Choose the smallest pipe diameter whose friction loss at the design flow still leaves at least 0.5 bar (5 m of head) at the tap, using the Hazen-Williams equation: 2.5 L/s through 50 mm plastic pipe loses about 3.3 m of head per 100 m at C = 150, while 1 L/s through the same pipe loses only 0.6 m. For a spring, time a 20 L bucket (20 L in 60 s is 0.33 L/s) and size the pipe to that flow and the head you have, not to a rule of thumb.
Part of Water Distribution
Pipe sizing is the calculation that balances diameter against friction loss, so that water arrives at each tap with usable pressure. It serves two readers: the person sizing every section of a village network, and the person matching a single pipe to the measured flow of a spring (Spring Capture).
Friction Eats Head, So The Pipe Is Sized By What Is Left At The Tap
A gravity line has one budget: the height of the water surface above the tap, minus what friction takes on the way. The distribution rules in this wiki require at least 0.5 bar (5 m of head) at each tap and no more than 6 bar (60 m) in any section. Everything between is the friction allowance. A bigger pipe loses less head, a smaller pipe costs less, and sizing is finding the cheapest pipe that stays inside the allowance.
The equation, as given in Distribution Network, is Hazen-Williams in SI units:
h = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87)
where h is friction loss in metres of head, L is length in metres, Q is flow in m³/s, C is the roughness coefficient and d is the internal diameter in metres. Handbook values for C are about 140–150 for smooth new plastic; this page designs with 140 to leave margin. Old cast iron is 100 and bamboo or wood is 80–100.
The Diameter Enters As The 4.87th Power, So Small Errors Are Costly
Plastic pipe is sold by outside diameter, and the bore is smaller by twice the wall. A pipe sold as 50 mm with a 45 mm bore has friction (50 ÷ 45)^4.87 = 1.111^4.87 = 1.67 times the figure for a 50 mm bore, 67% more. Always use the bore from the pipe’s data, not the label. The tables below treat nominal as bore, so real losses will be higher.
Friction Loss For The Standard Sizes
Inputs: flow from the sizing table in Water Distribution, C = 140, bore = nominal. Result is head lost per 100 m and the water speed (flow ÷ bore area).
| Pipe | Flow | Loss per 100 m | Speed |
|---|---|---|---|
| 25 mm | 0.5 L/s | 5.5 m | 1.02 m/s |
| 32 mm | 1 L/s | 6.0 m | 1.24 m/s |
| 50 mm | 2.5 L/s | 3.7 m | 1.27 m/s |
| 63 mm | 4 L/s | 2.9 m | 1.28 m/s |
| 75 mm | 6 L/s | 2.6 m | 1.36 m/s |
With C = 150 the 50 mm line at 2.5 L/s loses 3.3 m per 100 m. The sibling page distribution-network allows only about 1.0 L/s in a 50 mm main; at that flow the loss is 0.68 m per 100 m at C = 140 and 0.6 m at C = 150, at a speed of 0.51 m/s. The two pages do not contradict each other: they are different friction budgets from one equation. The higher flows suit short runs with ample head, the lower ones long runs.
What You Need
- Head available — measured height from the tank or source water level down to the tap, in metres
- Run length — measured along the pipe, not as the crow flies
- Design flow — see below
- The bore of the pipe you can actually buy — and its pressure class
Method: Budget The Head, Then Test Each Size
Step 1: Static head. Source level minus tap level, in metres. Ten metres is about 1 bar. If it exceeds 60 m, split the line with break pressure tanks.
Step 2: Friction allowance = static head − 5 m. This is the most the whole line may lose.
Step 3: Design flow. For a network, peak flow is not the daily total spread over 24 hours: the peak hour runs 2.5–3 times the average. For standpipes, count taps that can run at once and multiply by the flow of one tap. For a spring, use the measured flow.
Step 4: Try a diameter. Compute loss per 100 m from the equation, multiply by the run in hundreds of metres, compare with the allowance. Go up one size if it fails; “when in doubt go one size up” holds because a buried pipe cannot be enlarged later.
Step 5: Check the pressure class. The pipe’s rated pressure must exceed the static head, with margin for surges from closing a tap fast. A pipe class number is its nominal pressure in bar at 20 °C.
Worked Example: A Network Section
A standpipe is 300 m from a tank whose bottom is 12 m above it. Allowance = 12 − 5 = 7 m. Four taps can run at once at 0.33 L/s each (a 20 L bucket in 60 s), so the design flow is about 1.3 L/s.
- 32 mm at 1.3 L/s is already beyond its table figure; at 1 L/s it loses 6.0 m per 100 m × 3 = 18 m, far over 7 m. Fails.
- 50 mm at 1.3 L/s: loss per 100 m at C = 140 is 10.67 × 100 × (0.0013)^1.852 ÷ (140^1.852 × 0.05^4.87) = about 1.1 m, so 3.3 m over 300 m. Residual head = 12 − 3.3 = 8.7 m, about 0.87 bar. Passes.
Worked Example: A Spring
A bucket test gives 20 L in 60 s: 20 ÷ 60 = 0.33 L/s = 20 L/min = 28,800 L/day. The pipe runs 400 m and drops 14 m; allowance = 14 − 5 = 9 m, or 2.25 m per 100 m.
- 25 mm at 0.33 L/s loses 2.56 m per 100 m at C = 140. Slightly over; fails.
- 32 mm loses 0.77 m per 100 m, 3.1 m over the run. Passes with a wide margin and a speed of 0.41 m/s.
Size to the flow you will carry. Excess spring flow leaves through the box overflow. A drop pipe much larger than needed gives a slow speed that lets silt settle.
Common Mistakes
- Reading the label as the bore. Friction rises 67% for a 50 mm label with a 45 mm bore.
- Sizing to daily volume. The line must pass the peak, set by taps in use together.
- Mixing units. Hazen-Williams here takes m³/s and metres; 1 L/s is 0.001 m³/s and 50 mm is 0.05 m.
- Ignoring pressure class. The lowest point of a steep line sees the full static head.
Safety
Pressurised pipe fails violently, and a poor connection can contaminate the whole network
A burst or blown-off fitting under several bar throws fragments. Test new lines with water, with the air bled out first, and never with compressed air, which stores far more energy and can shatter plastic pipe. Keep every non-potable line physically separate from the drinking-water network: a cross-connection draws contaminated water in whenever pressure drops, which is the mechanism behind most historical network poisonings.
What’s Next
- Learn next: Pressure Zones — what to do when the static head exceeds the pipe’s limit
- Combine with: Leak Detection — losses reduce the flow the sizing assumed