diff --git a/x_fftw.f90 b/x_fftw.f90
index 7fc30b3..4efc0b2 100644
--- a/x_fftw.f90
+++ b/x_fftw.f90
@@ -298,7 +298,7 @@ CONTAINS
     ! Definition of the array ialias.
     ! The array ialias is just the number of wavenumbers at (i,j,k) that have
     ! their magnitude higher than nx/3.  This is needed in dealiasing procedures.
-    rnx3 = real(nx/3, 8)
+    rnx3 = real(2*nx, 8) / real(3*lx, 8)
     do k = 1,nz
        if (abs(aky(k)) .gt. rnx3) ialias(:,:,k) = 1
        do j = 1,ny