Fan / Blower Power Calculator - Air Power, Shaft Power & Motor kW (Free)
Fan and blower power is one of the most frequently mis-estimated quantities in HVAC and industrial ventilation design. Confusing air power with shaft power, mixing static pressure with total efficiency, or forgetting motor and belt losses can produce an error of 30–50 % in the predicted electrical demand — which propagates straight into your energy model, your operating-cost estimate and your motor selection. This page gives you a rigorous treatment of the fan power equation together with a free online Fan / Blower Power Calculator that computes air power, shaft (brake) power and motor input power in any unit system, and reports annual energy and cost.
Figure 1 Fan/Blower Power Calculation working steps
It pairs naturally with the site's CFM Calculator and Duct Sizing Calculator: size the duct, obtain the system pressure drop, then bring the airflow and pressure here to size the fan and its motor.
1. The Fan / Blower Power Calculator
Enter airflow and pressure rise in whatever units you have; the tool converts to SI, applies your efficiencies and returns the three power levels plus energy and cost. Nothing is sent to a server — all computation happens in your browser.
Fan / Blower Power Calculator
| Quantity | Value (SI) |
|---|---|
| Airflow Q | — |
| Pressure rise Δp | — |
| Air power (Q·Δp) | — |
| Shaft power | — |
| Motor input power | — |
| Overall efficiency | — |
2. Governing Equations
The useful power a fan imparts to the air stream — the air power — is simply the product of the volumetric flow rate and the total pressure rise it produces:
Because a real fan is not lossless, the impeller must absorb more than this. Dividing by the fan total efficiency gives the shaft (or brake) power that the drive shaft must deliver:
Finally the electric motor must supply the shaft power plus its own copper/iron losses and any belt-drive loss. The motor input (electrical) power is:
3. Air vs Shaft vs Motor Power
Three distinct power levels are involved, and each is larger than the previous one because of a cascade of losses. Selecting a motor on air power (instead of motor input power) is a classic under-sizing error.
| Level | Symbol | Definition | Losses included |
|---|---|---|---|
| Air power | P_air | Q × Δp | None (ideal) |
| Shaft / brake power | P_shaft | P_air / η_fan | Fan aerodynamic + mechanical |
| Motor input power | P_motor | P_shaft / (η_motor·η_drive) | + Motor + belt/drive |
4. How to Use It — 5 Steps
| # | Step | Detail |
|---|---|---|
| 1 | Enter airflow Q | Pick CFM, m³/s, m³/h or L/s — auto-converted to SI. |
| 2 | Enter pressure rise Δp | Total pressure in Pa, kPa, in.wg or mmH₂O. |
| 3 | Enter fan efficiency | See the typical values in Section 6. |
| 4 | Enter motor & drive efficiency | Direct-drive: set drive efficiency to 1.0. |
| 5 | Read the results | Air / shaft / motor power in kW & HP, plus energy and cost. |
5. Unit Conversions
| From | To SI | Multiply by |
|---|---|---|
| CFM | m³/s | 0.0004719 |
| m³/h | m³/s | 0.0002778 |
| L/s | m³/s | 0.001 |
| in. water gauge | Pa | 248.84 |
| mm H₂O | Pa | 9.80665 |
| W → HP | HP | 0.001341 |
6. Typical Fan Efficiencies
| Fan type | η_fan (total) | Notes |
|---|---|---|
| Airfoil centrifugal | 0.80–0.88 | Highest efficiency, clean air |
| Backward-curved centrifugal | 0.75–0.85 | Good general-purpose choice |
| Axial (vane) | 0.65–0.85 | High flow, low pressure |
| Forward-curved ("squirrel cage") | 0.55–0.70 | Compact, low cost |
| Propeller | 0.30–0.50 | Very low pressure only |
7. Worked Example
A supply fan delivers 2000 CFM against 2 in.wg total pressure. Fan efficiency 0.75, motor 0.90, belt drive 0.97.
Convert: Q = 2000 × 0.0004719 = 0.9438 m³/s; Δp = 2 × 248.84 = 497.7 Pa.
Overall efficiency ≈ 65.5 %. At 4000 h/yr and $0.15/kWh, annual cost ≈ $430. Selecting a motor on air power (0.63 HP) would have under-sized it by roughly a third.
8. Fan Laws & Air Density
The fan affinity laws relate performance to speed N and diameter D at constant density:
| Quantity | Scales as |
|---|---|
| Flow Q | ∝ N |
| Pressure Δp | ∝ N² |
| Power P | ∝ N³ |
Because power scales with the cube of speed, a 20 % reduction in fan speed (via a VFD) cuts power by roughly half — the fundamental reason variable-speed drives dominate modern energy-efficient ventilation. Note that the pressure a fan develops scales with air density; at altitude or elevated temperature the same fan produces less pressure, so density correction is essential during selection even though the P_air = Q·Δp formula itself uses the delivered pressure directly.
9. Frequently Asked Questions
What is the formula for fan power?
Air power P_air = Q × Δp (m³/s × Pa = W). Shaft power = P_air / η_fan; motor input power = shaft / (η_motor × η_drive).
What's the difference between air, shaft and motor power?
Air power is delivered to the air; shaft power is what the impeller absorbs (higher, via fan losses); motor input power is drawn from the mains (higher still, via motor and belt losses).
Static or total pressure?
Total pressure with total efficiency for correct air power. If only static pressure is known, use static efficiency to stay consistent.
How do I convert CFM and in.wg to power?
CFM × 0.0004719 → m³/s; in.wg × 248.84 → Pa; then Q × Δp. Shortcut: BHP ≈ (CFM × in.wg)/(6356 × η_fan).
What is a typical fan efficiency?
Backward-curved 0.75–0.85, airfoil up to 0.88, axial 0.65–0.85, forward-curved 0.55–0.70, propeller below 0.50.
How does air density affect fan power?
For fixed volumetric flow and pressure, Q·Δp is density-independent — but the pressure a fan actually develops scales with density, so altitude and temperature affect selection.
Educational tool for preliminary sizing. Verify against manufacturer fan curves and applicable standards (AMCA / ASHRAE) before final selection.
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