Enter duct dimensions, airflow, and length to estimate friction and documented fitting pressure losses.
Concepts behind this calculation
Calculate one uniform duct section, or sum documented fitting losses using each group’s own reference flow, area and density. Optional reporting-reference conversion prevents incompatible coefficient sums.
Straight-duct friction consumes total pressure through wall shear; fitting loss consumes total pressure through a geometry-dependent local process. Static pressure may also change with velocity, so static change is not synonymous with loss.
A fitting coefficient is tied to a named reference section. Adding raw coefficients referenced to different velocities is invalid; calculate each local loss first or normalize to one documented reporting velocity pressure.
How to use this calculator
Choose the uniform-section pressure calculation; enter duct shape, airflow, clear dimensions, straight length, roughness, air density, kinematic viscosity and a compatible sum of fitting coefficients.
Choose the fitting schedule when coefficients use different reference sections. Document the source and enter each group’s quantity, coefficient, local airflow, clear area and density.
Enable common reporting reference only to report a normalized coefficient. Define reporting-reference airflow, clear area and air density explicitly and retain the unchanged total fitting loss.
Calculation method and applicability
Uniform section: ΔP = f_D (L/D_h) VP + ΣK VP, with Colebrook friction. Fitting schedule: ΔP = Σ(nᵢ Cᵢ VPᵢ); optional C_report = Σ(nᵢ Cᵢ VPᵢ) / VP_report. Each VP uses its own actual flow, reference area and density.
f_D = dimensionless Darcy friction factor; ρ = actual air density; V = section-average velocity; D = reference diameter; L = straight length. Use consistent units in Darcy–Weisbach. The form accepts CFM, inch dimensions, ft length/roughness, lbm/ft³ density and ft²/s viscosity. Fitting C or K is dimensionless on its specified velocity-pressure reference; pressure results are in. w.g.
Input definitions and units
Pressure calculation
Choose a uniform-section calculation or a fitting-only schedule. Each coefficient must use its documented reference section. Choices: Uniform section — friction and compatible fitting ΣK; Fitting schedule — individual coefficient reference sections.
Duct section shape
The same selected section defines the velocity pressure reference for entered ΣK. Choices: Round; Rectangular. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK.
Airflow through this section (CFM)
Use one operating or design flow for the selected duct section and all entered fitting coefficients. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK.
Inside diameter (in)
Enter the clear inside diameter at the section where ΣK is referenced. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK; Duct section shape: Round.
Inside width (in)
Enter clear inside dimensions at the coefficient reference section. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK; Duct section shape: Rectangular.
Inside height (in)
Enter clear inside dimensions at the coefficient reference section. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK; Duct section shape: Rectangular.
Straight duct length (ft)
Use 0 for a fitting-only check. This is physical straight length, not total effective length. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK.
Absolute internal roughness (ft)
Enter a documented value for the installed surface. No material preset or universal default is assumed. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK.
Air density at the operating condition (lbm/ft³)
Use density matching this section and operating point; do not mix sea-level and altitude conditions. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK.
Air kinematic viscosity at the operating condition (ft²/s)
Use the matching air state. Example values are converted from the peer 20 °C case. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK.
Sum of fitting loss coefficients (ΣK)
Use only coefficients for the actual fitting geometry and flow ratio. Their reference velocity pressure must be based on this selected section. Enter 0 for straight duct only. Applies to Pressure calculation: Uniform section — friction and compatible fitting ΣK.
Fitting schedule and coefficient reference definitions
Identify a source/project schedule keyed to every row ID: exact construction, flow ratio, operating condition and coefficient reference plane. Actual coefficients must apply at those conditions. A source note does not authenticate the data. Applies to Pressure calculation: Fitting schedule — individual coefficient reference sections.
Coefficient and path applicability
Confirm all rows belong to the intended path/operating state, each C matches its source section and conditions, and repeated quantities are separately applicable. Include assembly/interaction losses once; no generic elbow or tee preset is selected. Choices: Not established; Documented coefficient references and path applicability established. Applies to Pressure calculation: Fitting schedule — individual coefficient reference sections.
Fitting groups and their own reference conditions (ID, quantity, C, actual CFM, reference area ft², density lbm/ft³)
One group per row, no header. Unique ID 1–10000; integer quantity 1–1000; nonnegative source coefficient; positive actual reference-section flow, clear reference area and density. Area must match the source coefficient plane, not an assumed grille Ak or free area. Quantities require identical applicable conditions; close-coupled fitting effects need a combined source. At most 100 groups. Applies to Pressure calculation: Fitting schedule — individual coefficient reference sections.
Common reporting reference
Total fitting pressure loss needs no arbitrary common section. Enable this option only to express all group losses as equivalent coefficients on one stated reporting velocity pressure. Choices: Total fitting loss only; Also report coefficients on one common reference. Applies to Pressure calculation: Fitting schedule — individual coefficient reference sections.
Reporting-reference actual airflow (CFM)
A selected reporting plane, not a claim that all fitting coefficient sections have this airflow. Reference conversion does not change the physical pressure loss. Applies to Pressure calculation: Fitting schedule — individual coefficient reference sections; Common reporting reference: Also report coefficients on one common reference.
