Pipe Flow Calculator

The Pipe Flow Calculator finds water flow rate, velocity, friction pressure drop or the pipe size you need for a full pipe, using the Hazen-Williams equation and the continuity equation Q = A × v.

How to Calculate Flow Rate in a Pipe

Flow rate is the bore area of the pipe multiplied by the average speed of the water.

$$Q=A\times v,A=\frac{\pi d^2}{4}$$

Always use the inside diameter. A 1-inch Schedule 40 pipe has a 1.049 in bore, while 1-inch Type L copper has a 1.025 in bore. The nominal size is only a name.

In US units, two shortcuts cover most field checks. Here d is the inside diameter in inches, v is velocity in ft/s, and Q is flow in GPM.

$$Q=2.448\times d^2\times v$$

$$v=\frac{0.4085\times Q}{d^2}$$

For example, 1-inch Schedule 40 pipe at 5 ft/s carries 2.448 × 1.049² × 5 = 13.5 GPM. In SI units the same relation is v = 1.273 × Q ÷ d², with Q in m³/s and d in meters.

Flow from Pipe Drop or Pressure: The Hazen-Williams Equation

Often you know how much head is available but not how fast the water will move. Friction sets that speed. The Hazen-Williams equation links velocity to pipe roughness, bore size and the energy slope.

$$v=k\times C\times R^{0.63}\times S^{0.54}$$

• v is the mean velocity, in m/s or ft/s.
• k is a unit factor: 0.849 for SI units and 1.318 for US units, per the Hazen-Williams equation reference.
• C is the roughness coefficient of the pipe material. Smoother pipe has a higher C.
• R is the hydraulic radius. For a round pipe running full, R = d ÷ 4.
• S is the slope of the energy line, which is the head lost divided by pipe length (Δh ÷ L).

For a full round pipe in US units, combining this with Q = A × v gives a one-line formula. Q is in GPM, d is the inside diameter in inches, and S is feet of head per foot of pipe. The 0.2815 constant comes from converting the 1.318 form to these units.

$$Q=0.2815\times C\times d^{2.63}\times S^{0.54}$$

A pressure entry is turned into head before S is found. One psi equals about 2.31 ft of water at room temperature, and the pipe flow calculator uses a water density of 998.2 kg/m³ (20 °C) for this step.

Hazen-Williams is empirical and fitted to water near 60 °F in turbulent flow. It does not suit oil, glycol, hot water near boiling, or slow laminar flow, so the pipe flow calculator warns when the Reynolds number falls below 4,000. Run the friction loss on its own with the Hazen Williams Equation Calculator.

The equation also assumes the pipe runs full. A sloped drain, sewer or culvert that runs partly full is open-channel flow, so use the Partial Pipe Flow Calculator, the Mannings Flow Calculator or the Culvert Flow Calculator for those.

Example 1: Gravity Flow Through a 2-Inch Pipe

With the pipe flow calculator defaults, a tank feeds 200 ft of 2-inch Schedule 40 PVC (inside diameter 2.067 in, C = 150). The outlet sits 8 ft below the tank water level.

1. Hydraulic radius R = 0.052502 m ÷ 4 = 0.013125 m. Slope S = 8 ÷ 200 = 0.04.
2. Velocity v = 0.849 × 150 × 0.013125^0.63 × 0.04^0.54 = 1.461 m/s, or 4.79 ft/s.
3. Area A = π × 0.052502² ÷ 4 = 0.0021649 m². Flow Q = A × v = 0.003162 m³/s, or 50.1 GPM.

That 50.1 GPM counts pipe friction only. When a pipe drains a tank into open air, water also loses head entering the pipe (K = 0.5) and leaving it (K = 1.0). Those 1.5 velocity heads take 0.50 ft of the 8 ft and cut the flow to 48.4 GPM, about 3.4% less. On a 100 ft run with 4 ft of drop, the same slope, the cut is about 7%, so the gap grows on short runs and fades on long ones. To see how long the tank lasts, find its volume with the Tank Volume Calculator.

