# Duct Calculator

Size HVAC ducts from CFM. Get round duct diameter, rectangular size, velocity, and pressure drop. Covers supply mains, branches, returns, sheet metal, and flex duct using Manual D methodology.

## What this calculates

Correctly sized ductwork delivers design airflow to every register without noise, pressure loss, or duct damage. This HVAC duct calculator takes your CFM requirement and application (supply trunk, branch, or return) and returns round duct diameter, equivalent rectangular size, velocity, and pressure drop using ACCA Manual D simplified methodology with a 0.08 in wc/100 ft friction target.

## Inputs

- **Airflow Required** (CFM) — min 0 — Cubic feet per minute of airflow needed (from Manual J or CFM calculator)
- **Duct Type** — options: Round Sheet Metal, Rectangular Sheet Metal, Flexible Duct
- **Application** — options: Supply Main / Trunk (800-900 FPM), Supply Branch (600-700 FPM), Return Main (700-800 FPM), Return Branch (500-600 FPM) — Supply branches should run slower than mains to minimize noise at registers
- **Friction Rate** (in wc/100ft) — min 0.05, max 0.2 — Standard residential is 0.08-0.1 in wc per 100 ft of duct (Manual D target)

## Outputs

- **Round Duct Diameter** (in) — Actual calculated round duct diameter
- **Standard Round Size** (in) — Nearest standard round duct size (4, 5, 6, 7, 8, 10, 12, 14, 16, 18 in)
- **Equivalent Rectangular Size** — formatted as text — Rectangular duct with equivalent capacity
- **Air Velocity** (FPM) — Feet per minute through the duct
- **Cross-Section Area** (sq in)
- **Pressure Drop (per 100 ft)** (in wc)

## Details

## Duct Sizing Formulas

The velocity method (Manual D standard for residential):

- **Cross-sectional area (sq ft) = CFM / Velocity (FPM)**
- **Round diameter (in) = sqrt((4 x Area in sq in) / pi)**

For 400 CFM at 700 FPM target velocity:
- Area = 400 / 700 = 0.571 sq ft = 82.3 sq in
- Diameter = sqrt(4 x 82.3 / pi) = 10.2 inches, rounded up to **10-inch round duct**.

## Duct Sizing Chart (CFM to Round Duct)

| CFM | Supply Trunk (850 FPM) | Supply Branch (650 FPM) | Return (750 FPM) |
|-----|------------------------|-------------------------|------------------|
| 100 | 5" | 6" | 5" |
| 200 | 7" | 8" | 7" |
| 300 | 8" | 9" | 9" |
| 400 | 9" | 10" | 10" |
| 600 | 11" | 13" | 12" |
| 800 | 13" | 15" | 14" |
| 1000 | 15" | 17" | 16" |

## Flex Duct CFM Chart

Flex duct has internal ribbing that increases friction, so it carries about 10-15% less air than smooth sheet metal at the same diameter:

| Flex Duct Diameter | Max CFM (0.08 in wc/100 ft) |
|--------------------|-----------------------------|
| 4" | 35 CFM |
| 5" | 60 CFM |
| 6" | 100 CFM |
| 7" | 160 CFM |
| 8" | 230 CFM |
| 10" | 425 CFM |
| 12" | 700 CFM |
| 14" | 1,000 CFM |
| 16" | 1,400 CFM |

Flex duct is cheaper and easier to install but should only be used on branches, kept short (under 10 feet), and pulled tight to avoid excessive pressure loss.

## Recommended Duct Velocities

- **Supply trunk (main):** 800-900 FPM. Higher velocities work but increase noise.
- **Supply branch:** 600-700 FPM. Keep branches slower to reduce register noise.
- **Return trunk:** 700-800 FPM.
- **Return branch:** 500-600 FPM.
- **Fresh air intake:** 400-500 FPM.

Commercial systems tolerate higher velocities (1,000-1,500 FPM in trunks) because duct runs are shorter relative to total airflow. Residential should stay below 900 FPM in occupied spaces to avoid noise complaints.

## Friction Rate

Manual D targets 0.08-0.10 inches of water column per 100 feet of equivalent duct length. The equivalent length accounts for elbows, takeoffs, and transitions (each adds 10-50 feet of "equivalent" straight duct). Multiply actual run length by 1.5-2x to get equivalent length for rough calculations.

## Frequently Asked Questions

**Q: How do I calculate duct size from CFM?**

A: Use the formula: duct area (sq ft) = CFM / velocity (FPM). For round duct, diameter = sqrt(4 x area / pi). At 700 FPM target velocity, 400 CFM needs a 10-inch round duct; 600 CFM needs a 12-inch duct; 1,000 CFM needs a 16-inch duct.

**Q: What size duct for 400 CFM?**

A: For 400 CFM at standard residential velocity (700 FPM), use a 10-inch round duct or roughly an 8x11 rectangular duct. Sheet metal can run at 900 FPM in a 9-inch round, but flex should stay at or below 700 FPM so the 10-inch is the safe choice.

**Q: How much CFM can 6 inch duct handle?**

A: A 6-inch round sheet metal duct at 700 FPM carries about 140 CFM. A 6-inch flex duct carries about 100 CFM at the same friction loss due to internal ribbing. Use 6-inch for individual bedrooms or small registers.

**Q: What is the friction rate for residential ductwork?**

A: Manual D targets 0.08 to 0.10 inches of water column per 100 feet of equivalent duct length. Higher friction rates let you use smaller ducts but increase blower load and noise. 0.08 is the standard residential target; commercial systems often use 0.10-0.12.

**Q: Is flex duct or metal duct better?**

A: Sheet metal is quieter, has less pressure loss, and lasts longer but costs more and takes more time to install. Flex duct is cheaper and fits tight spaces but should be kept to short, straight runs (under 10 feet pulled tight). Most modern homes use rigid trunks with flex branches.

**Q: What happens if my ducts are undersized?**

A: Undersized ducts cause high velocity (noisy registers), high static pressure (stressed blower, reduced airflow), uneven temperatures, and short equipment life. Static pressure above 0.5 in wc on a residential system usually means ducts are too small. Fix by adding a return, upsizing the trunk, or replacing flex runs with sheet metal.

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Source: https://vastcalc.com/calculators/construction/duct
Category: Construction
Last updated: 2026-04-08