diff --git a/cmdutils.c b/cmdutils.c
index 2349b7052e49b361c29150328db400c6dd890198..fcdebb2005bdaf4cadbc1004b8e8252b4b388c8b 100644
--- a/cmdutils.c
+++ b/cmdutils.c
@@ -36,6 +36,7 @@
 #include "libpostproc/postprocess.h"
 #include "libavutil/avstring.h"
 #include "libavutil/pixdesc.h"
+#include "libavutil/eval.h"
 #include "libavcodec/opt.h"
 #include "cmdutils.h"
 #include "version.h"
@@ -58,7 +59,7 @@ double parse_number_or_die(const char *context, const char *numstr, int type, do
 {
     char *tail;
     const char *error;
-    double d = strtod(numstr, &tail);
+    double d = av_strtod(numstr, &tail);
     if (*tail)
         error= "Expected number for %s but found: %s\n";
     else if (d < min || d > max)
diff --git a/doc/fftools-common-opts.texi b/doc/fftools-common-opts.texi
index 4fc271d3e052c0fd99ab5073dd2029f98e283f74..b011e73bd3916347d18cd1591a6d8e61ecf19206 100644
--- a/doc/fftools-common-opts.texi
+++ b/doc/fftools-common-opts.texi
@@ -1,3 +1,11 @@
+All the numerical options, if not specified otherwise, accept in input
+a string representing a number, which may contain one of the
+International System number postfixes, for example 'K', 'M', 'G'.
+If 'i' is appended after the postfix, powers of 2 are used instead of
+powers of 10. The 'B' postfix multiplies the value for 8, and can be
+appended after another postfix or used alone. This allows using for
+example 'KB', 'MiB', 'G' and 'B' as postfix.
+
 @section Generic options
 
 These options are shared amongst the ff* tools.