Start with 3 days, build toward a week

Japan's Cabinet Office recommends stockpiling at least 3 days - ideally one week - of water and food for large-scale disasters, with drinking water estimated at 3 liters per person per day. This checklist applies those figures automatically to the household size and duration you select. The numbers can look daunting, but nobody builds a stockpile in one shopping trip: start with 3 days, add toward a week at the next sale.

A one-month stockpile anticipates wide-area disasters - like a Nankai Trough earthquake - where logistics take long to recover. Storage space becomes the real constraint at that scale; see the Stockpile Quantity Guide for how to think about amounts and storage.

The easiest way to maintain any of this is to stop treating it as special shopping: buy a little extra of the everyday food you already eat and rotate through it. That is the rolling stock method.

Official simulators also exist - Tokyo's "Tokyo Bichiku Navi" is a good example - and pair well with this checklist.