$\delta N$ formalism is a useful method to calculate the curvature perturbation on superhorizon scales. Contrary to what it is typically done in the literature, we reformulate the $\delta N$ formalism by using the $e$-folding number $n$ counted forward in time. For a fixed initial time ${\overline{n}}_0$, the probability density function (PDF) of the field perturbation $\delta {\varphi }_0$ and its velocity $\delta {\pi }_0$ are specified by the solutions of the perturbation equation on subhorizon scales. As $\delta {\pi }_0$ is fully correlated with $\delta {\varphi }_0$ soon after horizon exit, we find a novel $\delta n$ formalism to calculate the curvature perturbation as well as its PDF. It can naturally incorporate those trajectories for which inflation never ends, which are lost when counting $N$ backward in time.

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• Inflation
• Primordial black holes