Size a household tank at 3 days of use and set its bottom 8-10 m above the highest tap: four people at 30-60 L each per day need 360 to 720 L, which contains the parent page’s 400-600 L, while a 3 m tower gives only 0.29 to 0.49 bar at ground level, far short of what indoor plumbing needs. The parent page’s 5,000-10,000 L for a community of 30 is about 1.9 times the 2,700 to 5,400 L that the same arithmetic gives, the difference being margin the page does not itemise.
Tank sizing sets the volume from daily use and the number of days of reserve; tank placement sets the height, and with it the pressure at every tap. The two interact, because a bigger tank is heavier and harder to lift.
Sizing: Use Per Person, Then Days
The parent page lists daily use per person: drinking and cooking 5-10 L, washing and hygiene 15-30 L, laundry 10-20 L; garden irrigation is separate and highly variable. Adding the three household items: 5 + 15 + 10 = 30 L to 10 + 30 + 20 = 60 L per person per day. Without laundry: 20 to 40 L.
The minimum storage is 3 days.
Worked Example: Household And Community
Family of four, three days, with laundry:
- Daily = 4 × 30 = 120 L to 4 × 60 = 240 L.
- Three days = 360 to 720 L.
- The parent page’s 400-600 L lies inside this range.
Community of 30:
- Daily = 30 × 30 = 900 L to 30 × 60 = 1,800 L.
- Three days = 2,700 to 5,400 L.
- The parent page gives 5,000-10,000 L. Dividing: 5,000 ÷ 2,700 = 1.85 and 10,000 ÷ 5,400 = 1.85, so about 1.9 times. The page does not say what the extra covers; plausible uses are garden water, losses and a longer reserve, but I have no source for them. Treat the arithmetic as the minimum and the page’s figure as a planning range with margin. Tank Sizing shows how to split volume into a peak buffer and an outage reserve.
Do Not Oversize: Stale Water Is A Hazard
A 6,000 L tank (the parent page’s 2 m × 2 m ferrocement tank) serving four people holds 6,000 ÷ 240 = 25 days to 6,000 ÷ 120 = 50 days of supply. Water that sits that long can recontaminate and grow bacteria (Water Storage). Size the tank to the real use plus the reserve you need, and size the supply to refill it: a spring giving 0.33 L/s fills 6,000 L in 6,000 ÷ 0.33 = 18,200 s = 5.1 hours, so the source is not the constraint.
Placement: Height Becomes Pressure
Pressure at a tap is 0.098 bar for each metre between the water surface and the tap, less friction (Calculating Head Pressure). The parent page’s minimum practical head for indoor plumbing is 8-10 m. Tank Placement sets the bottom of the tank at least 5 m above the highest tap, and the two fit together if friction takes 3-5 m.
The water level also changes. A tank 2 m deep has its surface 2 m above its bottom when full: a tap 8 m below the bottom has 8 m of head when the tank is empty (0.78 bar) and 10 m when full (0.98 bar), so the pressure swings by about 0.2 bar as the tank drains.
A Short Tower Gives Very Little Pressure
The parent page’s tower: a 3 m tower with a 2 m tank puts taps “3-5 m below the tank bottom”. More precisely, a tap at ground level is 3 m below the tank bottom and 5 m below the full water surface:
- Empty: 3 × 0.098 = 0.29 bar.
- Full: 5 × 0.098 = 0.49 bar.
That is at or below the parent page’s own 0.5 bar “trickle” figure and well short of its 8-10 m for indoor plumbing. So a 3 m tower serves a ground-level garden tap, not a house. For house plumbing, put the tank on a hillside, or build a much taller tower, or accept low flow.
The Weight Is The Constraint
Water weighs 1 kg per litre. A 6,000 L tank holds 6,000 kg of water, which is 6,000 × 9.81 = 59 kN, before the tank’s own weight. A rooftop tank needs a roof built for it. A tower needs posts and bracing for the full weight plus wind; four posts carry about 15 kN each, and the ground under them needs to carry it too. Hillside tanks put the weight on the ground, which is why they are preferred.
What You Need
- Daily use and household size (Storage Tanks & Cisterns)
- Highest tap elevation relative to the planned tank site
- A hose level or tube level for height (Water Tube Level)
- A site with firm ground and a clear route for the pipe
Method: Size, Then Place
Step 1: Count people and multiply by 30-60 L per day. Multiply by at least 3 days.
Step 2: Compare with the parent page’s 400-600 L (family) or 5,000-10,000 L (community) and choose a volume that is not wastefully large.
Step 3: Survey the heights. Find the highest tap and the highest ground within pipe reach.
Step 4: Place the tank bottom 8-10 m above the highest tap if you can; otherwise at least 5 m above it plus the friction loss.
Step 5: Check the weight and the base; add an overflow and a drain (Overflow Pipe).
Common Mistakes
- Measuring the head to the tank top instead of its bottom or surface.
- A short tower for a whole house.
- A huge tank for a small household.
- A weak support for a heavy tank.
- Ignoring that pressure falls as the tank drains.
Safety
A raised tank is a heavy load that can kill if it falls
6,000 L is about 6 tonnes. Build the base or tower for the full weight with margin, brace it against wind and never stand under a tank that is being lifted. Working at height needs a fall-arrest plan. The tank is a confined space once lidded: no one enters without ventilation, a harness and line and a second person outside who never climbs in; do not test the air with a flame.
What’s Next
- Learn next: Distribution & Fixtures — the taps and pipes the tank feeds
- Combine with: Pressure Zones — when the site spans a large drop