Plenty of engineers still size ducts on a Ductulator or in a small external tool, then type each number into the model by hand. Change the airflow of one room and the whole run has to be looked up again. Revit does the job inside the model, because every diffuser already carries an airflow and every length of duct knows where it sits in the system.
This article shows how to size ducts in revit 2027 in 13 steps on one apartment supply run: pick the run, pick a method (friction, velocity, equal friction, static regain) and Revit takes sizes from the project's size table. Release 2027 adds the MEP Sizing Calculator palette for sizing one length at a time. To learn the whole HVAC workflow, see the Institute's Revit MEP course.
What duct sizing revit works from
Three pieces of data; miss one and nothing is calculated.
Airflow. Autodesk's Duct Sizing page explains that the flow of each element comes from the Flow Configuration parameter of its connector. Diffusers default to Preset, so you type the number. Air handling equipment defaults to Calculated, so Revit adds up what is downstream. A System connector takes a share of the system flow through its Flow Factor.
Air properties. Air Density and Air Dynamic Viscosity live in Mechanical Settings, under Duct Settings.
A size table. Revit never invents a size; it picks from the project's table, where every size has two ticks: Used in Size Lists for the lists you draw from, and Used in Sizing for the sizing routine.
So an open system, a diffuser with no airflow, or an empty size table are the three reasons the command refuses to work.
Prepare the file you can follow along in
The article continues on the run drawn in how to draw ducts in Revit: a fan coil, a main, one branch and three supply diffusers on flex duct, inside the practice file from Revit MEP sample files. Any run will do as long as it is closed from the equipment to the diffusers. Systems themselves are explained in what Revit MEP is.
How to size ducts in Revit, steps 1–6: air flow in revit
- Open the plan that holds the run, here L3.
- Select a diffuser and scroll Properties down to Mechanical - Flow. That is where the airflow is entered. The SAG-600x600 grille in the Institute's file comes with Main Flow 446 L/s; change it to your design figure and click Apply.
- Repeat for the other diffusers. Quicker: select one, right-click, Select All Instances › Visible in View, and type the figure once.
- Select a length of the main and read Properties. Three things matter: the current size, the system it belongs to, and Size Lock. A length with Size Lock ticked is skipped by the sizing routine. The Flow box in Mechanical - Flow is the airflow of the diffusers downstream; if it still reads zero the system is not calculating, see the table near the end.
- Check that the system is closed: Analyze › MEP Connections › Check Duct Systems. The panel next to it, Reports & Schedules, is where the pressure loss report comes from in the last step.
- Open Mechanical Settings (MS). The Duct Settings branch holds Air Density and Air Dynamic Viscosity; the Institute's file keeps 1.2026 kg/m³ and 0.00002 Pa·s.
The Rectangular branch below it lists the project's rectangular sizes, each with two ticks.
The Calculation branch sets how pressure loss is calculated along straight lengths.
Steps 7–13: Duct Sizing and the MEP Sizing Calculator
- Hover over the main, press Tab so Revit picks up the whole chain of ducts and fittings, then click.
- On the contextual tab, in the Analysis panel, click Duct/Pipe Sizing. System Inspector sits right next to it.
- The Duct Sizing dialog opens. Pick the method in the top box and type the figure: the Institute's file comes set to Velocity 5.1 m/s and Friction 0.82 Pa/m. The Only, And and Or buttons decide how the two conditions combine.
- Branch Sizing decides how branches take their size. Where the ceiling is tight, tick Restrict Height and type the maximum; both locked boxes default to 2400.
- Click OK. Revit resizes every length in the run and swaps the fittings to match.
- To size one length by hand, use the palette new in 2027: select the duct and go to View › Windows › User Interface › MEP Sizing Calculator. The MEP Sizing palette opens on the right and names the duct type it is working on. The Sizing Method group holds Volumetric Flow, the three conditions Friction (≤), Velocity (≤) and Aspect Ratio (≤), each with an 80–150 % tolerance slider, and a Roughness row. The Constraints group holds Restrict Width, Restrict Height, Show Best Sizes and the Swap Height and Width button.
- Pick a row in the table and click Apply, and the duct changes size. Check the result with System Inspector and the pressure loss report below.
The four methods and the branch constraints
Autodesk's About Duct Sizing Methods page lists four.
| Method | Revit picks a size so that |
|---|---|
| Friction | pressure loss per metre stays under the figure you type |
| Velocity | air velocity in the duct stays under the figure you type |
| Equal Friction | pressure loss per metre stays roughly equal along the run |
| Static Regain | static pressure regained at each branch covers the loss of the next length |
The last two use the ASHRAE Duct Fitting Database 5.00.00 for losses through elbows, tees and transitions. Friction and Velocity combine three ways: Only for one of them, And to satisfy both, Or to satisfy either.
