Size a distribution tank as two parts added together: a buffer for the daily peaks and a reserve for outages. A source delivering a steady 1,500 L/day to 30 people with two daily peaks needs a buffer of about 360 L, and the rule of at least one full day, 1,500 L, covers that buffer plus roughly 1,140 L of reserve for repairs and dry spells.
Part of Water Distribution
Tank sizing is working out how much water a distribution tank must hold: enough to cover the gap between steady supply and peaked demand, plus a reserve for failures. The volume is set by the pattern of demand, not just the daily total.
Peaks Are Covered By The Difference Between What Arrives And What Is Drawn
A tank fills when supply exceeds demand and empties when demand exceeds supply. The buffer you need is the largest amount the tank ever falls behind. Plot the running total of (supply − demand) through the day: the tank must hold the distance from the highest point to the lowest point on that curve.
The water-distribution page says to hold at least one full day of demand. That figure covers more than the daily peaks; it also gives time to repair the source or a main.
What You Need
- Daily demand: people × litres per person (20 L minimum, 50 L comfortable)
- A pattern of use across the day: when people draw water. Count it for your own community by watching taps for a few days.
- The source’s steady delivery, measured in the dry season
- A decision on reserve: how many days you want to ride out a failure
Method: Run The Day Hour By Hour
Step 1: Write down demand for each hour.
Step 2: Write down supply for each hour (a steady source delivers the same each hour).
Step 3: Take the running total of supply minus demand.
Step 4: Read the buffer: highest value minus lowest value of that running total.
Step 5: Add the reserve for failures.
Worked Example
30 people × 50 L = 1,500 L/day. The source delivers a steady 1,500 ÷ 24 = 62.5 L/h. For illustration, assume 30% of the day’s water is drawn in each of two 2-hour peaks (06:00-08:00 and 17:00-19:00) and the remaining 40% is spread over the other 20 hours. (This pattern is an assumption; measure your own.)
- Peak hours: 30% × 1,500 = 450 L per peak = 225 L/h, against 62.5 L/h arriving, so the tank loses 162.5 L each hour
- Other hours: 40% × 1,500 = 600 L over 20 hours = 30 L/h, against 62.5 L/h arriving, so the tank gains 32.5 L each hour
Running total of supply minus demand, in litres, starting at midnight: it climbs 32.5 L/h through the early hours to a high of +195 L at 06:00, falls to −130 L at 08:00 (the morning peak), climbs to +162 L at 17:00, then falls to −162.5 L at 19:00, and recovers by midnight to zero.
Buffer = highest − lowest = 195 − (−162.5) = 357.5 L, about 360 L, which is 24% of the day’s demand. The rule of one full day (1,500 L) therefore gives the buffer plus 1,500 − 360 = 1,140 L, about 0.76 of a day, as a reserve. If the source can fail for longer than that, size the reserve for it. The water-distribution page gives 2,000-5,000 L as a comfortable range for a community of this size.
Common Mistakes
- Sizing to the daily total only. The peak sets the buffer.
- Counting the reserve as the buffer. A tank kept at a day’s level has no spare space unless you refill it.
- Taking the demand pattern from another village. Use yours.
- Sizing a tank for a source that is lower in the dry season. Use the dry-season flow.
What’s Next
- Learn next: Float Valves — keeping the tank topped up automatically
- Combine with: Reservoir Design — larger storage measured in days of supply