Reporting-reference clear area (ft²)
Defines the reporting velocity with its stated flow. Converted group coefficients, including quantities, may be summed on this reference; original coefficients at different planes may not. Applies to Pressure calculation: Fitting schedule — individual coefficient reference sections; Common reporting reference: Also report coefficients on one common reference.
Reporting-reference air density (lbm/ft³)
Use the stated reporting condition. With different local densities this is only a pressure normalization, not a source coefficient or density-correction model. Applies to Pressure calculation: Fitting schedule — individual coefficient reference sections; Common reporting reference: Also report coefficients on one common reference.
How to interpret the result
Separate straight friction rate, straight loss and fitting loss. A uniform-section result describes one compatible section; a fitting schedule excludes straight friction and does not solve an entire network.
Worked examples
Uniform section — friction and compatible fitting ΣK
Inputs: Vortia 3D published case converted to U.S. customary units: 1,000 m³/h through 250 mm round galvanized duct, 10 m, 20 °C, ΣK≈1.
Calculation: Convert the stated flow, geometry, roughness and air properties to the labeled U.S. inputs. Darcy-Weisbach/Colebrook gives Re≈93,689, f_D≈0.01993, 15.37 Pa straight loss and 19.28 Pa fitting loss.
Result: 0.139 in. w.g. total section loss; 0.188 in. w.g./100 ft straight friction rate; about 41.16 ft fitting equivalent length.
Inputs: Constructed path with 600 CFM at 0.075 lbm/ft³: two separate 8 in elbows using source C=0.11, plus two illustrative fitting groups with C=0.25 and 0.40 referenced to 12 × 8 and 10 × 8 in clear areas. The latter coefficients are constructed teaching values, not product data. The reporting reference is the 12 × 8 in plane at 600 CFM.
Calculation: Local velocities are 1,718.873, 900 and 1,080 FPM. Their velocity pressures are 0.183870868, 0.050409243 and 0.072589309 in. w.g. Quantity-weighted group losses are 0.040451591, 0.012602311 and 0.029035724 in. w.g.
Result: Total fitting loss is 0.082089625 in. w.g. The equivalent group coefficients on the 900 FPM reporting plane are 0.802463774, 0.25 and 0.576, totaling 1.628463774. Simply summing raw quantity-weighted C gives 0.87, which is not valid on that common reference. No straight friction, static regain or universal equivalent length is included.
Assumptions and limits
The uniform-section Colebrook method reports turbulent cases only (Reynolds number ≥4,000); laminar and transitional flows are withheld. The supplied-coefficient schedule requires its own source applicability and does not infer Reynolds or a friction factor.
Density, kinematic viscosity and absolute roughness are user-supplied and must match the air state and installed surface. No material preset or universal property is inferred.
Fitting coefficients depend on exact geometry, diameter, construction, flow ratio and source reference velocity. The schedule does not automatically select a generic elbow, tee, damper or transition value; source example coefficients are not transferred to arbitrary sizes.
In uniform-section mode, the selected section supplies friction geometry and the ΣK velocity-pressure reference. Set straight length to zero for fitting-only calculations with that one reference. Use the schedule mode when fitting coefficients have different reference sections.
Both methods require applicable low-speed incompressible conditions. The schedule sums only fitting total-pressure loss, not straight friction or static pressure change. Different densities in reporting normalization do not constitute automatic coefficient correction. Quantities cannot replace coupled-assembly data or justify double-counting interaction losses.
The calculation assumes low-speed incompressible air. It does not model compressibility, leakage, a complete branching network, system effect, fan operating point, sound, code compliance or fitting pressure loss inferred from geometry.
FAQs
Can all elbows use the same coefficient?
Only if the source supports the actual geometry, construction, size and reference velocity. An example elbow coefficient is not a universal preset.
Why does coefficient normalization change the sum but not pressure loss?
Changing the reporting velocity pressure changes the coefficient needed to express the same physical loss. Local quantity-weighted losses remain unchanged.
Sources
ASHRAE 2025 Fundamentals, Chapter 21: Duct DesignOfficial section method and fitting Equations 29–31. Equation 31 converts reference coefficients at matched density; the schedule’s VP-ratio normalization also permits explicitly supplied reporting densities without claiming coefficient transfer. The exact F25.22 locator and a universal fitting table were not verified.
McGill 2003 Duct System Design guideEquations 1.16–1.17 and named 8 in elbow / stated tee examples. Table 1.1 coefficients are typical and diameter/construction-specific, not generic presets. Printed elbow results use an intermediate VP rounded to 0.18; full-precision results can round differently.
Vortia 3D: Duct pressure drop calculator (Colebrook)Peer example directly observed and independently reproduced after unit conversion. Its fitting coefficients remain geometry-specific; peer output is not an authority or design approval.
MEP Tools: Duct Pressure Drop CalculatorPeer coverage includes Darcy-Weisbach straight loss, rectangular ducts, supplied ΣK and fitting schedules; it uses a different friction-factor implementation.
