Enter project inputs
Example data table
| Case | Available | Assist | Friction | Fittings | Equipment | Terminal | Margin | Expected NDP |
|---|---|---|---|---|---|---|---|---|
| Office corridor extract | 850 Pa | 0 Pa | 180 Pa | 90 Pa | 220 Pa | 120 Pa | 10% | ~155 Pa |
| Dust control negative pressure | 650 Pa | -40 Pa | 210 Pa | 120 Pa | 260 Pa | 90 Pa | 12% | Near 0 Pa |
| Long run with high filter drop | 900 Pa | 0 Pa | 260 Pa | 140 Pa | 420 Pa | 140 Pa | 10% | Negative |
Formula used
The calculator estimates remaining pressure after accounting for losses and a design margin:
- Gross available = Available + Assist
- Effective available = Gross available × (1 − Margin/100)
- Total losses = Friction + Fittings + Equipment + Terminal required
- Net driving pressure = Effective available − Total losses
Use a negative assist value to represent opposing forces (for example, adverse wind or stack effects).
How to use this calculator
- Select the unit system you use for pressure inputs.
- Enter available pressure at the start of the path.
- Enter assist/opposing pressure if it applies to your case.
- Enter friction, fitting, and equipment pressure drops.
- Enter the minimum terminal required pressure at the endpoint.
- Set a design margin to cover uncertainty and aging.
- Press Calculate and review the net driving pressure.
- Download CSV or PDF to attach to design notes.
Article
1) Net driving pressure as a design check
Net driving pressure (NDP) is the remaining pressure available to move air after all modeled losses are paid. In practical construction ventilation and HVAC paths, it confirms whether a fan, shaft, or duct run can deliver target flow to the endpoint while still meeting the terminal requirement.
2) Typical pressure magnitudes in buildings
Many comfort systems operate in the range of tens to a few hundred pascals across branches, while filters, coils, and temporary site screens can add significant drops. Because real installations include leakage, roughness changes, and partial damper positions, a 5–15% margin is commonly applied to avoid overpromising. For smoke control or industrial extract, budgets can be higher, and clean-versus-dirty filter drop should be verified.
3) Building the pressure budget
This calculator treats available pressure plus any assist or opposing effects as the starting budget. It then subtracts friction loss, fitting loss, equipment drop, and terminal required pressure. The result is the NDP, which should be positive for an adequate design. Negative values indicate a shortfall that must be corrected. If you have measured pressures, enter them directly to align with commissioning readings and avoid double-counting allowances.
4) Interpreting the loss breakdown
The breakdown table shows each loss component and its share of total losses. High fitting share usually points to excessive elbows, abrupt transitions, or crowded routing. High equipment share often indicates restrictive filters or protection screens. High friction share may suggest undersized ducts or rough interior surfaces.
5) Practical improvement actions
To recover deficit, start with the largest loss item. Simplify fittings, reduce duct velocity by resizing, select lower-drop equipment, or improve maintenance for dirty filters. If constraints remain, increase available pressure through fan selection or control strategy, then recheck NDP and document results for future site commissioning.
FAQs
1) What does a positive net driving pressure mean?
A positive NDP means your available pressure exceeds modeled losses and the terminal requirement. It suggests the path can meet flow targets, assuming inputs reflect real installation conditions.
2) When should I use a negative assist value?
Use a negative assist value when external effects oppose flow, such as adverse wind, stack effects, or backpressure from connected spaces. It reduces the gross available pressure.
3) How do I choose an appropriate margin percentage?
Common practice is 5–15%, depending on uncertainty, cleanliness, and change risk. Higher margins suit temporary installations, unknown fittings, or systems expected to clog over time.
4) Why include terminal required pressure as a loss?
The terminal requirement is the minimum pressure needed at the endpoint to deliver performance. Treating it as part of total losses ensures the remaining pressure is assessed correctly.
5) Which loss component should I reduce first?
Start with the highest share in the breakdown table. It delivers the biggest improvement per effort, whether by rerouting fittings, resizing ducts, or selecting lower-drop equipment.
6) Can I use different units for inputs and outputs?
Yes. Choose an input unit for your entries and an output unit for reporting. The calculator converts internally and presents results consistently in the selected output unit.
7) How can I validate the calculation on site?
Measure pressure at key locations and compare with your assumptions. Check for obstructions, leaks, and dirty filters. Update loss values and rerun the calculation to match actual conditions.