How We Calculate

The two anchors

Water and food baselines follow published Ready.gov household guidance: one gallon of water per person per day covering drinking and sanitation, and a two-week supply at home where storage allows. Calorie figures use the 2,000 kcal/day adult reference from US nutrition labelling.

Everything beyond those two anchors is a modelling choice we made. Each one is stated below and repeated in the basis line under the relevant number, so you never have to guess where a figure came from.

Water

• 1 gallon per person per day. Drinking plus sanitation, per Ready.gov.
• Hot or arid climate multiplies by 1.5. Ready.gov notes hot climates can require doubling. We use the midpoint between the baseline and that ceiling as a planning figure.
• Dogs at 0.5 gal/day, cats at 0.06 gal/day. Dogs vary enormously by body weight. The dog figure models a medium dog and is deliberately generous.
• Rural properties get a manual well pump line. A well is a water source only while it has power.

Food

• 2,000 kcal/day per adult, 1,500 per child. The child figure is a midpoint across the age range.
• Cold climate multiplies calories by 1.15. Cold raises metabolic demand, and a household without heat burns more still. Applied to food only, never to water.
• Familiar food is listed separately from bulk storage. Freeze-dried buckets are depth. Rotating grocery food covers the first stretch and keeps morale intact.

Power

Battery storage is sized to one day of autonomy, then paired with solar for recharge. This is the most important modelling choice in the tool and the one most likely to surprise you.

Sizing a battery bank to the full duration would recommend something absurd and unaffordable at any duration past a few days. The honest answer past day one is generation, not storage. So the tool sizes storage for a day and generation for the rest.

• 300 Wh/day baseline for lights, phones, and a radio.
• 1,200 Wh/day for one efficient full-size refrigerator, derated to 60 percent in apartments where fridges are typically smaller.
• 400 Wh/dayfor a CPAP-class medical device, plus a second independent power source, because a single battery is a single point of failure when someone's health depends on it.
• Four usable peak-sun hours per day for solar sizing.

Why apartments get a different plan

Dwelling type is not cosmetic. Selecting an apartment removes fuel generators from the plan entirely and adds a self-contained toilet as a critical item.

Combustion generators vent carbon monoxide back into occupied space from balconies, stairwells, and garages, and most leases prohibit stored fuel outright. Carbon monoxide is a leading cause of post-disaster death in multi-unit housing. Meanwhile no yard means no alternative once building water or sewer stops, which is the requirement apartment households discover far too late.

What we deliberately do not do

• We never store your plan.No account, no saved household profile, no record of what you keep on hand. A stored list of a household's supplies is a registry. The tool computes, renders, and prints. Nothing leaves your browser.
• We do not link products for the things that are not products. The prescription buffer, the cash reserve, and the written family communications plan have no affiliate links, because attaching one would be gross.
• Affiliate relationships never reorder the plan. Requirements are computed first and products are matched to them afterward. The waterfall picks a merchant per line item; it cannot add, remove, or resize a line.

Limits

These are planning figures for an ordinary household. They are not medical, engineering, or legal advice. Local code governs generator placement, fuel storage, and water tanks. Anyone with a medical dependency should plan with their clinician rather than with a calculator.