Ductwright HVAC Calculator

What it does

Ductwright is an HVAC and sheet metal calculator for HVAC technicians, installers, sheet metal workers and apprentices. It works like a hardware calculator: a galvanized-grey chassis, real keys and a clear LCD, with each answer labelled so you know what you are reading.

Worked examples

• Duct size. 1000 cfm at 0.1 in wg per 100 ft -> 13 7/8 in round (then 949 fpm).
• Sensible load. 1200 cfm, 20 F rise -> 25920 BTU/h sensible (then tons, kW).
• Superheat. R410A at 118 psig, 55 F suction line -> superheat 15.2 F.
• Fan laws. 1000 to 1200 rpm, 2000 cfm -> new airflow 2400 cfm (then SP, BHP).
• Taper pattern. Round taper 12 in to 6 in over 8 in -> slant 8 9/16 (then pattern radii, sweep).
• Metric duct. Metric: 1000 L/s -> 469 mm round (then 5.79 m/s).

How it's calculated

Every trade key and the formula behind it, as shown in the app's key guide. A second legend is the shifted function.

CFM L/sAirflow / Convert airflow

Stores the typed airflow (cfm, or L/s in metric), or solves Q = Area × Velocity from the duct and velocity; shifted converts between cfm and L/s.

Vel VPAir velocity / Velocity pressure

Stores the typed velocity (ft/min, or m/s in metric), or solves V = Q ÷ Area. Shifted: a typed velocity pressure gives V = 4005 × √VP (or √(Pv ÷ 0.6) m/s); blank gives the velocity pressure of the stored velocity.

Duct RectRound duct size / Rectangular duct

Stores the typed round diameter, or sizes round galvanized duct by equal friction: D = (0.109136 × Q^1.9 ÷ friction)^(1 ÷ 5.02), friction 0.1 in wg per 100 ft unless stored. Shifted stores side A, or solves side B for the stored diameter.

Frict EqØFriction rate / Equivalent diameter

Stores the typed friction rate (in wg per 100 ft, or Pa/m), or solves it for the stored airflow and duct. Shifted stores side B, or gives the round equivalent De = 1.30 × (A × B)^0.625 ÷ (A + B)^0.25.

Fan LawFan speed / New fan speed

Stores the present fan rpm; shifted stores the new rpm. Blank steps through the fan laws: flow × N2/N1, static pressure × (N2/N1)², brake power × (N2/N1)³.

ΔT LoadTemperature difference / Sensible load

Stores the air temperature rise or drop (°F, or K in metric). Shifted gives sensible heat Q = 1.08 × cfm × ΔT BTU/h (1.2 × L/s × ΔT W), then tons and kW; a typed load solves the airflow instead.

Fitting PatternFitting / Taper pattern

Stores a typed offset (or angle). Blank gives offset travel √(O² + L²) and angle, and radius elbow throat r × θ, heel (r + A) × θ and centreline (r + A ÷ 2) × θ. Shifted stores the length, or lays out a round taper: slant, pattern radii and sweep 360 × R ÷ Rp.

Bend KBend allowance / K-factor and radius

Stores the typed sheet thickness (or angle). Blank gives bend allowance (π × θ ÷ 180) × (Ri + K × T), bend deduction and outside setback. Shifted stores a typed K-factor, or a radius typed with a unit key; blank steps K 0.33 and 0.44.

Temp DP/WBAir temperature / Dew point and wet bulb

Stores the dry-bulb temperature. Shifted stores the typed relative humidity; blank gives dew point (Magnus) and wet bulb (Stull 2011, valid 5 to 99 % RH and −20 to 50 °C).

Refrig SH/SCRefrigerant / Superheat or subcooling

A typed gauge pressure (psig, or bar in metric) gives the saturation temperature; blank steps through R410A, R32, R454B, R134A, R290 and R22. Shifted: a typed line temperature gives superheat above dew or subcooling below bubble.

Results are computed from the formulas shown for each function and are provided for reference only. They do not replace a load calculation, duct design or the equipment maker's data. Verify every result against the code in force and the authority having jurisdiction, and follow safe refrigerant handling practice. The developer accepts no liability for loss or damage arising from use.