Free subwoofer box calculator from EM Audio. Pick one of our hand-built subs or type in any subwoofer’s Thiele/Small specs, tell it your vehicle and what you want out of it, and it designs the enclosure with you: sealed or ported volume, port tuning, slot or flared aero port sizing, a modeled response curve, outside dimensions and a ¾″ MDF cut sheet. Built for your phone as much as your desk.

How the subwoofer box calculator works

Every design starts from the driver. For EM Audio subs the calculator uses the enclosure range printed on each product page. For any other sub you enter Fs, Qts and Vas (plus Xmax, Sd, mounting depth and cutout if you have them) and it runs the classic Thiele/Small alignments: EBP to decide sealed vs. ported, a Qtc 0.707–0.9 sealed volume, and a ported volume and tuning from Qts and Vas. Then it sizes the box around your space, builds a slot port from the box walls with Helmholtz tuning, checks port air velocity, adds the driver, port and bracing displacement on top of the net volume, models the in-box frequency response, and writes the cut sheet.

Sealed or ported?

A sealed box is smaller, tighter and more forgiving — it rolls off gently and works with cabin gain, so it is the safe choice for tight trunks and sound quality. A ported box gives roughly 3 dB more output around its tuning frequency and digs deeper, at the cost of a bigger box and a subsonic filter set a few hertz under the tune. Daily music usually lands at 30–34 Hz, demos and SPL at 35–45 Hz, deep bass and SQ at 26–30 Hz.

Port tuning, port area and chuffing

The port length and area set the tuning: a longer port or a bigger box tunes lower, and more port area lowers air velocity (less chuffing) but needs a longer port for the same tune. A good slot port runs 12–16 square inches per cubic foot of net volume and stays under about 25–30 m/s at full excursion. Flared aero ports (3″–8″ round tubes) get away with roughly half that area because the flares keep the air quiet, so an aero box can be smaller for the same net volume and is easy to re-tune by swapping tube length; the calculator picks the port count and diameter for you, tells you if the tube will stick out the back, and reminds you to keep each port a full diameter off the inside walls. Say “aero ports” in the chat to switch.

Want the why behind the numbers? Read our ported subwoofer box guide on tuning, volume and port size, then see how to set your amp gain once the box is in.

Sub box calculator FAQ

How do I calculate subwoofer box volume?

Multiply the inside width × height × depth in inches and divide by 1,728 to get cubic feet. That is the gross volume. Subtract what the sub, the port and any bracing take up to get the net volume the sub actually sees. The calculator does both and gives you the outside dimensions in ¾″ MDF.

How big of a box does a 12 inch subwoofer need?

It depends on the sub, not just its size. Many 12s land around 1–1.5 ft³ sealed and 1.75–3 ft³ ported, but high-power competition subs often want more. Pick the sub in the calculator, or type in its Fs, Qts and Vas, and it designs around that driver instead of a rule of thumb.

What should I tune my ported box to?

Daily music usually sounds best at 30–34 Hz, demos and SPL at 35–45 Hz, and deep bass or SQ at 26–30 Hz. Set your subsonic filter a few hertz below the tune.

Does it work with subs that aren’t EM Audio?

Yes. Type in the Thiele/Small specs from the manufacturer’s spec sheet: Fs, Qts and Vas at minimum, plus Xmax, Sd, mounting depth and cutout if you have them.

Is the box calculator free?

Yes. No account needed, and it works on your phone. When you like the design, send it to EM Audio and we will look it over before you cut wood.

A calculator is a starting point, not a substitute for a designer. When you like the numbers, hit “send to EM Audio” and we will double-check the design against your sub and your vehicle before you cut wood.