diff --git a/swash/config-nb.ini b/swash/config-nb.ini
index fd56912..af53a8b 100644
--- a/swash/config-nb.ini
+++ b/swash/config-nb.ini
@@ -12,6 +12,5 @@ inp=inp_post/spec_nb
 compare=inp_post_nb
 out=out_post
 #nperseg=1024
-dt=0.25
 x0=-1250
 t0=180
diff --git a/swash/config.ini b/swash/config.ini
index 1a58cea..43f34d8 100644
--- a/swash/config.ini
+++ b/swash/config.ini
@@ -12,6 +12,5 @@ inp=inp_post/spec
 compare=inp_post_nb
 out=out_post
 #nperseg=1024
-dt=0.25
 x0=-1250
 t0=180
diff --git a/swash/processing/post.py b/swash/processing/post.py
index ca6a785..0d1a0c1 100644
--- a/swash/processing/post.py
+++ b/swash/processing/post.py
@@ -38,7 +38,7 @@ x0 = config.getint("post", "x0")
 arg_x0 = np.abs(x - x0).argmin()
 t0 = config.getfloat("post", "t0")
 arg_t0 = np.abs(t - t0).argmin()
-dt = config.getfloat("post", "dt")
+dt = np.diff(t).mean()
 f = 1 / dt
 nperseg = config.getint("post", "nperseg", fallback=None)
 log.info(f"Computing reflection coefficient at x={x0}")