The Water Horizon: A Calculator for Your Household
How many days of water do you actually have? Not a guess — a number. Five inputs, one minute, and a personal target that beats every generic recommendation.
Why a generic number is the wrong number
Every preparedness guide tells you "store 3 days of water." Most cite "3 litres per person per day." The 3-litre figure is the WHO emergency minimum — drinking only, not washing, not cooking, not flushing. For most European households the real consumption is 8–15 litres per person per day, and "3 days" is the absolute floor of resilience, not the target.
This article gives you a small calculation that produces your water horizon — the number of days your current household supply could realistically last under crisis conditions. It is not a survival figure; it is a number you can act on.
The five inputs
- People in the household: count adults and children. Children: 1.5–2 litres/day each. Adults: 2.5–3 litres/day each for drinking + cooking only.
- Hygiene baseline: the absolute minimum to prevent disease — 1 litre/person/day for hand-washing, food prep cleanup, basic dental care. Below this, gastrointestinal illness rates rise sharply.
- Climate adjustment: in 30°C+ heat, drinking water needs rise 40–60%. Add 1.5 litres/adult/day in summer heatwaves.
- Pets: dog ~50 ml/kg/day. Cat ~100 ml/day. A medium dog (15 kg) needs 0.75 litres/day.
- Special needs: infant formula preparation (1 litre/feeding for a 2-month-old × 6 feedings = significant). Dialysis patients have specific water requirements. Add these explicitly.
Worked example: family of four
2 adults (3 L drinking + 1 L hygiene = 4 L each) + 2 children (2 L drinking + 0.5 L hygiene = 2.5 L each) + 1 dog (0.75 L) = 14 L/day baseline in mild conditions. In a 32°C summer: closer to 19 L/day.
A 24-litre stored reserve (about 16 standard 1.5 L bottles) gives this family 1.7 days of comfortable consumption — or 4 days of survival-only drinking water with no hygiene. The honest reading: most "well-stocked" households have a water horizon of 2–3 days, not the 7+ they imagine.
Extending the horizon — three multipliers
Add direct storage. Cheap, dense, no expiry maintenance: 5-litre rigid containers in a cool dark place. €1.50 each. Three per household member triples your horizon overnight.
Add collection capacity. A rainwater barrel (~200 L) is functional in northern and central Europe most of the year. Combined with a 0.1-micron filter and purification tablets, it pushes the horizon into double-digit days.
Add the bathtub habit. At the first sign of a major disruption, fill the bath. 150 litres of usable water for hygiene and toilet flushing. The household that does this in the first 20 minutes after the warning has a fundamentally different week from the one that does not.
Your target
For most European households the practical target is a 7-day comfortable horizon — about 100 litres of stored or quickly accessible water per person. That feels like a lot. In practice it is twenty-four 4-litre bottles in a corner of a cupboard. €40 spent once, rotated annually.
One thing this week: run the calculation for your household. Write the number down. If your horizon is <3 days, add one €1.50 container to the next shopping trip.