Example 2: Pressure Drop at a Known Flow

Now push 60 GPM through the same 100 ft of 2-inch Schedule 40 pipe. Velocity is 0.4085 × 60 ÷ 2.067² = 5.74 ft/s. Hazen-Williams gives 5.58 ft of head loss, which is 2.42 psi.

In US units the pressure form gives the same answer, with P in psi, L in ft, Q in GPM and d in inches.

$$P=\frac{4.52\times L\times Q^{1.852}}{C^{1.852}\times d^{4.87}}$$

This is friction in straight pipe only. Add 0.433 psi for every foot the pipe rises, and add the equivalent length of elbows, tees and valves to L before you trust the total.

Example 3: Sizing a Pipe for 60 GPM

To size a pipe, pick the fastest velocity you allow and solve for the smallest bore.

$$d=\sqrt{\frac{4Q}{\pi v_{max}}}$$

At 60 GPM with a 7 ft/s limit, the smallest bore is 1.871 in. The first Schedule 40 size above that is 2-inch, with a 2.067 in bore and 5.74 ft/s. A 1½-inch Schedule 40 pipe (1.610 in bore) would push the water to 9.46 ft/s.

Hazen-Williams C Values by Pipe Material

C drops as pipe ages, scales or corrodes, and a lower C means more friction. The ranges below follow published Hazen-Williams tables from Engineering ToolBox and ToroAg.

Pipe Material	New Pipe C	Older or Design C
PVC, CPVC	150	150
Polyethylene (PE)	140	140
Copper	130 to 140	130
Ductile iron, cement lined	140	140
Cast iron, unlined	130	89 to 100 at 20 years
Steel, new unlined	140 to 150	100
Galvanized iron	120	120
Concrete	100 to 140	100 to 110

The pipe flow calculator presets cover Plastic 150, Copper 140, Steel new 140, Steel used 120, Cast iron new 130, Cast iron aged 100 and Concrete 110. For pipe that is scaled or decades old, enter a lower custom C.

Material matters more than most people expect. Here is the friction loss for 60 GPM through 100 ft of 2-inch Schedule 40 pipe at different C values.

C Value	Head Loss	Pressure Loss
150	5.58 ft	2.42 psi
140	6.34 ft	2.74 psi
130	7.28 ft	3.15 psi
120	8.44 ft	3.65 psi
100	11.83 ft	5.12 psi

Going from C = 150 to C = 100 more than doubles the loss at the same flow.

Maximum Flow Rate by Pipe Size

A pipe has no fixed maximum flow, since more pressure pushes more water. The practical limit is velocity, because fast water causes noise, erosion and water hammer.

UPC 610.12.1 caps copper and copper-alloy tube at 8 ft/s for cold water and 5 ft/s for hot water. Section 610.12.2 applies the same caps to copper fittings in other tubing. Above 140 °F, Watts cites Copper Tube Handbook guidance of 2 to 3 ft/s for copper.

So a 2-inch Schedule 40 pipe tops out near 83.7 GPM at 8 ft/s and 52.3 GPM at 5 ft/s. A 1-inch pipe carries 21.6 GPM and 13.5 GPM, and a 4-inch pipe carries 317 GPM and 198 GPM.

Schedule 40 Steel or PVC Pipe

Nominal Size	Inside Diameter	GPM at 5 ft/s	GPM at 8 ft/s
½ in	0.622 in	4.7	7.6
¾ in	0.824 in	8.3	13.3
1 in	1.049 in	13.5	21.6
1¼ in	1.380 in	23.3	37.3
1½ in	1.610 in	31.7	50.8
2 in	2.067 in	52.3	83.7
2½ in	2.469 in	74.6	119.4
3 in	3.068 in	115.2	184.3
4 in	4.026 in	198.4	317.4
6 in	6.065 in	450.2	720.4
8 in	7.981 in	779.6	1,247
10 in	10.020 in	1,229	1,966
12 in	11.938 in	1,744	2,791

