Fan Energy Index calculator
Calculate FEI at any duty point per ANSI/AMCA Standard 208-18. The calculator builds the full reference fan chain — shaft power, belt drive, motor — so you see not just the index but where the reference power comes from, and how much power you have left before you fall below the code threshold.
What FEI measures
The Fan Energy Index answers one question: at this exact duty point, how much electrical power does this fan draw compared with a defined reference fan?
FEI = FEPreference / FEPactual
Both powers are electrical, in kilowatts, evaluated at the same airflow, the same pressure and the same air density. An FEI of 1.20 means the fan draws about 17 % less power than the reference. An FEI of 0.90 means it draws about 11 % more. Because the comparison is anchored to the duty point rather than to the fan's peak, an oversized fan running far off its best efficiency point cannot hide behind a good catalogue number.
The reference fan
The reference is not a real machine — it is a defined chain of efficiencies that scales with the duty point. Its shaft power comes from a deliberately penalised air power:
Hi,ref = (Q + 250) · (P + 0.40 · ρ/ρstd) / (6343 · η0) [hp]
Q airflow, CFM
P pressure, in. wg η0 = 0.66 total basis
ρstd 0.075 lb/ft³ η0 = 0.60 static basis
The +250 CFM and +0.40 in. wg terms are what make the metric fair across sizes. At 50 000 CFM they are negligible; at 500 CFM they dominate, which is why the reference wire-to-air efficiency ranges from roughly 13 % for a very small fan up to about 49 % for a large one. Small fans are not asked to match large ones.
From the reference shaft power the chain continues through a V-belt drive and an IE3-class motor:
ηtrans,ref = 0.96 · (Hi,ref / (Hi,ref + 2.2))0.05
Ht,ref = Hi,ref / ηtrans,ref
ηmtr,ref = −0.003812x⁴ + 0.025834x³ − 0.072577x² + 0.125559x + 0.850274
where x = log₁₀(Ht,ref in kW), and 0.962 at or above 185 kW
ηctrl,ref = 1.00 the reference fan is constant speed
FEPref = Hi,ref / (ηtrans,ref · ηmtr,ref) · 0.7457 [kW]
The actual fan
AMCA 208 permits four routes to FEPactual. The cleanest is direct measurement of electrical input power under AMCA 210/230/260 or ISO 5801 — and for a fan with a speed controller, that measurement is taken at the controller input, not at the motor. Where no measurement exists, the standard allows the same chain to be reconstructed from shaft power with a known motor efficiency, or with a default motor efficiency curve. The default route is not permitted when a speed controller is present.
This is why FEI replaced FEG. A fan efficiency grade described the impeller. FEI describes what the building actually pays for: the drive losses, the motor losses and the drive controller losses, at the point where the fan really runs.
What the codes require
| Code | Section | Requirement |
|---|---|---|
| ASHRAE 90.1-2019 and later | 6.5.3.1.3 | FEI ≥ 1.00, VAV systems ≥ 0.95 |
| IECC 2021 | C403.8.3 | FEI ≥ 1.00 at the design point, VAV ≥ 0.95 |
| ASHRAE 189.1-2020 | — | FEI ≥ 1.10 |
A fan falls under the requirement once its motor nameplate output exceeds 1.0 hp (0.75 kW), or its nameplate electrical input power exceeds 0.89 kW. For arrays and series or parallel combinations the thresholds are 5.0 hp combined motor power or 4.1 kW combined electrical input. The 1 hp cut-off is a large drop from the 5 hp threshold that applied under FEG, which is why the metric now reaches deep into small equipment.
Exemptions include third-party-certified equipment packages such as cataloged air handlers, blower coils, fan coils and rooftop units; ceiling fans, which DOE regulates separately; high-temperature fans; explosive-atmosphere fans; reversible tunnel ventilation fans; emergency-only fans; and anything outside the scope of AMCA 208.
Where FEI stands in 2026
There is no edition newer than 208-18; the method has been harmonised internationally as ISO 12759-6:2024. The federal picture is less settled: the DOE energy conservation standard for commercial and industrial fans and blowers was withdrawn in early 2025, and DOE has proposed removing these products from EPCA coverage. The test procedure at 10 CFR 431.174, which incorporates ANSI/AMCA 214-21, remains in force. Meanwhile ASHRAE 90.1-2025 and the proposed IECC 2027 shift their citation from AMCA 208 to the DOE test procedure — the underlying arithmetic is unchanged, but the reference in the code text will move.
Practically: about seventeen US states still run FEG-era codes, and everyone on 90.1-2019 or IECC 2021 and later is on FEI. If you sell into the United States, your catalogue needs FEI at every published duty point, not a single headline number.
Frequently asked questions
Can a fan have different FEI values?
Yes, and it will. FEI is a property of the fan at a duty point, not of the fan alone. AMCA 208 requires manufacturers to calculate FEI for each duty point offered for sale, which is why FEI belongs in the selection program rather than in a datasheet footnote.
Does a VFD improve FEI?
Not at the design point. At full speed the drive adds losses and the measurement is taken at the drive input, so FEI at the design point is slightly worse. The benefit appears in part-load operation, which is why VAV systems get the relaxed 0.95 threshold.
Does FEI apply in Europe?
No — Europe uses the ecodesign framework instead, now Regulation (EU) 2024/1834. If you sell into both markets you have to publish both, which is a strong argument for computing them from a single catalogue dataset. Check the European requirement →