  Our understanding of observed Gravitational Waves (GWs) comes from matching
data to known signal models describing General Relativity (GR). These models,
expressed in the post-Newtonian formalism, contain the mathematical constant
$\pi$. Allowing $\pi$ to vary thus enables a strong, universal and
generalisable null test of GR. From a population of 22 GW observations, we make
an astrophysical measurement of $\pi=3.115^{+0.048}_{-0.088}$, and prefer GR as
the correct theory of gravity with a Bayes factor of 321. We find the variable
$\pi$ test robust against simulated beyond-GR effects.
