How many days would you hold out?

The principle: the weakest link is what counts

A household holding three months of canned food and four days of water has four days of supply, not three months. That is why this food storage calculator does not average the axes but keeps the smallest of them, and above all why it names which one. A global figure with no limiting factor attached never tells you what to buy first, it only reassures or worries you.

Every axis is worked out the same way, by dividing what you have by what your household gets through in a day, rounded down. An axis with no declared need, typically medication in a home where nobody is on a daily treatment, is ignored rather than counted as zero, otherwise it would flatten everyone's result.

Water, the axis that almost always decides

In the vast majority of homes, water is the limiting factor, and by a wide margin. The reason is arithmetic: an adult needs two to three liters a day, which is two to three kilos to store every single day, when the same day of calories fits into five hundred grams of rice. Two weeks of water for four people is seventy liters, a pallet of packs that has to physically live somewhere.

The figure used here, 2.5 liters per adult per day, covers drinking and ordinary use in cooking. It covers neither hygiene, nor washing up, nor sanitation, all of which easily double or triple the real need. If you want to account for those, the simplest approach is to treat the result as a floor rather than a forecast.

Calories, and why weight alone is not enough

Two pantries of thirty kilos can last two weeks or six, depending on what is in them. A kilo of oil delivers around nine thousand kilocalories, a kilo of canned vegetables delivers a thousand, nine times less for exactly the same shelf space. That is why the form above asks for a weight per family rather than one total weight, and why the densities stay editable.

These default values are averages, useful to get a first read and nothing more. The label on your products is what counts, and the gap between two brands of the same product is enough to move the result by several days. A calculation built from your own labels, item by item, reaches a completely different level of precision, and that is exactly what the app does when it reads a barcode.

Medication, the axis everyone forgets

A daily treatment changes how the result should be read entirely. A household with three months of supplies and ten days of medication has ten real days of supply, and no amount of extra rice will change that. It is the easiest axis to overlook, because it does not show up in a cupboard, and the hardest to make up for in an emergency.

What this calculator does not do

• It knows nothing about your expiration dates. Thirty kilos of canned food with half of it past its shelf life gives the same result as thirty usable kilos, which is plainly wrong.
• It looks at neither protein, nor fat, nor carbohydrates. A stock that covers the calories can still be badly unbalanced, and that shows over a few weeks.
• It keeps nothing. Nothing is sent, nothing is stored, nothing is dropped into your browser, so everything has to be typed in again on your next visit.
• It is no substitute for an inventory. A figure that is accurate once a year ages very fast, at the pace of your shopping and your meals.

The same thing, kept up to date on its own

The Hardtack app runs this calculation continuously, from your real inventory rather than from a form. It reads nutrition values when you scan a barcode, follows the expiration date of every item, warns you seven days then one day before each deadline, and adds three more axes, protein, fat and carbohydrates, as soon as your inventory is detailed enough for that to be reliable. All of it on the device, with no account and no server.

The full feature breakdown is on the Features page, the free versus full comparison is on the Download page, and the most common questions are gathered in the FAQ.