Calculation sequence
Step-by-step method
- Calculate velocity pressure at the coefficient's reference section.
- Confirm the coefficient represents the actual fitting geometry and flow path.
- Multiply velocity pressure by the entered loss coefficient.
MechanicalIntermediateDuct Systems
Duct Fitting Loss Calculator
Calculate fitting pressure loss from an entered loss coefficient and local velocity pressure.
Why it matters: Elbows, transitions, branches, dampers, and entrances can represent a large share of fan-system pressure loss.
Calculated result
Engineering evaluation
Fitting pressure loss (ΔP)0.07 in. w.g.
- Reference velocity pressure (Pv)
- 0.14 in. w.g.
- Entered loss coefficient (K)
- 0.5 ratio
Engineering interpretation
The entered K = 0.5 at 1500 fpm gives a calculated fitting loss of 0.07001 in. w.g..
Important limitations
- Loss coefficients depend on fitting geometry, upstream conditions, reference section, Reynolds number, and flow split. Use the applicable published value.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Pv = ρV²/2
ΔP = K·Pv
Current substitution
Pv = 0.14 in. w.g.; ΔP = 0.5×0.14 = 0.07001 in. w.g..
Equation legend
Inputs and calculated quantities
- K
- Fitting loss coefficient — ratio
- V
- Reference air velocity — fpm or m/s
- ρ
- Air density — lb/ft³ or kg/m³
- ΔP
- Fitting pressure loss — in. w.g.
- Pv
- Reference velocity pressure — in. w.g.
- K
- Entered loss coefficient — ratio
Applications
How this calculation is used
Building design
Apply a published fitting loss coefficient to a design velocity.
Construction
Evaluate the impact of a field-installed fitting or geometry change.
Commissioning
Screen whether measured local pressure loss is consistent with an approved coefficient.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe one stable operating condition and use consistent measurement boundaries.
- Fluid or air properties and geometric dimensions are representative of the evaluated system.
Limitations
- This is a preliminary evaluation and field-verification tool, not a final sizing or code-compliance determination.
- Manufacturer data, adopted codes, project criteria, and professional engineering review remain controlling.
Field use
Verification procedure
- Confirm the approved reference, measurement plane, and instrument calibration.
- Stabilize the system and record dimensions and operating data.
- Compare the calculated result with project documents and manufacturer data before disposition.
Common engineering mistakes
- Using outside dimensions rather than clear inside dimensions.
- Combining values from different loads, device positions, or measurement conditions.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsCurrent handbook chapters covering duct design, fluid flow, heat transfer, insulation, and measurement; verify the licensed edition during professional review.
- SMACNA Technical StandardsIndustry duct-design, construction, leakage, and air-system reference context; protected tables are not reproduced.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement