Do the math before you cut the hole

Start with the math

Every calculator shows its formula, renders a worked example into the page itself, and returns nothing rather than a guess when the inputs fall outside the published relations.

• Enclosure

  Sealed box volume Vb = Vas / ((Qtc / Qts)^2 - 1), and Fc = Fs x (Qtc / Qts)

• Enclosure

  Ported box volume Vb = 20 x Vas x Qts^3.3, Fb = 0.42 x Fs x Qts^-0.96, F3 = 0.26 x Fs x Qts^-1.4

• Enclosure

  Port length Lv = (1.463e7 x R^2) / (Fb^2 x Vb) - (k x R)

• Wiring

  Wiring impedance Series: R total = R1 + R2 + ... Parallel: 1 / R total = 1/R1 + 1/R2 + ...

• Electrical

  Fuse size I = P_rms / (efficiency x volts), then fuse at or below the conductor ampacity

• Electrical

  Wire gauge Vdrop = 2 x run length x amps x (ohms per 1000 ft / 1000)

All 16 calculators

Where most people start

A dual voice coil 12 with published specifications, which is what makes the enclosure math on this site possible in the first place.

All 12 inch subwoofers compared

The questions people actually ask

What size box does my subwoofer need?

The volume the manufacturer publishes for that exact driver, and nothing else. Enclosure volume follows the driver Vas and Qts rather than its diameter, so two 12 inch drivers can genuinely want volumes that differ by a factor of two. If the manufacturer publishes Thiele/Small parameters, the sealed and ported calculators here turn them into a real answer. If it publishes nothing, a figure borrowed from a different driver of the same size is not a substitute.

What ohm load does a dual 4 ohm subwoofer present?

Two ohms wired in parallel or eight ohms wired in series, and never four. A dual voice coil driver is named for the impedance of one coil, so a D4 has two separate 4 ohm coils. This is the single most common misunderstanding in subwoofer wiring, and it matters because presenting a load below what an amplifier is rated for is how amplifiers die.

What size fuse does my amplifier need?

Whatever the power wire can carry, because the fuse protects the wire rather than the amplifier. A 1,000 watt class D amplifier draws about 87 amps, and on 1/0 gauge that takes a 100 amp fuse comfortably. The amplifier already has onboard fuses that protect the amplifier. Fit the main fuse within 18 inches of the battery positive terminal, because every inch of cable before it is unprotected.

Does a capacitor stop my headlights dimming?

Barely. One farad at 14.4 volts stores about 104 joules, and a 1,000 watt amplifier drawing 87 amps consumes that in roughly a tenth of a second, which is shorter than a bass note. The one farad per thousand watts figure is a convention with no measurement behind it. The fixes that address the cause, in order, are the big 3 upgrade, a clean ground, a second battery and a higher output alternator.

How do I set my amplifier gain?

With a multimeter and the square root of power times impedance. A 500 watt RMS channel into 2 ohms should read 31.6 volts AC on a sine test tone with every equalizer band and bass boost flat and the head unit at about three quarters volume. Gain is an input sensitivity control rather than a volume knob, and clipping rather than wattage is what burns voice coils.

Is a ported box louder than a sealed one?

Near its tuning frequency, by roughly 3 decibels for the same power, which is the same increase as doubling the amplifier. The trade is a larger box, a port that has to be the right length and fit inside the enclosure, and a subsonic filter that is not optional because the cone unloads below tuning. Sealed is smaller, flatter and far more forgiving of a volume error.