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Water and food

Tank capacity and days of reserve

Litres held by a cylindrical or rectangular tank and how many days it supplies a group of people.

m

If you can only measure the circumference, diameter = circumference ÷ 3.1416.

m

From the outlet or tap up to the overflow: water below the outlet cannot be used.

L

Sphere: at least 15 L for drinking, cooking and basic hygiene. Drinking only: 2.5-3 L (more in heat or with exertion).

Result

Usable capacity
— L
Volume
— m³
Group consumption per day
— L
Days of reserve
—

How it is calculated

Vertical cylinder: V (m³) = π × (D ÷ 2)² × h. Rectangular: V = length × width × h. Litres = V × 1000. Days = litres ÷ (people × litres per person per day).

Example. A cylindrical tank 2 m in diameter with 1.5 m of usable water: 3.1416 × 1² × 1.5 = 4.71 m³ = 4712 L. For 5 people at 15 L: 4712 ÷ 75 = 62.8 days.

Rectangular. 2 × 1.5 m with 1.2 m of water: 3.6 m³ = 3600 L; 4 people drinking only 3 L: 3600 ÷ 12 = 300 days.

Keep in mind

  • Sphere (2018): at least 15 L per person per day for drinking, cooking and hygiene; survival needs (drinking and food) 2.5-3 L, rising with heat, physical work, pregnancy and breastfeeding.
  • Measure inside dimensions. A horizontal cylindrical tank is not linear with depth: this tool does not calculate it.
  • The days assume the tank starts full and is not refilled: allow for leaks, evaporation from open tanks and the water used to clean the tank itself.
  • Keep the tank covered, opaque and with the outlet above the bottom; re-chlorinate stored water if it loses its chlorine smell.

See also

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