diff --git a/fs/config/configflags/configflags.go b/fs/config/configflags/configflags.go
index f15fbf2bf..9e9ca06a3 100644
--- a/fs/config/configflags/configflags.go
+++ b/fs/config/configflags/configflags.go
@@ -40,7 +40,7 @@ func AddFlags(flagSet *pflag.FlagSet) {
 	flags.IntVarP(flagSet, &fs.Config.Transfers, "transfers", "", fs.Config.Transfers, "Number of file transfers to run in parallel.")
 	flags.StringVarP(flagSet, &config.ConfigPath, "config", "", config.ConfigPath, "Config file.")
 	flags.StringVarP(flagSet, &config.CacheDir, "cache-dir", "", config.CacheDir, "Directory rclone will use for caching.")
-	flags.BoolVarP(flagSet, &fs.Config.CheckSum, "checksum", "c", fs.Config.CheckSum, "Skip based on checksum & size, not mod-time & size")
+	flags.BoolVarP(flagSet, &fs.Config.CheckSum, "checksum", "c", fs.Config.CheckSum, "Skip based on checksum (if available) & size, not mod-time & size")
 	flags.BoolVarP(flagSet, &fs.Config.SizeOnly, "size-only", "", fs.Config.SizeOnly, "Skip based on size only, not mod-time or checksum")
 	flags.BoolVarP(flagSet, &fs.Config.IgnoreTimes, "ignore-times", "I", fs.Config.IgnoreTimes, "Don't skip files that match size and time - transfer all files")
 	flags.BoolVarP(flagSet, &fs.Config.IgnoreExisting, "ignore-existing", "", fs.Config.IgnoreExisting, "Skip all files that exist on destination")
diff --git a/fs/operations/operations.go b/fs/operations/operations.go
index e7a3b3ad9..60e56b712 100644
--- a/fs/operations/operations.go
+++ b/fs/operations/operations.go
@@ -104,6 +104,8 @@ func sizeDiffers(src, dst fs.ObjectInfo) bool {
 	return src.Size() != dst.Size()
 }
 
+var checksumWarning sync.Once
+
 func equal(src fs.ObjectInfo, dst fs.Object, sizeOnly, checkSum bool) bool {
 	if sizeDiffers(src, dst) {
 		fs.Debugf(src, "Sizes differ (src %d vs dst %d)", src.Size(), dst.Size())
@@ -125,6 +127,9 @@ func equal(src fs.ObjectInfo, dst fs.Object, sizeOnly, checkSum bool) bool {
 			return false
 		}
 		if ht == hash.None {
+			checksumWarning.Do(func() {
+				fs.Logf(dst.Fs(), "--checksum is in use but the source and destination have no hashes in common; falling back to --size-only")
+			})
 			fs.Debugf(src, "Size of src and dst objects identical")
 		} else {
 			fs.Debugf(src, "Size and %v of src and dst objects identical", ht)