How do I read a fan P-Q curve?
A P-Q curve plots a fan's airflow (Q) against static pressure (P). The two endpoints are easy: maximum flow at zero pressure (free air) and maximum pressure at zero flow. Everything a design review cares about happens between them — especially where the fan curve crosses your enclosure's resistance curve, which is the only operating point the fan will actually settle at.
The four landmarks on the curve
From right to left: the free-air endpoint (maximum flow, zero pressure), the knee where the curve bends upward, the stall or saddle region on some axial fans, and the shut-off endpoint (zero flow, maximum static pressure). Catalogue CFM is always the free-air endpoint — a number the fan can only reach in an empty room, never inside your product.
- Free-air flow: flow with zero restriction — never achievable in an enclosure.
- Maximum static pressure: the pressure the fan develops with the outlet fully blocked.
- Knee/stall region: unstable zone on many axial fans; avoid operating here — noise and flow oscillation rise.
Where the real operating point comes from
Your enclosure adds a resistance curve that rises roughly with the square of flow: ΔP = K·Q². Draw it on the same axes. The intersection of the fan curve and the system curve is the operating point — the only flow the installation will deliver. If a datasheet claims 100 CFM and your review target is 80 CFM at 0.10 inH₂O, check the intersection, not the headline number.
| Point on curve | Flow | Static pressure | Meaning |
|---|---|---|---|
| Free air | 100% | 0 inH₂O | Unrestricted test value |
| Design target | ≈60–80% | system-dependent | Where most products should run |
| Shut-off | 0 | max | Blocked outlet — no cooling |
Common reading mistakes
Reading one fan curve against several candidate enclosures without re-plotting the system curve each time; comparing fans at free-air flow; ignoring that the curve was measured at a stated density and voltage per test standards such as ISO 5801 or AMCA 210. Curves are measured at fixed voltage (e.g. 12.0 V DC) and stated air density — a hotter, thinner site shifts everything.
- Always overlay the system curve — never read flow off the fan curve alone.
- Check voltage: at 10.8 V on a 12 V rail the whole curve drops; PWM curves are published per duty cycle.
- Stay right of the stall region; if your point lands in the saddle, change fan or resistance.
Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.
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