Water Tank Size Calculator

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.

This calculator takes the number of people, daily use per person and reserve days, and returns the tank size you need in litres.