Mains Water Pipe Sizing
Pipe Sizing Procedure
1. Reference the pipe section.
2. Calculate flow rates from Table below.
3. Estimate flow rates in each section.
Keep velocity below 2 m/s. See also CIBSE Guide G (2003) part 2, Table 2.19.
4. Estimate pipe diameter from pipe sizing tables in CIBSE Guide C.
5. Measure the pipe run from drawings.
6. Calculate length of pipe equal to resistance of fittings.
The Total equivalent length of a fitting = Equivalent Length x Pressure Loss factor z(Zeta).
7. Calculate effective pipe length.
8. Determine pressure loss due to friction from CIBSE Tables.
9. Calculate pressure consumed due to friction (Pa) = effective pipe length (m) x pressure loss due to friction (Pa/m)
10. Calculate total pressure consumed = Friction loss + Static pressure loss
11. Determine pressure at start of section.
12. Calculate pressure available at end of section = Pressure at start of section  Total pressure consumed
If pressure available at end of section is less than the maximum allowable pressure drop then we can accept this pipe size.
13. Determine pressure required at end of section, this can be the minimum pressure that is required for terminal equipment.
14. If the pressure available at the end of the section is more than or equal to the pressure required at the end of the section then the pipe size is correct.
Water Flow Rates
Cold water flow rates for
sanitary appliances for small installations may be found from the table below.
Approximate
hot or cold water demand 
Flow rate
(l/s) 
Basin
(spray tap) 
0.05 
Basin
(tap) 
0.15 

0.30 

0.60 

0.10 
Shower
(nozzle) 
0.15 
Shower
(100mm rose) 
0.40 
Sink
(15mm tap) 
0.20 
Sink
(20mm tap) 
0.30 
Wash
fountain 
0.40 
In
larger more complex buildings where many sanitary appliances are installed
simultaneous demand should be considered from tables CIBSE Guide B (1986) B4.20
and B4.21.
Notes:
An
alternative method of pipe sizing is to use a nomogram.
This
can be found in CIBSE Guide G (2004) Public Health
Engineering Figure 2.21.
Pipe Sizing table
Pressurised Cold Water Pipe Sizing Table 


1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 


Ref 
Demand Units if required 
Flow Rate
(l/s) 
Estimated Pipe Dia.
(mm) 
Measured Pipe Run
(m) 
Length of Pipe Equal to Fittings Resistance’s (m) 
Effective Pipe Length (m) 
Pipe Pressure Loss (Pa/m) From CIBSE Tables 
Pressure Consumed due to Friction (Pa) 
Friction loss + Static pressure loss = Total Pressure Consumed (Pa) 
Pressure at Start of Section
(Pa) 
Pressure Available at End of Section (Pa) 
Pressure Required at End of Section
(Pa) 
Final Pipe Size
(mm) 


































































































































































































































































Example 1
Calculate
an appropriate pipe size for the system shown above.
Use Copper
Table X pipework for water at 10^{o}C..
Answer
The
maximum allowable pressure drop along the length of pipe = 300,000
Pa – 250,000 Pa = 50,000 Pa
Pressurised Cold Water Pipe Sizing Table 

1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 

Ref 
Demand Units if required 
Flow Rate
(l/s) 
Estimated Pipe Dia.
(mm) 
Measured Pipe Run
(m) 
Length of Pipe Equal to Fittings Resistance’s (m) 
Effective Pipe Length (m) 
Pipe Pressure Loss (Pa/m) From CIBSE Tables 
Pressure Consumed due to Friction (Pa) 
Friction loss + Static pressure loss = Total Pressure Consumed (Pa) 
Pressure at Start of Section
(Pa) 
Pressure Available at End of Section (Pa) 
Pressure Required at End of Section
(Pa) 
Final Pipe Size
(mm) 

A 
none 
0.8 
22 
50 
none 
50 
3500 
175,000 
175,000 
300,000 
125,000 
250,000 
Too small 

A 
none 
0.8 
28 
50 
none 
50 
1000 
50,000 
50,000 
300,000 
250,000 
250,000 
28 
















