Duct Static Pressure Calculator

1) Velocity

V = Q / A, where Q is flow rate and A is cross‑sectional area.

2) Velocity pressure

q = ρ V² / 2, where ρ is air density.

3) Straight duct loss (Darcy–Weisbach)

ΔP_straight = f (L / Dh) q. For rectangular ducts, Dh = 2ab/(a+b).

4) Fitting loss (K-method)

ΔP_fittings = (Σ K) q.

5) Total static loss

ΔP_total = (ΔP_straight + ΔP_fittings + ΔP_equipment) × SafetyFactor.

1. Choose a unit system that matches your drawings and schedules.
2. Enter flow rate, duct size, and total straight length.
3. Select duct material roughness or enter a custom value.
4. Add fitting counts and adjust K values if you have project data.
5. Enter equipment drops from product submittals and shop drawings.
6. Click Calculate and review straight, fittings, and equipment breakdown.
7. Use CSV or PDF export for estimates, reviews, and commissioning.

1) Why static pressure drives fan selection

Supply and return fans must overcome total system resistance, which is the sum of duct friction, fitting turbulence, and component pressure drops. In many comfort systems, total external static pressure commonly falls between 0.5 and 2.5 in.wg, depending on duct velocity, filtration level, and accessories.

2) Velocity is the hidden cost multiplier

Pressure losses scale with velocity pressure q = ρV²/2. If velocity increases by 20%, velocity pressure rises by 44%. That increase affects both straight runs and fittings, so “small” layout changes can materially shift total pressure and fan brake horsepower.

3) Straight duct loss and surface roughness

The calculator uses Darcy–Weisbach for straight duct loss and estimates friction factor from Reynolds number and relative roughness. Smooth materials reduce friction, while flexible or ductboard sections typically increase resistance. When mixing materials, model the highest-loss segments explicitly.

4) Fittings and transitions often dominate

Elbows, tees, entries, exits, and abrupt transitions convert flow energy into turbulence. Their loss is captured with the K‑method: ΔP = (ΣK)q. For high-velocity mains, a handful of fittings can add as much loss as tens of meters of straight duct, especially on branch takeoffs.

5) Commissioning and reporting

Use measured device drops from submittals (filters, coils, silencers, dampers) and apply a modest safety factor when routing is uncertain. Export the results to document assumptions, compare alternates, and support TAB targets. If measured field pressure deviates, adjust K values and lengths to calibrate.

1) What is duct static pressure?

It is the resistance a fan must overcome to move air through ducts, fittings, and devices. It is commonly reported in Pa or inches of water column.

2) Why do fittings matter so much?

Fitting losses rise with velocity pressure, so higher air speed increases turbulence losses quickly. Elbows, tees, and abrupt transitions can dominate total pressure in compact layouts.

3) When should I apply a safety factor?

Use it when routing is not finalized, when future accessories may be added, or when filter loading is expected to increase. Keep it modest and document the basis.

4) Can I use this for return air ducts?

Yes. The same loss methods apply to supply and return. Enter the correct flow rate and include grilles, dampers, filters, and any terminal devices that add pressure drop.

5) What happens at very low airflow?

Flow can approach laminar conditions. The calculator uses a laminar friction relation when Reynolds number is below 2300, which changes how straight-duct loss scales with velocity.

6) Are K values always the same?

No. K depends on geometry, radius, angle, and fabrication quality. Use handbook or manufacturer values when available, then adjust K here to match project assumptions or field findings.

7) Does this replace full duct design?

It supports rapid loss estimates and documentation. Detailed design may still require equal‑friction or static‑regain methods and verified fitting data for each branch, terminal, and control device.

1) What is duct static pressure?

It is the pressure a fan must overcome to move air through ducts, fittings, and devices. It is typically reported as Pa or inches of water column.

2) Why do fittings matter so much?

Fittings create turbulence and separation. Their loss rises with velocity pressure, so high air speed can make elbows, tees, and transitions dominate total pressure.

3) When should I use a safety factor?

Use it when duct routing is uncertain, when accessories may be added, or when cleanliness will change over time. Keep it modest and document the basis.

4) Can I use this for return air systems?

Yes. The same loss methods apply to supply and return runs. Make sure you use the correct flow rate and include devices such as grilles, dampers, and filters.

5) What if my airflow is very low?

At low Reynolds numbers, flow may be laminar and friction behavior changes. The calculator switches to a laminar friction relation when Reynolds number is below 2300.

6) Are K values universal?

No. K depends on geometry, radius, angle, and fabrication. Use manufacturer or handbook values when available, then adjust here to match your project assumptions.

7) Does this replace a full duct design?

It helps estimate losses quickly, but detailed design may require full equal‑friction or static‑regain methods and verified fitting data for each branch and terminal.