Calculate pipe head loss, flow rate, diameter, length, or friction factor from the other four values using common engineering units.
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Head Loss Formula
The calculator uses DarcyโWeisbach for straight-pipe friction loss. Choose the value to solve and enter the other four. Internally, Q is in mยณ/s, D and L in meters, velocity in m/s, and head loss in meters. GPM means US gallons per minute. The supplied Darcy factor is held fixed in inverse calculations; verify that it suits the resulting Reynolds number and relative roughness.
- hL = head loss, in meters internally
- f = Darcy friction factor, unitless
- L = pipe length, in meters internally
- D = internal pipe diameter, in meters internally
- V = mean flow velocity, in m/s internally
- Q = volumetric flow rate, converted to mยณ/s internally
- A = pipe cross-sectional area, in mยฒ internally
- g = standard gravity, 9.80665 m/sยฒ
Select Head loss, Flow rate, Internal diameter, Pipe length, or Darcy friction factor. Enter the four displayed physical inputs and choose the result unit where offered. Use positive values; inverse calculations exclude zero-flow and zero-loss cases because the unknown can be nonunique. This models distributed friction loss only; add fittings, valves and other losses separately.
Friction Factor and Unit Reference
Friction factor is not a fixed property of pipe material. For fully developed laminar flow in a circular pipe, Darcy f = 64/Re. For turbulent flow, obtain f from Reynolds number and relative roughness using a suitable correlation or Moody chart; material alone does not specify it.
| Pipe or flow condition | Typical Darcy friction factor | Notes |
|---|---|---|
| Laminar flow | 64 / Re | Applies when Reynolds number is below about 2,000 |
| Smooth pipe, turbulent flow | Determine from Re and relative roughness | Depends on Reynolds number even for hydraulically smooth tubing |
| PVC or copper water pipe | Determine from Re and relative roughness | Use internal diameter, fluid viscosity and an appropriate roughness estimate |
| Commercial steel pipe | Determine from Re and relative roughness | Depends strongly on age and roughness |
| Rough or aged pipe | Determine from Re and relative roughness | Scaling, corrosion, or deposits can increase losses |
| Input type | Supported units | Internal base unit |
|---|---|---|
| Flow rate | GPM, LPM, mยณ/h | mยณ/s |
| Diameter | in, cm, m | meters |
| Length | ft, m, yd | meters |
| Head loss | ft, m, in | meters |
Head Loss Examples
Example 1: Calculate head loss
Choose Head loss and enter 100 US GPM, internal diameter 4 in, pipe length 100 ft, and Darcy friction factor 0.020. Select ft as the result unit.
The calculated head loss is about 0.6078 ft.
Example 2: Calculate friction factor
Choose Darcy friction factor. Using the same pipe and flow, enter 100 US GPM, 4 in, 100 ft, and the unrounded head loss 0.6077917219 ft. Rounded head inputs give an approximately equal factor.
The calculated friction factor is about 0.020000.
Head Loss Calculator FAQ
What does head loss mean?
Head loss is the loss of mechanical energy in a flowing fluid, expressed as an equivalent height of fluid. A result of 5 ft of head loss means the system loses energy equal to lifting the fluid 5 ft. It is not the same as pressure drop, but it can be converted to pressure drop if you know the fluid density or specific weight.
Should I use Darcy friction factor or Fanning friction factor?
Use the Darcy friction factor. The Darcy friction factor is four times the Fanning friction factor. If you enter a Fanning value by mistake, the calculated head loss will be too low by a factor of about 4.
How do I choose the unknown?
Choose the unknown from Solve for, then enter the four displayed inputs. The omitted field is the calculated result; changing mode preserves your other entries.