Pipe Sizing Procedure
1. Reference
the pipe section  section A.
2. Calculate
demand units or loading units from Tables in CIBSE guide (attached). – not required, see No.3 below.
3. Estimate
flow rates in each section. Keep velocity below 2 m/s.
 given
4. Estimate
pipe diameter from pipe sizing tables in CIBSE Guide C. – 22mm (velocity too high at approx 2.4 m/s) or 28mm (velocity is 1.5
m/s).
5. Measure
the pipe run from drawings. – 50m
6. Calculate
length of pipe equal to resistance of fittings. – no
fittings
7. Calculate
effective pipe length.  50m
8. Determine
pressure loss due to friction from CIBSE Tables. See
Table 4.18 in Guide C (CD version).
9. Calculate
pressure consumed due to friction (Pa) = effective pipe
length (m) x pressure loss due to friction (Pa/m). Column 7 x 8 in Pipe Sizing Table.
10. Calculate
total pressure consumed = Friction loss + Static pressure loss. There are no vertical pipe sections and therefore no static
pressure loss.
11. Determine
pressure at start of section. Given in drawing as
300,000
12. Calculate pressure
available at end of section = Pressure at start of
section  Total pressure consumed.
300,000 – 175,000 =
125,000
If pressure available at end of section is less than the
maximum allowable pressure drop then we can accept this pipe size.
13. Determine
pressure required at end of section, this can
be the minimum pressure that is required for terminal equipment. Given in drawing as 250,000
14. If
the pressure available at the end of the
section is more than or equal to the pressure
required at the end of the section then the pipe size is correct.
28mm pipe is correct, 22mm is too
small since there is not enough pressure available
at the end of the section and the water velocity is also too high.
Example 2
Calculate
an appropriate pipe size for the system shown above.
Use Copper
Table X pipework for water at 10^{o}C..
Answer
The
maximum allowable pressure drop along the length of pipe = 120,000
Pa – 90,000 Pa = 30,000 Pa
Pressurised Cold Water Pipe Sizing Table 


1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 


Ref 
Demand Units if required 
Flow Rate
(l/s) 
Estimated Pipe Dia.
(mm) 
Measured Pipe Run
(m) 
Length of Pipe Equal to Fittings Resistance’s (m) 
Effective Pipe Length (m) 
Pipe Pressure Loss (Pa/m) From CIBSE Tables 
Pressure Consumed due to Friction (Pa) 
Friction loss + Static pressure loss = Total Pressure Consumed (Pa) 
Pressure at Start of Section
(Pa) 
Pressure Available at End of Section (Pa) 
Pressure Required at End of Section
(Pa) 
Final Pipe Size
(mm) 


A 
none 
0.5 
22 
14 
1.6 
15.6 
1500 
23,400 
23,400 
120,000 
96,600 
90,000 
22 


































Pipe Sizing Procedure
1. Reference
the pipe section  section A.
2. Calculate
demand units or loading units from Tables in CIBSE guide (attached). – not required
3.
Estimate flow rates in each section. Keep velocity below 2 m/s.  given
4.
Estimate pipe diameter from pipe sizing tables in CIBSE Guide C. – 22mm (velocity is 1.5 m/s).
5.
Measure the pipe run from drawings. – 14m
6.
Calculate length of pipe equal to resistance of fittings. – 2 bends.
The Total equivalent length of a
fitting = Equivalent Length x Pressure Loss factor z(Zeta).
See Pipe Sizing Heating
Section  page 4  pipe fitting losses.
Copper pipe elbow z(Zeta) = 1.0 x 2 bends = 2.0
Determine equivalent
length from CIBSE table C4.18, le = 0.8
Total equivalent length of fittings
= 0.8 x 2.0 = 1.6 metres.
7.
Calculate effective pipe length.  15.6m
8.
Determine pressure loss due to friction from CIBSE Tables. See Table 4.18 in Guide C (CD version).
9.
Calculate pressure consumed due to friction (Pa) = effective
pipe length (m) x pressure loss due to friction (Pa/m). Column 7 x 8 in Pipe Sizing Table.
10.
Calculate total pressure consumed = Friction loss + Static pressure
loss. There are no vertical pipe sections and
therefore no static pressure loss.
11.
Determine pressure at start of section. Given in
drawing as 120,000
12.
Calculate pressure available at end of
section = Pressure at start of section  Total
pressure consumed. 120,000 – 23,400
= 96,600
If pressure
available at end of section is less than the maximum allowable pressure
drop then we can accept this pipe size.
13.
Determine pressure required at end of
section, this can be the minimum pressure that is required for terminal
equipment. Given in drawing as 90,000Pa.
14. If
the pressure available at the end of the
section is more than or equal to the pressure
required at the end of the section then the pipe size is correct.
22mm pipe is correct.