Water Tank Sizing (Ground & Roof)

Every storage calculation starts with demand, and demand starts with people. The daily figure is simply the number of occupants multiplied by how much water each one uses in a day. That per-person figure is where the engineering judgement lives: it is set by the building's function and the local standard, not by the calculator. A residential occupant might use 120 to 200 litres a day, an office worker far less, a hotel guest or a hospital bed considerably more.

Getting that figure right matters more than any other input, because it scales everything. Use an office allowance for a hotel and the storage will be a fraction of what is needed; use a hospital allowance for apartments and you will over-build. The sensible move is to take the number straight from the applicable standard — SNI 8153 in Indonesia, or the equivalent local code — or from the owner's brief, and only fall back to a generic default for the earliest screening.

Once the daily demand is fixed, the autonomy factor decides how much of it to store. Storing a full day means the building can ride out a 24-hour interruption on tank alone. Some projects design for less where the supply is dependable; others want more resilience. The safety factor on top is a modest cushion for under-estimation and growth, commonly around ten percent.

Most buildings store water in two places, and the split is deliberate. The ground tank — usually the larger of the two — sits at the bottom and holds the main reserve. It is filled from the incoming supply whenever that supply is available, so it absorbs outages and levels out the difference between a steady fill and a peaky demand. It is also easy to build large, since it sits on or below grade where structure is cheap.

The roof tank is smaller and serves a different purpose. Perched at the top of the building, it provides water to the fixtures by gravity or through a booster, holding just a few hours of supply. That buffer smooths short-term demand peaks and, crucially, keeps water flowing for a while if the transfer pump that lifts water from the ground tank stops. It is the building's short-term insurance, where the ground tank is its long-term reserve.

The calculator captures this by sizing the roof tank from the hours of supply you want it to hold, then leaving the rest of the total in the ground tank. The roof figure is intentionally modest — a large roof tank is heavy, demands serious structure at the top of the building, and offers little extra benefit once it covers the realistic peak. Keep the roof tank lean and let the ground tank carry the bulk.

The occupancy-based number is a clean estimate of domestic demand, but a real project carries draws it does not see. The most important is fire water. Fire-fighting reserve is governed by its own codes and is often a large, dedicated volume that domestic use is not allowed to touch — typically held in its own tank or in a clearly reserved low section of a shared tank with the domestic draw-off raised above it. It is added on top of the figure here, never blended into it.

Other draws depend on the project. Landscape irrigation can be a significant seasonal load, cooling-tower make-up runs continuously in some buildings, and a small allowance for leakage and system losses is prudent on any sizable installation. None of these are in the basic occupancy calculation, so the engineer adds them where they apply rather than assuming the headline number covers everything.

There is also the question of water quality over time. A tank sized generously for a small population turns its water over slowly, and slow turnover invites stagnation and the loss of disinfectant residual. Where storage is large relative to demand, the design has to keep the water moving — through circulation, sensible inlet and outlet placement, or supplementary treatment. Sizing is the start of the storage story, not the end of it; this calculator gives you the volume, and good detailing keeps that volume healthy.