Duct Sizing for Home HVAC: Room by Room Guide
Duct Sizing for Your Home
Sizing HVAC ducts for a home requires calculating the airflow needs for each room and then selecting the right duct diameter for each branch and the main trunk line. This guide walks through the practical process.
Room by Room CFM Requirements
Using the 1 CFM per square foot rule for a typical 2,000 sq ft home:
| Room | Size | CFM Needed | Duct Size |
|---|---|---|---|
| Master bedroom | 250 sq ft | 250 CFM | 8 to 10 inch |
| Bedroom 2 | 150 sq ft | 150 CFM | 7 to 8 inch |
| Bedroom 3 | 130 sq ft | 130 CFM | 6 to 7 inch |
| Living room | 350 sq ft | 350 CFM | 10 to 12 inch |
| Kitchen | 200 sq ft | 200 CFM | 8 inch |
| Dining room | 180 sq ft | 180 CFM | 8 inch |
| Bathroom 1 | 60 sq ft | 60 CFM | 5 to 6 inch |
| Bathroom 2 | 80 sq ft | 80 CFM | 6 inch |
| Home office | 150 sq ft | 150 CFM | 7 to 8 inch |
| Hallway | 100 sq ft | 100 CFM | 6 inch |
| Total | 1,650 sq ft | 1,650 CFM |
Note: Not all square footage is conditioned (garages, closets). The total CFM for ductwork may be less than total home square footage.
Trunk Line Sizing
The main trunk must carry the total system CFM from the air handler. For our 2,000 sq ft example:
- Total CFM: approximately 2,000
- At 700 FPM velocity: trunk needs approximately 20 inch round duct or 24 × 12 rectangular
Reducing Trunk Design
As the trunk extends and branches take airflow, reduce the trunk size:
| Trunk Section | CFM Remaining | Duct Size |
|---|---|---|
| At air handler | 2,000 CFM | 20 inch round |
| After living room branch | 1,650 CFM | 18 inch round |
| After kitchen branch | 1,450 CFM | 16 inch round |
| After master bed branch | 1,200 CFM | 16 inch round |
| After bedroom 2 branch | 1,050 CFM | 14 inch round |
| Final sections | 500 to 700 CFM | 12 inch round |
Branch Duct Sizing Tips
Keep Runs Short
Every foot of duct adds friction. Route branches as directly as possible from the trunk to the register. Avoid unnecessary detours around obstacles when a shorter path exists.
Minimize Bends
Each 90° bend adds 10 feet of equivalent length (15 feet for flex). Plan routes that minimize direction changes. Use 45° elbows instead of 90° where possible.
Use the Right Material
- Metal ducts for main trunk and long branch runs
- Flex ducts acceptable for short final connections (under 15 feet)
- Never kink or compress flex duct
Match Register Sizes
The supply register should match the duct size. An 8 inch duct feeding a 4 inch register creates a bottleneck that restricts airflow and increases noise.
Return Duct Sizing
Return ducts are often overlooked but equally important:
- Minimum: One return per floor (central location)
- Better: One return per zone or wing of the house
- Best: Dedicated return in each bedroom with transfer grilles
Return duct sizing: use 0.06 in/wg friction rate and 500 to 700 FPM velocity. Returns should be 1.25 to 1.5 times larger than the largest supply branch in the area they serve.
Common Home Duct Sizing Mistakes
- One size fits all — Using the same duct size for every room regardless of CFM needs
- Ignoring flex duct friction — Flex duct needs larger sizes than metal for the same CFM
- Too many bends — Complex routing with 3 or 4 bends dramatically increases TEL
- Undersized returns — The number one cause of HVAC performance problems
- Not insulating — Ducts in attics, crawl spaces, or garages lose significant capacity without insulation
Quick Sizing Chart
For a fast reference at 0.08 in/wg friction rate and approximately 700 FPM velocity:
| Room CFM | Recommended Duct |
|---|---|
| 50 to 80 | 5 to 6 inch |
| 80 to 150 | 6 to 7 inch |
| 150 to 250 | 8 inch |
| 250 to 400 | 10 inch |
| 400 to 600 | 12 inch |
| 600 to 900 | 14 inch |
Calculate Your Home’s Duct Sizes
Use our HVAC Duct Calculator to size each duct run precisely. For whole house sizing based on square footage, check our home sizing guides.