diff --git a/common/json.c b/common/json.c
index ce68543b9..5f36499dd 100644
--- a/common/json.c
+++ b/common/json.c
@@ -411,6 +411,11 @@ void json_add_snum(struct json_result *result, const char *fieldname, int value)
 	json_start_member(result, fieldname);
 	result_append_fmt(result, "%d", value);
 }
+void json_add_double(struct json_result *result, const char *fieldname, double value)
+{
+	json_start_member(result, fieldname);
+	result_append_fmt(result, "%f", value);
+}
 
 void json_add_u64(struct json_result *result, const char *fieldname,
 		  uint64_t value)
diff --git a/common/json.h b/common/json.h
index 6fc1cdde3..805a270a5 100644
--- a/common/json.h
+++ b/common/json.h
@@ -90,6 +90,9 @@ void json_add_literal(struct json_result *result, const char *fieldname,
 void json_add_snum(struct json_result *result, const char *fieldname,
 		   int value);
 /* '"fieldname" : value' or 'value' if fieldname is NULL */
+void json_add_double(struct json_result *result, const char *fieldname,
+		     double value);
+/* '"fieldname" : value' or 'value' if fieldname is NULL */
 void json_add_num(struct json_result *result, const char *fieldname,
 		  unsigned int value);
 /* '"fieldname" : value' or 'value' if fieldname is NULL */