Type L Copper Tube

Nominal Size	Inside Diameter	GPM at 5 ft/s (Hot)	GPM at 8 ft/s (Cold)
½ in	0.545 in	3.6	5.8
¾ in	0.785 in	7.5	12.1
1 in	1.025 in	12.9	20.6
1¼ in	1.265 in	19.6	31.3
1½ in	1.505 in	27.7	44.4
2 in	1.985 in	48.2	77.2
2½ in	2.465 in	74.4	119.0
3 in	2.945 in	106.2	169.9
4 in	3.905 in	186.6	298.6

Inside diameters come from ASME B36.10M wall thickness for Schedule 40 steel (PVC Schedule 40 under ASTM D1785 shares the same outside diameters and walls) and ASTM B88 for Type L copper. The UPC velocity caps apply to copper, so for PVC or PE check the pipe maker's velocity limit.

Flow Rate for Metric Pipes: 25 mm, 32 mm and 100 mm

Metric plastic and copper pipes are named by outside diameter, so the bore is smaller than the name. A 25 mm PE pipe can have a bore near 20 mm, depending on its pressure class. Use the true bore in the table below.

Bore	1 m/s	1.5 m/s	2 m/s	3 m/s
15 mm	10.6 L/min	15.9 L/min	21.2 L/min	31.8 L/min
20 mm	18.8 L/min	28.3 L/min	37.7 L/min	56.5 L/min
25 mm	29.5 L/min	44.2 L/min	58.9 L/min	88.4 L/min
32 mm	48.3 L/min	72.4 L/min	96.5 L/min	144.8 L/min
40 mm	75.4 L/min	113.1 L/min	150.8 L/min	226.2 L/min
50 mm	117.8 L/min	176.7 L/min	235.6 L/min	353.4 L/min
100 mm	471.2 L/min	706.9 L/min	942.5 L/min	1,414 L/min

A 25 mm bore at 2 m/s carries 58.9 L/min (15.6 GPM). A 100 mm bore at the same speed carries 942 L/min (249 GPM). For reference, the UPC copper caps work out to 2.4 m/s cold and 1.5 m/s hot.

Water Flow Rate from Pressure and Pipe Diameter

Pressure alone does not set the flow. What matters is the pressure difference the pipe uses up between its two ends. A static gauge reading taken with every tap closed is not the pressure left once water starts moving.

To work from pressure, pick psi, kPa or bar for the drop entry and type the pressure you can afford to lose along the pipe. The pipe flow calculator treats that figure as measured at both pipe ends, so no entrance or exit loss is subtracted, and it shows the flow one pipe size up for comparison.

In a pumped system, the pump must cover static lift plus friction. Convert between feet of head and psi with the Pump Pressure Calculator.

How to Measure Water Flow Rate

The simplest check is a bucket test. Open the tap or valve fully and time how long it takes to fill a container of known size.

$$\text{GPM}=\frac{\text{gallons}\times 60}{\text{seconds}}$$

A 5-gallon bucket that fills in 25 seconds means 5 × 60 ÷ 25 = 12 GPM. In metric, litres × 60 ÷ seconds gives L/min. The Gallons Per Minute Calculator (GPM) and the Liters per Minute Calculator do this math for you.

Compare the measured flow with the pipe flow calculator result. A large shortfall usually points to a partly closed valve, scale inside old pipe, or long runs of small fittings.

How to Calculate Nozzle Flow Rate

Flow from a nozzle or open hydrant outlet depends on the pressure at the tip, not on pipe friction. NFPA 291 fire flow testing uses the Freeman formula, with Q in GPM, d as the outlet inside diameter in inches, and p as the pitot pressure in psi.

$$Q=29.83\times c\times d^2\times \sqrt{p}$$

The coefficient c is 0.90 for a smooth, rounded outlet, 0.80 for a square, sharp outlet and 0.70 for a square outlet that projects into the barrel. A 2½-inch outlet with c = 0.90 at 23 psi flows 29.83 × 0.90 × 6.25 × 4.796 = 805 GPM. For required building fire flow, use the Fire Flow Calculator.

Flow Rate Unit Conversions

Flow rate is volume per unit of time, so every unit converts with a single factor. Water mass figures below use 998.2 kg/m³ at 20 °C.

