diff --git a/pyModeS/adsb.py b/pyModeS/adsb.py
index d98a04a..fbbd479 100644
--- a/pyModeS/adsb.py
+++ b/pyModeS/adsb.py
@@ -678,9 +678,9 @@ def altitude_diff(msg):
 
     msgbin = util.hex2bin(msg)
     sign = -1 if int(msgbin[80]) else 1
-    value = util.bin2int(msgbin[81:88]) * 25
+    value = util.bin2int(msgbin[81:88])
 
     if value == 0 or value == 127:
         return None
     else:
-        return sign * value
+        return sign * (value - 1) * 25    # in ft.
diff --git a/tests/test_adsb.py b/tests/test_adsb.py
index 48fa274..40c88bb 100644
--- a/tests/test_adsb.py
+++ b/tests/test_adsb.py
@@ -61,7 +61,7 @@ def test_adsb_velocity():
     assert vgs == (159, 182.9, -832, 'GS')
     assert vas == (376, 244.0, -2304, 'AS')
     assert vgs_surface == (19.0, 42.2, 0 , 'GS')
-    assert adsb.altitude_diff('8D485020994409940838175B284F') == 575
+    assert adsb.altitude_diff('8D485020994409940838175B284F') == 550
 
 
 def test_nic():