📖 How To Use
How to Use This Calculator
Sizing a water tank for an apartment building takes 30 seconds with the right inputs:
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Enter the number of flats
Count the total residential units in the building — not bedrooms, not floors, just distinct apartments or flats. Include all units, even rarely occupied ones; you're sizing for peak occupancy.
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Enter the number of floors
This affects pump head pressure requirements and is used by the visual display. It doesn't directly change volume but helps identify if your building needs a gravity-fed rooftop tank or a pressurized underground cistern.
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Enter residents per flat
Use the average household size for your context. In South Asia, 4–5 is common. In European cities, 1.8–2.5 is typical. When in doubt, use 3.5 as a conservative default.
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Select a usage level
Basic (100 L/person/day) covers drinking, cooking, and minimal hygiene. Standard (150 L) is the WHO urban benchmark. Comfort (200 L) includes daily bathing. Luxury (250 L) accounts for dishwashers, frequent laundry, and garden taps.
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Set your backup days
This is how many days of full consumption the tank must cover without a refill — in case of supply interruptions. Municipal areas in reliable-supply cities use 1–2 days. Buildings in areas with frequent outages should use 3–5 days minimum.
Tip: The recommended size already includes a 15% safety buffer above the raw calculated storage. Never size a tank to the exact demand — thermal expansion, sediment accumulation, and unexpected peak usage all eat into nominal capacity.
📐 The Formula
Apartment Water Tank Size Formula
The calculation is straightforward but the inputs matter enormously:
Total Residents = Flats × Residents per Flat
Daily Demand (L) = Total Residents × Usage Rate (L/person/day)
Base Storage (L) = Daily Demand × Backup Days
Recommended Tank = Base Storage × 1.15
The 1.15 multiplier adds a 15% safety buffer — the absolute minimum any serious engineer will tell you to build in. For buildings in low-supply-reliability areas, bump it to 1.25 or even 1.3.
Usage Rate Reference Table
| Usage Level | L / Person / Day | Typical Context | WHO Alignment |
|---|
| Basic | 100 L | Low-income urban, water-scarce regions | Above survival minimum (50 L) |
| Standard | 150 L | Typical urban apartment, developing world | WHO urban benchmark |
| Comfort | 200 L | Middle-income apartment, daily bathing | European average |
| Luxury | 250 L | High-end residential, full appliance use | US/Australian average |
Backup Storage by Supply Reliability
| Supply Situation | Backup Days | Regions |
|---|
| Highly reliable mains supply | 1 day | Most of Western Europe, North America |
| Generally reliable, occasional outage | 2 days | Urban GCC, parts of Eastern Europe |
| Regular scheduled rationing | 3–4 days | Urban South Asia, parts of MENA |
| Unreliable / frequent outages | 5–7 days | Rural South Asia, Sub-Saharan Africa |
📏 Quick Reference
Typical Apartment Building Tank Sizes
Based on standard usage (150 L/person/day), 4 residents per flat, and 2 days backup storage with 15% buffer:
| Building Size | Flats | Total Residents | Daily Demand | Recommended Tank |
|---|
| Small block | 4 | 16 | 2,400 L | ~5,500 L |
| Small block | 8 | 32 | 4,800 L | ~11,000 L |
| Mid-rise | 16 | 64 | 9,600 L | ~22,100 L |
| Mid-rise | 24 | 96 | 14,400 L | ~33,100 L |
| High-rise | 40 | 160 | 24,000 L | ~55,200 L |
| Large complex | 80 | 320 | 48,000 L | ~110,400 L |
| Large complex | 120 | 480 | 72,000 L | ~165,600 L |
Actual requirements depend on local regulations, roof load limits, and supply conditions. Always verify with a licensed plumber or civil engineer before procurement.
🏢
Rooftop vs. Underground
Rooftop tanks rely on gravity for pressure — cheap but structurally demanding. Underground sumps need a pump but avoid roof load issues. For buildings over 6 floors, a combined system (underground sump + overhead tank) is standard practice.
⚖️
Roof Load Reality
1,000 litres = 1,000 kg of water. A 10,000-litre rooftop tank adds 10 tonnes to your structure. Always get a structural engineer to sign off before installation.
