HesapHanem

Water Tank Size Calculator

Enter the number of people, daily use per person and reserve days to size your water tank.

How it is calculated

The calculation is three multiplications, all in litres.

daily demand = people × litres_per_person
tank size = daily demand × reserve_days
m³ = tank size ÷ 1000

people is the number of occupants and must be a whole number, Litres per Person per Day is the daily use of one person in litres, and Reserve Days is how many days the tank must last with no refill; the last two may have decimals. All three must be greater than zero, so a fractional number of people, zero or a negative value gives no result. Results are rounded to whole litres and to two decimals in m³. The tool does not add a safety margin, dead volume below the outlet, evaporation or rainfall inflow, so round the answer up to the next real tank size.

Worked example

A household of 4 people uses 150 L each per day and wants 5 days of reserve. Daily demand: 4 × 150 = 600 L. Tank size: 600 × 5 = 3,000 L, which is 3.00 m³. A standard tank of 3,000 L or slightly more would cover the five days. With only 2.5 reserve days the same household would need 1,500 L.

Try this example →

Common mistakes

  • Entering the household's daily use in the per-person field. If the family uses 600 L a day in total, type 150 for four people, not 600, or the tank comes out four times too large.
  • Estimating only drinking and cooking water (a few litres) when the tank also feeds showers, toilets and laundry.
  • Counting reserve days as days of rainfall rather than days without any inflow. The tank must hold the whole dry spell.
  • Typing a fraction of a person, such as 2.5 for a couple and a part-time lodger. The number of people must be whole, so round it up before entering.
  • Choosing a tank exactly equal to the result. Usable volume is lower than the nominal size, so go up a size.

About this calculator

Water tank sizing multiplies the number of people in a household by their daily water use per person, then by the number of reserve days the tank needs to cover — required volume (litres) = people × daily use per person × reserve days — giving a storage capacity that covers normal consumption through a chosen buffer period without needing a refill or mains connection.

This calculation is standard for rainwater harvesting systems, off-grid or rural properties without reliable mains water, and backup water supply planning where a mains outage or dry season needs to be bridged. Reserve days are typically set higher for properties with unreliable water delivery or infrequent rainfall, and lower for a simple top-up buffer on an otherwise mains-connected property; daily use per person also varies widely depending on whether the estimate includes just drinking and cooking or full household use including bathing and laundry.

The tank size you need in litres follows from the number of people, daily use per person and reserve days.

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