TOOL GUIDE
Fitting loss — elbows, tees & valves
Calculate fitting losses from K, flow, reference section and quantity.
Purpose & scope
Calculate fitting losses from K, flow, reference section and quantity.
Selected flow path only; negative mixing-junction K is unsupported. This is not a K library or fabrication drawing.
Inputs
Use drawings, design conditions and manufacturer data. Units and example values follow the calculator; example values are not universal design requirements.
| Parameter | Value / interpretation |
|---|---|
| Fluid / system | Air ducts / Water pipes |
| Fitting type (illustration) | 90° elbow / 45° elbow / Tee — one selected flow path / Reducer / expansion / Valve / device / Other fitting |
| Number of identical fittings | Number of identical fittings with the same K and reference section. |
| K loss coefficient per fitting | Dimensionless loss coefficient K for one fitting at the correct reference section/velocity. |
| K source / fitting code / applicable conditions | Record the K source, fitting identifier and conditions for later review. |
| Reference cross-section shape | Rectangular / Round |
| Flow at the reference section | Volumetric flow at the calculation location; check the selected unit. |
| Flow unit | m³/h / L/s / CFM (air only) |
| Width a / internal diameter D · mm | Internal diameter for round sections or reference-section width. |
| Height b (rectangular duct) · mm | Height of the rectangular reference section. |
| Air density · kg/m³ | Density at design conditions, distinct from mass flow rate. |
| Pure water temperature · °C | Water temperature used for fluid properties. |
Steps
- Select air/water and fitting illustration; the illustration does not populate K.
- Enter a documented K, its reference section/velocity and identical fitting count.
- Read Δp or equivalent head; transfer to fan/pump path without double counting.
Worked example
This reproducible training example uses the values below. Select the appropriate mode and overwrite restored project values. Example rates, K values and equipment losses are assumptions, not mandatory design criteria.
Examples use a decimal point and no thousands separator.
All example inputs
| Parameter | Value / interpretation |
|---|---|
| Fluid / system | Air ducts |
| Fitting type (illustration) | 90° elbow |
| Number of identical fittings | 1 |
| K loss coefficient per fitting | 0.5 |
| K source / fitting code / applicable conditions | |
| Reference cross-section shape | Rectangular |
| Flow at the reference section | 2400 |
| Flow unit | m³/h |
| Width a / internal diameter D | 400 mm |
| Height b (rectangular duct) | 300 mm |
| Air density | 1.2 kg/m³ |
| Pure water temperature | 7 °C |
Expected results (rounded)
| Parameter | Value / interpretation |
|---|---|
| Reference velocity | 5.56 m/s |
| Total loss | 9.26 Pa |
Small differences in displayed rounding are normal. A result that passes the entered limits does not establish suitability for the complete system.
Method & limitations
Selected flow path only; negative mixing-junction K is unsupported. This is not a K library or fabrication drawing.
Single-fitting Δp = K × ρv²/2; v = Q/A at the section used by the K source. Total loss = count × Δp. Straight-run friction is excluded.
The initial K = 0.5 is an equation example, not a universal 90° elbow coefficient. Fitting type changes the illustration, not K. Enter a coefficient for the actual geometry, bend radius, area ratio and flow split.
For tees, check one selected flow path; do not add straight-through and branch losses. Use the reference flow and area specified by the coefficient source. This version accepts nonnegative K; mixing-junction negative-K cases are excluded.
Add to path transfers count × K, reference flow and dimensions as a zero-length fitting row. The path recalculates with its own fluid properties. Session drafts preserve the path for multiple additions; review the critical path and calculation conditions.
The illustration is schematic, not to scale or a fabrication drawing. Water mode uses pure water only. Switching fluid loads suitable sample flow and dimensions.
Common mistakes
Wrong K reference area; treating illustration as catalogue data; summing parallel tee paths; double counting summed and individual K values.
Projects & reports
- Choose or create an engineering project and enter your actual design inputs.
- Project inputs/results are autosaved in this browser. Where a saved-list panel is available, name and add the selected section or alternative to compare multiple options.
- Review the report before exporting PDF. In the print dialog select Save as PDF. Export JSON for backup and use Open file to restore it; PDF cannot restore project data.
Saved results do not automatically recalculate when another tool changes. Review any dependency warning, recalculate and update the affected results.
References & related tools
ASHRAE — Duct Design: Fitting Losses
The tool implements the methods and limits stated above; linking a standard does not establish complete compliance with that standard.