For any practical superconductor the magnitude of the critical current density, $J_textrm{c}$, is crucially important. It sets the upper limit for current in the conductor. Usually $J_textrm{c}$ falls rapidly with increasing external magnetic field but even in zero external field the current flowing in the conductor generates a self-field which limits $J_textrm{c}$. Here we show for thin films of thickness less than the London penetration depth, $lambda$, this limiting $J_textrm{c}$ adopts a universal value for all superconductors - metals, oxides, cuprates, pnictides, borocarbides and heavy Fermions. For type I superconductors, it is $H_{textrm{c}}/lambda$ where $H_textrm{c}$ is the thermodynamic critical field. But surprisingly for type II superconductors we find the self-field $J_textrm{c}$ is $H_{textrm{c}1}/lambda$ where $H_{textrm{c}1}$ is the lower critical field. $J_textrm{c}$ is thus fundamentally determined and this provides a simple means to extract absolute values of $lambda(T)$ and, from its temperature dependence, the symmetry and magnitude of the superconducting gap.
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