Index: usr.sbin/mixer/mixer.8
===================================================================
--- usr.sbin/mixer/mixer.8
+++ usr.sbin/mixer/mixer.8
@@ -21,7 +21,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 20, 2022
+.Dd April 29, 2022
 .Dt MIXER 8
 .Os
 .Sh NAME
@@ -112,8 +112,8 @@
 .It Sy Name Ta Sy Value
 .It Ar dev Cm .volume Ta Xo
 .Ar vol |
-.Oo Cm \&+ | Cm \&- Oc Ar lvol
-.Oo Cm \&: Oo Cm \&+ | Cm \&- Oc Ar rvol Oc
+.Oo Cm \&+ | Cm \&- Oc Ar lvol Oo % Oc
+.Oo Cm \&: Oo Cm \&+ | Cm \&- Oc Ar rvol Oo % Oc Oc
 .Xc
 .It Ar dev Cm .mute Ta Cm 0 | 1 | ^
 .It Ar dev Cm .recsrc Ta Cm ^ | + | - | =
@@ -128,10 +128,10 @@
 and/or
 .Ar rvol
 values have to be specified.
-The values have to be normalized 32-bit floats, from 0.0 to 1.0 inclusively.
-If no
-.Ql \&.
-character is present, the value is treated like a percentage, for backwards compatibility.
+The values should typically be normalized 32-bit floats, from 0.0 to 1.0
+inclusively.
+A trailing percent sign indicates that the value should be treated as a percent,
+rather than an absolute value.
 If the left or right volume values are prefixed with
 .Cm +
 or
Index: usr.sbin/mixer/mixer.c
===================================================================
--- usr.sbin/mixer/mixer.c
+++ usr.sbin/mixer/mixer.c
@@ -341,7 +341,7 @@
 	mix_ctl_t *cp;
 	mix_volume_t v;
 	const char *val;
-	char lstr[8], rstr[8];
+	char *endp, lstr[8], rstr[8];
 	float lprev, rprev, lrel, rrel;
 	int n;
 
@@ -357,19 +357,25 @@
 	if (n > 0) {
 		if (*lstr == '+' || *lstr == '-')
 			lrel = rrel = 1;
-		v.left = strtof(lstr, NULL);
+		v.left = strtof(lstr, &endp);
+		if (*endp != '\0' && *endp != '%') {
+			warnx("invalid volume value: %s", lstr);
+			return (-1);
+		}
 
-		/* be backwards compatible */
-		if (strstr(lstr, ".") == NULL)
+		if (*endp == '%')
 			v.left /= 100.0f;
 	}
 	if (n > 1) {
 		if (*rstr == '+' || *rstr == '-')
 			rrel = 1;
-		v.right = strtof(rstr, NULL);
+		v.right = strtof(rstr, &endp);
+		if (*endp != '\0' && *endp != '%') {
+			warnx("invalid volume value: %s", rstr);
+			return (-1);
+		}
 
-		/* be backwards compatible */
-		if (strstr(rstr, ".") == NULL)
+		if (*endp == '%')
 			v.right /= 100.0f;
 	}
 	switch (n) {