Flow Unit	Equals
1 GPM	3.785 L/min · 0.2271 m³/h
1 L/min	0.2642 GPM · 0.06 m³/h
1 L/s	15.85 GPM · 3.6 m³/h
1 m³/h	16.67 L/min · 4.403 GPM
1 CFM	1.699 m³/h · 7.481 GPM
1 ft³/s	448.8 GPM · 101.9 m³/h
1 MGD	694.4 GPM · 157.7 m³/h
1 GPM of water	8.33 lb/min
1 m³/h of water	998.2 kg/h

Frequently Asked Questions

Which equation does the pipe flow calculator use?

It uses the Hazen-Williams equation for friction and Q = A × v for flow. That suits water in pipes running full. For partly full drains, sewers and culverts, Manning's equation is the right method.

Does the pipe flow calculator work for PVC and steel pipe?

Yes. Pick Plastic (C = 150) for PVC, or Steel new (C = 140) or Steel used (C = 120) for steel. The size list holds Schedule 40 and Schedule 80 inside diameters for both, plus Type L copper.

What is the flow rate of a 1 inch pipe?

A 1-inch Schedule 40 pipe (1.049 in bore) carries 13.5 GPM at 5 ft/s and 21.6 GPM at 8 ft/s. A 1-inch Type L copper tube carries 12.9 GPM and 20.6 GPM at the same speeds. How much it actually delivers depends on the pressure you have and the length of the run.

How do you calculate water flow rate in litres per minute?

Multiply the bore area by the velocity. With d in millimeters and v in m/s, L/min = 0.04712 × d² × v. A 25 mm bore at 2 m/s gives 0.04712 × 625 × 2 = 58.9 L/min.

Does a 22mm pipe increase flow rate?

Yes, compared with 15 mm pipe. A 22 mm copper tube has a bore of roughly 20 mm, about twice the area of a 15 mm tube. At 1.5 m/s it carries around 28 L/min against about 16 L/min, though the supply pressure and the rest of the system still limit the final flow.

What is a normal water flow rate?

At a single fixture, US federal rules cap showerheads at 2.5 GPM at 80 psi and kitchen and bathroom faucets at 2.2 GPM at 60 psi. Many states set lower limits. For a pipe, a good flow rate is one that keeps velocity under the limit for the material, such as 8 ft/s for cold water in copper.

What is the difference between flow rate and velocity?

Flow rate is the volume that passes per unit time, such as GPM or m³/h. Velocity is how fast the water moves, such as ft/s. The same 50 GPM moves at 4.8 ft/s in 2-inch Schedule 40 pipe but at 18.6 ft/s in 1-inch Schedule 40 pipe.

What is m³ in flow rate?

m³ is the cubic meter, a volume of 1,000 litres. Flow rates in m³ are written per hour (m³/h) for pumps and plumbing, or per second (m³/s) for large mains and engineering math. One m³/h equals 16.67 L/min.

Is 1 m³ of water equal to 1 kg?

No. One cubic meter of water weighs about 1,000 kg, not 1 kg. It is 998.2 kg at 20 °C and very close to 1,000 kg at 4 °C. So 1 kg of water takes up about 0.001 m³, which is 1 litre.

How do I convert m³/h of water to kg/h?

Multiply by the water density in kg/m³. At 20 °C, 1 m³/h is 998.2 kg/h, so 3 m³/h is about 2,995 kg/h. For other liquids, use their own density.

How do you calculate total flow area?

Flow area is π × d² ÷ 4 using the inside diameter. For several pipes in parallel, add their areas. Two 2-inch Schedule 40 pipes give 6.71 in² of flow area, less than the 7.39 in² of one 3-inch pipe. To find the water a pipe holds, use the Pipe Volume Calculator.

Why does a longer pipe carry less water?

Friction acts along the full length of the pipe. With the same drop or pressure spread over more length, the slope S gets smaller and the velocity falls. Doubling the length at the same head cuts the flow by about 31% under Hazen-Williams.