diff --git a/lib/width.c b/lib/width.c
index 37c3afa..b4b6bda 100644
--- a/lib/width.c
+++ b/lib/width.c
@@ -47,7 +47,7 @@ using std::isnan;
  * Olivero & Longbothum [1977]
  * Journal of Quantitative Spectroscopy and Radiative Transfer. 17:233
  */
-double hwhm0(double sigma, double gamma)
+static double hwhm0(double sigma, double gamma)
 {
     return .5*(1.06868*gamma+sqrt(0.86743*gamma*gamma+4*2*log(2)*sigma*sigma));
 }
diff --git a/test/widthtest.c b/test/widthtest.c
index b395d34..a44b83f 100644
--- a/test/widthtest.c
+++ b/test/widthtest.c
@@ -28,7 +28,7 @@
 #include "testtool.h"
 
 // excellent approximation [Olivero & Longbothum, 1977], used as starting value in voigt_hwhm
-double hwhm0(double sigma, double gamma)
+static double hwhm0(double sigma, double gamma)
 {
     return .5*(1.06868*gamma+sqrt(0.86743*gamma*gamma+4*2*log(2)*sigma*sigma));
 }