Branch Sizing decides how branches take their size.
| Constraint | Meaning |
|---|---|
| Calculated Size Only | whatever the calculation gives |
| Match Connector Size | the branch takes the equipment connector size up to the first junction |
| Larger of Connector and Calculated | the bigger of the two above |
When the size constraint and the flow constraint disagree, Revit keeps the size and warns that not every parameter could be satisfied. Any length with Size Lock ticked is left alone.
Reading the result: System Inspector and the duct pressure loss report
Sizes are not the end of the job. Two tools read the result back inside the model.
System Inspector sits beside the sizing button, on the Analysis panel. Select the system, click System Inspector, then Inspect: arrows show the direction of flow, and hovering over a length shows its airflow, static pressure and pressure loss. The worst path is drawn in red. The button only wakes up for a closed system, and the tool is not available for fire protection systems.
The duct pressure loss report: click Analyze › Reports & Schedules › Duct Pressure Loss Report, choose the systems, then choose the fields to print. The setup dialog can add system information, the critical path, detail for every straight length and a table of fitting loss coefficients. Click Generate and save as HTML or CSV. The system must have Calculations set to All; fire protection and gravity systems have no such report.
From Duct Checker and printed tables to Revit
HVAC engineers in Vietnam are used to sizing ducts in a small tool such as Duct Checker, or from a printed table: enter the airflow, set a velocity or a pressure loss per metre, read off the size. The thinking is the same in Revit; only where you type and where you read change.
| Outside Revit | Inside Revit 2027 |
|---|---|
| Enter the airflow of each length | Enter the airflow once at the diffuser and Revit adds it up back to the equipment |
| Set a maximum velocity or pressure loss per metre | Pick the method in the Duct Sizing dialog, or type it in the MEP Sizing Calculator |
| Read the size off a table and type it on the drawing | Revit writes the size straight onto the duct and updates the fittings |
| Sizes come from the manufacturer's table | Sizes come from the project table, from the Used in Sizing column |
| Check with a separate calculation sheet | System Inspector and the pressure loss report, inside the model |
Autodesk says plainly that the MEP Sizing Calculator exists to replace external sizing tools, physical Ductulators included.
When the sizing button is greyed out, or the sizes look odd
| Symptom | Cause | Fix |
|---|---|---|
| Duct/Pipe Sizing is greyed out | The duct is not part of a closed system | Close the gaps, check with Analyze › Check Duct Systems |
| Nothing changes after sizing | Airflow is still zero, or the length has Size Lock ticked | Enter the diffuser airflow; clear Size Lock |
| The duct Flow box stays at zero although the diffusers carry airflow | The system type has Calculations set to None, or the diffuser family does not pass its airflow to the connector | Check Calculations in the system type properties; if it already reads All, check the diffuser family |
| Revit warns that not all parameters could be satisfied | The size constraint beat the flow constraint | Loosen Restrict Height or Restrict Width, or change method |
| Odd sizes nobody orders | Every size in the project table is ticked Used in Sizing | Leave the tick only on the sizes you actually buy |
| A branch as big as the main | Branch Sizing is on Match Connector Size | Switch to Calculated Size Only and size again |
| No pressure loss report | The system has Calculations other than All, or it is open | Set Calculations to All and close the gaps |
| System Inspector greyed out | The system is open, or it is a fire protection system | Fix the gaps; fire protection has no inspector |
Three things people get wrong
| Often heard | What Revit 2027 does |
|---|---|
| Revit knows the airflow of each diffuser by itself | You type it at the diffuser, with Flow Configuration on Preset; Revit only adds it up |
| Sizing also gives you the pressure loss of the system | Sizing is one job; for pressure figures use System Inspector or the report |
| Sizing a duct means opening an external tool | Release 2027 has the MEP Sizing Calculator inside Revit |
| The sizing routine resizes every length in the run | Lengths with Size Lock and sizes without Used in Sizing are skipped |
Frequently asked questions
Where do I change the airflow unit?
Manage › Project Units, the HVAC discipline, the Air Flow row. Switch it to m³/h and every Flow box and tag follows.
Should I size the whole system or one run at a time?
Work in pieces: one floor, one zone, one branch. Sizing a whole large system is slow and hard to check.
Do I have to fix the fittings after sizing?
No. Elbows, tees and transitions follow the new size through the duct type's Routing Preferences, covered in how to draw ducts in Revit.
Which release added the MEP Sizing Calculator?
The duct side arrived in 2027.1 and the pipe side in 2027.2.
Is pipe sizing the same?
The same Duct/Pipe Sizing button opens a Pipe Sizing dialog, with Friction and Velocity as the two methods.