Subwoofer Box Volume Calculator

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Getting the Enclosure Volume Right Before You Build

A subwoofer's published specifications — recommended sealed or ported enclosure volume — are only useful if the box actually built matches that target. Wood panels have thickness, and that thickness eats into the usable internal air space, which is what actually matters for the driver's performance. This calculator converts either internal or external panel dimensions into the internal volume in cubic feet, the standard unit subwoofer specs are given in.

The Formulas

Internal dimensions given:
Volume (ft³) = (L × W × H) / 1728

External dimensions given:
Internal Dim = External Dim − (2 × Wall Thickness)
Volume (ft³) = (Internal L × Internal W × Internal H) / 1728

Dividing by 1728 converts cubic inches to cubic feet (12³ = 1728). The external-dimension mode subtracts wall thickness from both sides of each dimension, since a panel of a given thickness reduces the interior span on two opposing faces.

A Worked Example

For an external box measuring 16.5 × 13.5 × 11.5 inches with ¾-inch (0.75 in) MDF walls, each internal dimension shrinks by 2 × 0.75 = 1.5 inches, leaving internal dimensions of 15 × 12 × 10 inches. That gives an internal volume of 1,800 cubic inches, or 1.042 cubic feet.

Note: This is gross internal volume before any deductions. Real net volume should also subtract driver displacement, port volume, and internal bracing — this calculator does not account for those.

Where This Gets Used

  • Matching a driver's Thiele/Small specs — subwoofer datasheets list a recommended sealed or ported volume range, and box dimensions need to hit that target net volume.
  • Planning material dimensions before cutting — working backward from a target internal volume to the external panel dimensions a cut list needs.
  • Comparing enclosure options — checking whether a given trunk or cabinet space can physically fit a box built to the recommended volume.

How to Use This Calculator

  1. Choose whether you're entering Internal or External dimensions.
  2. Enter length, width, and height in inches.
  3. If using external dimensions, enter the wall thickness in inches (defaults to 0.75 in for standard ¾-inch MDF).
  4. Select Calculate to see the internal volume in cubic feet.

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Once volume is confirmed, check speaker wiring impedance with the Speaker Impedance Calculator, or size the amplifier cable run with the Speaker Wire Gauge Calculator.