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Sones to Decibels Converter: How Loud Is a Fan Rating Really?
A common approximation for fan noise is dB ≈ 33.22 × log10(sones) + 28, so 1 sone is about 28 dB, 2 sones about 38 dB and 4 sones about 48 dB. Every doubling of sones adds about 10 dB and sounds twice as loud. Going the other way, sones ≈ 10^((dB − 28) ÷ 33.22).
Sones and decibels converter
What a sone is
Decibels describe sound pressure on a logarithmic scale. Sones describe perceived loudness on a linear scale: 2 sones sounds twice as loud as 1 sone, and 4 sones twice as loud again. That makes sones easy to compare on a product box, which is why the Home Ventilating Institute and manufacturers use them for bathroom fans, range hoods and inline ventilators.
Loudness perception roughly doubles for every 10 dB increase. The 33.22 factor in the formula is exactly what makes a doubling of sones equal 10 dB: 33.22 × log10(2) = 10.0.
The conversion formulas
dB ≈ 33.22 × log10(sones) + 28
sones ≈ 10^((dB − 28) ÷ 33.22)
Treat the result as an approximation. A true sone value is defined from loudness level in phons and depends on the frequency content of the sound, and fan ratings are measured in a lab at a set distance. The +28 offset is a practical fit that relates typical fan sone ratings to A-weighted decibels heard in a room. It is good for comparing products and setting expectations, not for code compliance or acoustic design.
Worked example
You are choosing between a 1.0 sone fan and a 3.0 sone fan.
- 1.0 sone: 33.22 × log10(1) + 28 = 0 + 28 = 28 dB, the calculator's default.
- 3.0 sones: 33.22 × 0.477 + 28 = 15.8 + 28 = 43.8 dB.
- Difference: about 16 dB, and the 3-sone fan sounds three times as loud.
Going the other way, a 40 dB reading converts to 10^((40 − 28) ÷ 33.22) = 10^0.361 = 2.3 sones.
Common sone levels
| Sones | Approx. dB | Typical comparison |
|---|---|---|
| 0.3 | 10.6 | Close to inaudible |
| 0.5 | 18.0 | Whisper-quiet ventilation fan |
| 1.0 | 28.0 | Quiet refrigerator hum |
| 1.5 | 33.8 | Quiet bathroom fan |
| 2.0 | 38.0 | Standard bathroom fan |
| 3.0 | 43.8 | Budget bathroom fan, low range hood |
| 4.0 | 48.0 | Range hood on medium, active office |
| 6.0 | 53.9 | Range hood on high |
| 8.0 | 58.0 | Loud range hood, conversation level |
The dB column is calculated from the formula above; comparisons are typical and for orientation only.
How quiet should a fan be?
- Continuous ventilation fans: ASHRAE 62.2 limits them to 1.0 sone, because a fan that runs all day must not be noticed.
- Intermittent bathroom fans: the same standard allows up to 3.0 sones. Quieter is better, because loud fans get switched off too soon. Size the fan with the bathroom fan CFM calculator.
- Bedrooms and offices: 1 sone or less if the fan or purifier will run while you sleep or work.
Sone ratings are measured at the fan's rated airflow. A restrictive duct makes many fans louder, so good bathroom fan venting also helps with noise.
Other noise sources in HVAC
Outdoor condensers and heat pumps are usually rated directly in decibels, typically somewhere in the mid-50s to mid-70s dB depending on the model and speed. Air purifiers list dB at each fan speed; check the speed you will actually use, which ties into air purifier sizing. If a system that used to be quiet becomes loud, it is a maintenance signal rather than a rating issue; see AC making noise.
Why the conversion is only approximate
Sound level meters measure pressure and usually apply A-weighting, a filter that roughly matches how the ear hears quiet sounds. Sones come from a loudness model that accounts for how the ear responds across the whole frequency range. Two fans with the same dBA reading can have different sone ratings if one has a low rumble and the other a high whine, because the ear does not treat those equally.
Measurement distance also matters. Lab sone ratings for ventilating fans are taken under controlled conditions in a test chamber, while a dB reading on your phone is taken wherever you happen to stand, in a room with hard tile surfaces that reflect sound. Expect a real bathroom to sound a little louder than the rating suggests. For buying decisions, compare products using the same kind of number: sones against sones, or dBA against dBA at the same distance. Use the converter to translate between the two when a product only lists one of them, and treat differences of a couple of decibels as within the uncertainty of the method.
A difference of 3 dB is just noticeable; 10 dB sounds about twice as loud. So 1.5 sones versus 2.0 sones (about 4 dB apart) is a small but audible step.
Frequently asked questions
How many decibels is 1 sone?
About 28 dB using the common fan approximation. In strict acoustic terms, 1 sone is defined as the loudness of a 1 kHz tone at 40 dB, which is why sources differ.
Is 1.5 sones quiet for a bathroom fan?
Yes. 1.5 sones (about 34 dB) is quiet. Most people barely notice a fan at 1 sone or less.
Why do fan makers use sones instead of decibels?
Sones are linear in perceived loudness, so shoppers can compare them directly: 2 sones is twice as loud as 1. Decibels are logarithmic and less intuitive.
How many sones is 50 dB?
Using the fan approximation, 10^((50 − 28) ÷ 33.22) = 10^0.662 ≈ 4.6 sones. That is about the level of a range hood on a medium setting and too loud for a fan that runs continuously.
Last reviewed 2026-10-28