🔧
Split Tanks
Large requirements are often met with multiple tanks in parallel. This also enables maintenance shutdowns without completely cutting off supply to the building.
🏠 Use Cases
When Do You Need This Calculator?
New Building Construction
Architects and developers use this during design-stage planning to spec the right tank size and check whether the rooftop structure can bear the load, or whether an underground cistern is needed. Getting this wrong at design stage is expensive to fix later.
Replacing an Undersized Existing Tank
If your building is running out of water before daily supply arrives, this calculator helps you quantify the deficit. Compare the recommended output against your current tank size to understand how much extra capacity you need to add.
Post-Construction Renovation
Building managers who have converted additional flats, added floors, or seen occupancy increase above original design specs can use this to check whether the original tank is still adequate for the current resident count.
Emergency Preparedness Planning
Adjust the backup days slider to see how much storage you'd need to survive a 3-day, 5-day, or 7-day supply disruption — relevant for buildings in areas prone to water supply outages, natural disasters, or civil emergencies.
Housing Societies & Facility Managers
Use the output as a benchmark when requesting quotes from tank suppliers. The recommended tank size gives you a starting point; factor in your local building codes and any regulatory minimum storage requirements before finalising the spec.
❓ FAQ
Frequently Asked Questions
How do I calculate water tank size for an apartment building?
Multiply the total number of residents by the daily water usage per person (typically 135–200 litres), then multiply by your desired backup days. Add a minimum 15% safety buffer on top. The formula: Tank Size = (Residents × Daily Usage × Backup Days) × 1.15. This calculator does all of that for you automatically.
What is the standard water tank size for a 10-flat apartment building?
With 4 residents per flat (40 total), standard usage of 150 L/person/day, and 2 backup days, the recommended size is roughly 13,800 litres (including 15% buffer). In practice, you'd install two 7,000-litre tanks or one 15,000-litre tank depending on available space and structural constraints.
How many litres of water does an apartment building use per day?
It depends heavily on the number of residents and their usage habits. A rough benchmark is 135–200 litres per person per day for urban apartment living. A 20-flat building with 4 residents each (80 people) at 150 L/day consumes around 12,000 litres per day.
Should I use a rooftop tank or an underground sump for my apartment?
For low-rise buildings (up to 4–5 floors), a rooftop tank is the simplest and cheapest option — gravity feeds water pressure without a pump. For taller buildings, a combined system is standard: a large underground sump stores the main reserve and a pump fills a smaller overhead tank, which gravity-feeds the building. Buildings with very large requirements (10,000+ litres) often need underground storage regardless of height, since rooftop structural limits typically cap out at 3,000–5,000 litres safely.
What is the recommended water storage per person per day?
The WHO minimum for basic needs is 50 litres per person per day (drinking, cooking, basic hygiene). The WHO urban benchmark for apartment living is around 135–150 litres. Most comfort-standard urban apartments use 150–200 litres per person. Use the usage level selector in this calculator to match your building's real-world consumption pattern.
How many backup days should I size my apartment water tank for?
In cities with reliable municipal supply, 1–2 days is usually sufficient. In regions with scheduled water rationing or frequent outages — common in parts of South Asia, MENA, and Sub-Saharan Africa — 3–5 days is the practical minimum. For off-grid or emergency scenarios, 7 days is the standard preparedness target. When in doubt, go bigger — oversized tanks hold you through outages, undersized tanks don't.
Can I split one large tank into multiple smaller ones?
Yes, and for large buildings it's often the better approach. Multiple tanks in parallel give you redundancy — one can be taken offline for cleaning or repair without cutting off the whole building. They also distribute roof load more evenly. Use the recommended litres output from this calculator as your total target and split it across as many tanks as makes structural and logistical sense.
Does the number of floors affect how big the water tank needs to be?
The number of floors doesn't directly affect storage volume — that's determined by residents and usage. However, floors do affect pressure requirements and pump sizing. A rooftop tank on a 10-floor building needs to deliver adequate pressure to all floors, which may require a pressurisation system or a taller header tank. Use our Minimum Tank Height for Shower Pressure calculator for pressure planning.