argparse.ArgumentParser(description='Display progress and time estimates for arbitrary tasks')
-parser.add_argument('-f', '--final', type=str, help='Expected final size/value at completion.')
+parser.add_argument('-f', '--final', type=str, help='Expected final size/value at completion, optionally with a K/M/G/T suffix')
 parser.add_argument('-i', '--interval', type=int, default=10, help='Interval in seconds between samples (default 10)')
+parser.add_argument('-w', '--window', type=int, default=100, help='Number of samples to keep for the sliding window (default 100)')
 tracker_types = parser.add_mutually_exclusive_group(required=True)
 tracker_types.add_argument('-p', '--path', type=str, help='Track total disk usage of a given path')
-tracker_types.add_argument('-c', '--command', type=str, help='Track value returned by a shell command; this should return a single number, optionally followed by K/M/G/T')
+tracker_types.add_argument('-c', '--command', type=str, help='Track value returned by a shell command; this should return a single number, optionally with a K/M/G/T suffix')
 
 args = parser.parse_args()
 
@@ -67,8 +66,8 @@ while True:
     samples.append(current)
     if len(samples) < 2:
         continue
-    if len(samples) > WINDOW_SIZE:
-        samples = samples[-WINDOW_SIZE:]
+    if len(samples) > args.window:
+        samples = samples[-args.window:]
 
     rate = (current - samples[0])/((len(samples)-1)*args.interval)