diff --git a/src/lib/libcmd/head.c b/src/lib/libcmd/head.c
index d6c5f744d..415b1b3d9 100644
--- a/src/lib/libcmd/head.c
+++ b/src/lib/libcmd/head.c
@@ -39,14 +39,17 @@ static const char usage[] =
     "with the \b-q\b and \b-v\b options.]"
 "[+?If no \afile\a is given, or if the \afile\a is \b-\b, \bhead\b "
     "copies from standard input starting at the current location.]"
-"[+?The option argument for \b-c\b, and \b-s\b can optionally be "
-    "followed by one of the following characters to specify a different unit "
-    "other than a single byte:]"
-    "{"
-        "[+b?512 bytes.]"
-        "[+k?1-kilobyte.]"
-        "[+m?1-megabyte.]"
-    "}"
+"[+?The option argument for \b-n\b, \b-c\b and \b-s\b may end in one of the following "
+	"case-insensitive unit suffixes, with an optional trailing \bB\b ignored:]{"
+		"[+b?512 (block)]"
+		"[+k?1000 (kilo)]"
+		"[+Ki?1024 (kibi)]"
+		"[+M?1000*1000 (mega)]"
+		"[+Mi?1024*1024 (mebi)]"
+		"[+G?1000*1000*1000 (giga)]"
+		"[+Gi?1024*1024*1024 (gibi)]"
+		"[+...?and so on for T, Ti, P, Pi, E, and Ei.]"
+	"}"
 "[+?For backwards compatibility, \b-\b\anumber\a is equivalent to \b-n\b "
     "\anumber\a.]"
 "[n:lines?Copy \alines\a lines from each file.]#[lines:=10]"
diff --git a/src/lib/libcmd/tail.c b/src/lib/libcmd/tail.c
index 2316dd7e2..f69ed7664 100644
--- a/src/lib/libcmd/tail.c
+++ b/src/lib/libcmd/tail.c
@@ -41,13 +41,16 @@ static const char usage[] =
 "[+?If no \afile\a is given, or if the \afile\a is \b-\b, \btail\b "
 	"copies from standard input. The start of the file is defined "
 	"as the current offset.]"
-"[+?The option argument for \b-c\b can optionally be "
-	"followed by one of the following characters to specify a different "
-	"unit other than a single byte:]{"
-		"[+b?512 bytes.]"
-		"[+k?1 KiB.]"
-		"[+m?1 MiB.]"
-		"[+g?1 GiB.]"
+"[+?The option argument for \b-n\b and \b-c\b may end in one of the following "
+	"case-insensitive unit suffixes, with an optional trailing \bB\b ignored:]{"
+		"[+b?512 (block)]"
+		"[+k?1000 (kilo)]"
+		"[+Ki?1024 (kibi)]"
+		"[+M?1000*1000 (mega)]"
+		"[+Mi?1024*1024 (mebi)]"
+		"[+G?1000*1000*1000 (giga)]"
+		"[+Gi?1024*1024*1024 (gibi)]"
+		"[+...?and so on for T, Ti, P, Pi, E, and Ei.]"
 	"}"
 "[+?For backwards compatibility, \b-\b\anumber\a is equivalent to "
 	"\b-n\b \anumber\a and \b+\b\anumber\a is equivalent to "