One coefficient. A defined velocity.
The dimensionless loss coefficient K expresses a fitting’s irreversible hydraulic loss relative to a specified velocity head. It is also called a resistance coefficient or minor-loss coefficient. “Minor” describes the loss type, not its size.
hL = K Vref² / (2g)
Use density ρ in kg/m³ and reference velocity V in m/s for pressure loss in Pa. Head loss is in meters of the flowing fluid when g = 9.80665 m/s².
Typical fixed K values
Generic screening values from the US EPA’s EPANET 2.2 manual, Table 3.3. They are not guaranteed product data. Use the connected pipe velocity for these simple cases. For actual dividing or combining tees, use a junction correlation with a stated flow split and reference leg.
13 reference entries
| Fitting | K | Category | Selection note |
|---|---|---|---|
| Globe valve, fully open | 10.0 | Valve | Generic open-valve estimate. |
| Angle valve, fully open | 5.0 | Valve | Match the actual flow path. |
| Swing check valve, fully open | 2.5 | Valve | Requires the disc to reach full opening. |
| Gate valve, fully open | 0.2 | Valve | Do not use for throttled service. |
| Short-radius elbow | 0.9 | Bend | Generic geometry; verify radius and connection. |
| Medium-radius elbow | 0.8 | Bend | Generic geometry; verify radius and connection. |
| Long-radius elbow | 0.6 | Bend | A named radius class alone does not define every product. |
| 45° elbow | 0.4 | Bend | Use a matching angle. |
| Closed return bend | 2.2 | Bend | Return geometry matters. |
| Standard tee, flow through run | 0.6 | Junction | Screening value, not a flow-split correlation. |
| Standard tee, flow through branch | 1.8 | Junction | Screening value, not a flow-split correlation. |
| Square entrance | 0.5 | Entrance | Sharp-edged tank-to-pipe entry; pipe velocity. |
| Exit | 1.0 | Exit | Discharge into a large reservoir with negligible bulk velocity. |
No matching fittings. Try a broader term.
Why your calculator may show a different K: the pressure-drop calculator also uses size-dependent fT correlations and geometry-specific equations. Those are separate models. Do not replace them with a generic fixed value solely to make the results match.
Ball valves, butterfly valves, lift or tilting-disc checks, strainers, and proprietary devices need more specific data. Use the manufacturer’s loss curve or the matching geometry in the pressure-drop calculator. Full-port and reduced-port ball valves are not interchangeable.
Choose the appropriate loss method
| Available information | Use | Check first |
|---|---|---|
| Product pressure-loss curve or tested K | Manufacturer data | Exact size, trim, opening, fluid, flow range, and pressure-tap basis. Prefer product data when applicable. |
| Generic fitting, preliminary estimate | Fixed K | Matching geometry and turbulent service; examine sensitivity if fitting losses dominate. |
| Specified n × fT correlation | K = n fT | Use the correlation’s reference friction factor, size, and bore corrections. |
| Equivalent length Le or Le/D | K = fD Le/D | Use the specified Darcy factor and diameter. Identify how a published equivalent length was derived. |
| Low Reynolds number / viscous fluid | Validated Reynolds-dependent model | Use applicable test data, 2K/3K, or another documented correction with its validity range. |
| Valve Cv or Kv | Valve sizing relation | Cv and Kv are capacity coefficients, not dimensionless K. Use the actual opening and appropriate liquid or gas equation. |
How this site uses fT
The current pipe pressure-drop calculator calculates fT from a fixed reference roughness of 0.0018 in (0.04572 mm) and the pipe inside diameter. The operating Darcy factor fD is calculated separately for straight-pipe loss. Changing the fluid viscosity or actual pipe roughness therefore does not directly change a simple n × fT fitting value.
K = n fT
For example, its standard 90° elbow uses 30 fT and its long-radius 90° elbow (r/D ≈ 1.5) uses 14 fT. At an illustrative fT = 0.020, these give K = 0.60 and 0.28. These explain the site’s implementation, not universal elbow constants.
Do not enter 30 as K when the source says 30 fT. Multiply first, or select the calculator’s custom n × fT option. fT is not generally equal to the operating fD.
