How to size a subwoofer
A subwoofer has to do two things at once. It must reach low enough to cover the part of the spectrum your main speakers cannot, and it must play loud enough at those frequencies to match the level of the rest of the system. Both goals depend on the room. This calculator estimates driver size and amplifier power from the volume of your room, how low you want to play and how loud you listen, and also shows where your room is most likely to boom.
Room volume sets the workload
Low frequencies carry long wavelengths, and bass pressure in a closed room is related to the volume of air the subwoofer has to energize. A small bedroom can sound deep with a modest driver, while a large open living room needs far more displacement. The calculator starts with the room volume and scales it for your chosen listening level, low-frequency target and placement.
Volume (ft³) = length × width × height Base power (W) ≈ Volume ÷ 10 Required power = Base × level factor × extension factor × placement factor
Listening level doubles the factor each time peaks rise by roughly 3 dB, because doubling amplifier power gives about +3 dB. Extension factor rises if you want to play very low, since cone excursion rises quickly as frequency drops. Corner placement gains efficiency because walls reinforce bass; free-standing placement does not.
Driver size guide
| Required power (per sub) | Typical driver | Best for |
|---|---|---|
| Up to 120 W | 8" | Small rooms, music, desktop systems |
| 120 – 250 W | 10" | Medium rooms, mixed use |
| 250 – 500 W | 12" | Large rooms, movies |
| 500 – 900 W | 15" | Large or open rooms, high levels |
| Over 900 W | 18" or dual 15" | Dedicated theaters |
If you want response below 20 Hz, we move the recommendation up a size, because deep bass needs more cone area. A bigger driver is not automatically better: a well-engineered 10-inch sub will beat a poorly built 15-inch model.
Room modes: why bass sounds uneven
Every room has resonant frequencies where standing waves form between parallel surfaces. The lowest axial mode for a dimension is half the speed of sound divided by that length. In a room that is 18 feet long the first mode is near 31 Hz, so notes close to that frequency can boom in some seats and vanish in others.
Axial mode (Hz) = 565 ÷ dimension in feet (343 ÷ (2 × meters)) Schroeder frequency ≈ 2000 × √(RT60 ÷ Volume in m³)
Below the Schroeder frequency the room is dominated by individual modes; above it, reflections blend together. Moving the subwoofer, moving your seat or using two subs in different positions are the most effective ways to smooth the response.
Crossover and enclosure type
The crossover tells the receiver which low frequencies go to the sub. A good rule is to set it 20–30 Hz above where your main speakers lose output. For many bookshelf speakers this means 80 Hz. Sealed enclosures are tighter and a little less deep, which many music listeners prefer, while ported designs are louder and reach lower for the same size, which suits movies.
Placement checklist
- Try the subwoofer crawl: put the sub at your seat, play bass-heavy music, and crawl around the room to find where the bass sounds best. Place the sub there.
- Avoid exactly the middle of a wall if your room has a strong mode in that dimension.
- Use two subs on opposite walls for smoother bass across multiple seats.
- Set the sub’s phase to match your main speakers, then use room correction if your receiver supports it.