Duct Sizing Calculator

The Duct Sizing Calculator sizes a duct run by the equal-friction method. You give it the airflow, the friction rate you are designing to, and the shape you want, and it returns the required round diameter, the next standard size up, the equivalent rectangular dimensions if you asked for those, and the velocity, Reynolds number and pressure drop that result.

It can also check an existing duct rather than size a new one, and it works in either IP or SI units.

It offers two friction models, and they do not use the same inputs. The default ASHRAE power-law fit ignores the duct material you select; only the Darcy-Weisbach model uses it. That is explained below and it changes answers by more than a nominal size.

Purpose and model boundary

This model sizes a round duct, sizes one side of a rectangular duct, or checks an existing round or rectangular duct. It reports diameter or equivalent diameter, velocity, friction rate, Reynolds number, friction factor, and straight-run pressure drop using either a published equal-friction power-law fit or a Darcy-Weisbach/Colebrook calculation.

It is not a complete duct-system design. It excludes fittings, equivalent length, leakage, fan selection, total external static pressure, balancing, noise, vibration, fire/smoke requirements, and installation constraints.

Inputs and units

The model accepts IP or SI values, but changing the unit-system selector changes the meaning and labels—it does not convert numbers already typed. Airflow is entered in cfm or L/s; dimensions and run length in in/ft or mm/m; air temperature in deg F or deg C; altitude in ft or m; and target friction in in.wg/100 ft or Pa/m.

The calculation mode controls which dimension fields are visible. Material selects an absolute roughness. Relative humidity, temperature, and altitude affect air properties in the Colebrook branch. The ASHRAE power-law branch is calibrated to standard air and galvanised steel.

Governing relationships

All internal calculations use SI units. Standard-atmosphere pressure follows ASHRAE Chapter 1 Eq.3. Moist-air density is calculated from saturation pressure, humidity ratio, and the ideal-gas specific-volume relationship. Dynamic viscosity follows Sutherland's correlation.

The ASHRAE equal-friction fit is evaluated in its native IP form:

d_in = (0.109136 x Q_cfm^1.9 / FR_inwg_per_100ft)^(1/5.02).

For the physics branch, the workbook applies

deltaP/L = f x rho x V^2 / (2 x D)

with V = Q/A and Re = rho x V x D / mu. A Swamee-Jain estimate seeds the friction factor. Three fixed-point diameter updates, three Colebrook refinements, and one final diameter update are written out in cells; the workbook does not enable circular iteration.

For rectangular ducts the equivalent diameter relationship is

De = 1.30 x (a x b)^0.625 / (a + b)^0.25.

When sizing rectangular duct, the workbook rearranges this expression and performs five explicit fixed-point updates for side b. Round sizing is rounded upward through the standard-size list. At the installed geometry, area, velocity, Reynolds number, friction factor, friction rate, velocity pressure rho V^2/2, and straight-run pressure drop are recalculated.

Calculation sequence

1. Resolve unit system, calculation mode, friction model, and material.
2. Convert the applicable input arm to SI and calculate barometric pressure, density, viscosity, and roughness.
3. In a sizing mode, solve the required round or equivalent diameter from the selected friction model; in a check mode, use the entered geometry.
4. Round a selected standard dimension upward where the model defines a size list.
5. Recalculate geometry and pressure-loss quantities at that installed size.
6. Convert all reported values back to the selected display system.
7. Evaluate Model_Status in the order below.

Outputs and interpretation

The decision result is the governing round/equivalent diameter, supported by exact required diameter and next standard round size where applicable. Rectangular sizing returns exact and rounded side b. Velocity, Reynolds number, friction factor, friction rate, velocity pressure, pressure drop, aspect ratio, density, viscosity, and roughness explain the result.

Flow_Regime and warning status should be read together. The friction calculation is intended for turbulent duct flow; a low Reynolds number is reported as a review condition rather than silently treated as ordinary turbulent design.

Validation and status logic

The workbook returns the first matching status.

Assumptions and limitations

The ASHRAE power-law fit is a standard-air, galvanised-steel correlation and deliberately does not respond to material roughness or actual air density. The Colebrook branch responds to those properties but still treats a uniform straight duct. The two methods can legitimately disagree because one is a fitted ductulator relation and the other is a physics calculation.

Air-property correlations and standard-atmosphere pressure are approximations. Rectangular equivalent diameter matches friction behavior; it does not prove equal acoustic, clearance, or fitting performance. Standard-size and roughness lists are workbook data, not product specifications. Numeric inputs must be re-entered after switching unit systems.

Restrictions and non-computing states

Altitude is explicitly restricted to -1500 through 20,000 ft (-457 through 6096 m). Airflow and applicable duct dimensions must be positive. Target friction must be positive in a sizing mode. Input checking rejects declared negative values before calculation; the workbook performs the stronger mode-dependent and physical checks above.

Errors and warnings

NOT VALID suppresses reliance on a result when atmospheric range, airflow, friction target, or geometry is unusable. CHECK means a numeric result exists but the selected correlation basis, flow regime, or rectangular aspect ratio needs review. Rejected entries, connection failures, and calculation-service failures are separate from these workbook messages.

The workbook derives its relations rather than reproducing any table, chart or figure from a handbook or standard. The power-law correlation and the Colebrook/Darcy-Weisbach path are both computed rather than read from a duct calculator or friction chart.

• ASHRAE. Handbook: Fundamentals, Duct Design chapter, the source of the friction relations and equivalent-diameter methods this tool implements. https://handbook.ashrae.org/Handbooks/F17/SI/f17_ch21/f17_ch21_si.aspx
• Wikipedia. Darcy-Weisbach equation. https://en.wikipedia.org/wiki/Darcy%E2%80%93Weisbach_equation
• Wikipedia. Darcy friction factor formulae, for the Colebrook equation. https://en.wikipedia.org/wiki/Darcy_friction_factor_formulae

ACCA Manual D is the governing residential duct-design procedure in United States practice. This tool does not implement it and a result here is not a Manual D calculation. Roughness values shipped with the workbook are typical published figures and carry no authority. No trademark or organisation name appearing here implies endorsement by its owner.

Additional source notes migrated from Methodology

The workbook cites the ASHRAE Handbook—Fundamentals duct-design and psychrometric relationships; Colebrook (1939) for turbulent friction; Swamee and Jain (1976) for the explicit seed; Huebscher (1948) for rectangular equivalent diameter; and Sutherland's air-viscosity correlation. Its fixed-point and Colebrook steps are explicitly unrolled in the workbook.