diff --git a/README.md b/README.md
index c908537..96eff7e 100644
--- a/README.md
+++ b/README.md
@@ -121,6 +121,11 @@ Pop the value `a` and push its arc tangent.
 Pop two values `a` and `b` and push the arc tangent of `b / a`,
 using the signs of `a` and `b` to determine the quadrant.
 
+### Error function
+
+`erf`
+Pop the value `a` and push its error function.
+
 ### Summation
 
 `sum`
diff --git a/clac.1 b/clac.1
index 7ad8a18..ad6a412 100644
--- a/clac.1
+++ b/clac.1
@@ -116,6 +116,13 @@ Pop two values `a` and `b` and push the arc tangent of `b / a`,
 using the signs of `a` and `b` to determine the quadrant.
 .El
 .
+.Ss Error function
+.
+.Bl -tag -width Fl
+.It Ic erf
+Pop the value `a` and push its error function.
+.El
+.
 .Ss Summation
 .
 .Bl -tag -width Fl
diff --git a/clac.c b/clac.c
index 49394bc..24d30bb 100644
--- a/clac.c
+++ b/clac.c
@@ -369,6 +369,10 @@ static void process(sds word) {
 		if (count(s0) > 0) {
 			push(s0, log10(pop(s0)));
 		}
+	} else if (!strcasecmp(word, "erf")) {
+		if (count(s0) > 0) {
+			push(s0, erf(pop(s0)));
+		}
 	} else if (!strcasecmp(word, "!")) {
 		if (count(s0) > 0) {
 			a = pop(s0);