--- a/tools/gfxconv.c	Tue Mar 05 14:51:02 2019 +0200
+++ b/tools/gfxconv.c	Tue Mar 05 15:09:18 2019 +0200
@@ -1424,21 +1424,13 @@
 
     // Determine number of planes, if paletted
     if (spec->format == DM_COLFMT_PALETTE &&
-        spec->nplanes == 0)
-    {
-        for (int n = 8; n >= 0;)
-        {
-            if ((image->pal->ncolors - 1) & (1 << n))
-            {
-                spec->nplanes = n + 1;
-                break;
-            }
-            else
-                n--;
-        }
-    }
+        spec->nplanes == 0 && image->pal != NULL)
+        spec->nplanes = dmGetNPlanesFromNColors(image->pal->ncolors);
+    else
+    if (image->format == DM_COLFMT_GRAYSCALE)
+        spec->nplanes = 8;
 
-    if (spec->nplanes == 0)
+    if (spec->nplanes <= 0)
         spec->nplanes = 4;
 
     spec->fmtid = fmt->fmtid;