Equivalent length
This equality matches pressure loss at the selected conditions and reference diameter. If K = 0.60, D = 0.050 m, and fD = 0.025, then Le = 1.20 m. A conversion at one flow condition does not prove that a fixed equivalent length will be accurate at every flow rate.
K ↔ equivalent length calculator · Darcy friction factor · Liquid valve Cv calculator
Use the right velocity basis
For a circular bore, V = 4Q/(πD²). Use the actual inside diameter, not nominal pipe size or outside diameter.
At the same incompressible flow rate in circular pipes, this becomes Knew = Kold (Dnew / Dold)⁴. A K of 0.5 based on a 25 mm bore becomes 8.0 when expressed using velocity in a 50 mm bore. Both predict the same loss.
Reducers and enlargements
Check whether the coefficient uses the small-pipe, large-pipe, upstream, or downstream velocity. Specify the diameter ratio and transition angle. Static pressure can recover across an enlargement even though mechanical energy is lost; fitting loss alone is not the entire static pressure change when velocity or elevation changes.
Tees and wyes
Select the actual path through the junction, its dividing or combining direction, area ratio, and flow split. The pipe pressure-drop calculator’s detailed tee/wye equations reference the combined-leg velocity. Model legs with different flows separately. Some path-specific junction coefficients can be negative because of the reference convention and energy exchange between streams; this does not represent energy generation.
Adding several fittings
Only use (ΣK)ρV²/2 directly when all K values share that velocity and density basis. Count each physical fitting once. Closely spaced fittings can interact, so independent coefficients are an approximation.
Worked example: four elbows and a gate valve
Assume water at ρ = 998 kg/m³, a common velocity of 2.0 m/s, four short-radius elbows at K = 0.9 each, and one fully open gate valve at K = 0.2. These are the generic fixed values above.
Δploss = 3.8 × 998 × 2.0² / 2 = 7,584.8 Pa
= 7.585 kPa ≈ 1.100 psi
hL = 3.8 × 2.0² / (2 × 9.80665) = 0.775 m
This is the fitting loss only. Add straight-pipe friction once, and account separately for elevation, velocity changes, equipment losses, and pumps when writing the system energy balance.
At an assumed constant K, doubling velocity would make this loss four times larger. Verify that the chosen K model remains applicable at the new flow.
Common mistakes and limits
- Darcy versus Fanning: fD = 4 fF. Using Fanning f in a Darcy equivalent-length formula causes a factor-of-four error.
- Counting the same loss twice: use either K or its equivalent length for a fitting, not both. Check whether vendor data already includes reducers or end connections.
- Valve position: fully open values do not describe a throttled valve. Check-valve opening depends on flow and valve design.
- Low Reynolds number: fixed turbulent K values can miss important viscous losses. Do not assume laminar pipe friction fixes a turbulent fitting coefficient.
- Gas and steam: a constant-density liquid loss calculation is inadequate when density changes materially. Use compressible methods and check choking where applicable.
- Cavitation, flashing, or two-phase flow: use suitable equipment data and sizing methods rather than a generic single-phase K.
- Different meanings of K: sprinkler discharge K, meter calibration factors, valve Kv, and thermal conductivity are different quantities.
Pipe pressure-drop calculator · Pipe network solver · Water properties · Pipe roughness
Sources and scope
- US EPA: EPANET 2.2 User Manual, Section 3.1, Table 3.3: fixed reference coefficients and velocity-head loss definition.
- AFT: Adjusted Turbulent K Factor Method: equivalent-length relationships and Reynolds-number dependence.
- AFT: Area Change Loss Model: transition geometry and upstream/downstream coefficient bases.
- Swagelok: Valve Sizing Technical Bulletin: product flow coefficients and liquid/gas sizing.
The fT examples describe the existing Frictionless Calc implementation; the EPA fixed-K table is a separate reference dataset. Source differences, geometry, and reference velocity can all produce different valid coefficients. For detailed handbook correlations, consult the applicable edition of Crane TP-410 or Idelchik with its diagrams and validity limits.
Sources checked September 30, 2026. Intended for preliminary engineering calculations and method selection. Document the selected source, geometry, valve position, and velocity basis in your calculation.