Ported box size for 3 8" subs
3 8" subs usually want 1.8–3 ft³ net (51–85 L) in a ported box. Your sub’s manual wins: if it gives a volume, build to that.
Example: 2.4 ft³ (68 L) net in 3/4" MDF is a box 32 1/2" × 11" × 20" outside (width × height × depth), with a 3 9/16" × 9 1/2" slot port 32 1/16" long for 32 Hz. The port takes 0.76 ft³ (21.4 L), already counted in that size.
The 32 1/16" port is longer than the box is deep inside (18 1/2"): fold it into an L along the side or the back.
Example box for 2.4 ft³ (68 L) net
Example box for 2.4 ft³ (68 L) net| What | Size |
|---|
| Outside (W × H × D) | 32 1/2" × 11" × 20" (826 × 279 × 508 mm) |
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| Inside (W × H × D) | 31" × 9 1/2" × 18 1/2" |
|---|
| Gross volume | 3.16 ft³ (89.4 L) |
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| Slot port | 3 9/16" wide × 9 1/2" tall (33.6 sq in) |
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| Port length | 32 1/16" (814 mm) for 32 Hz |
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| Port takes up | 0.76 ft³ (21.4 L) |
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| Net volume | 2.4 ft³ (68 L) |
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Cut sheet for the example box (3/4" MDF)
Cut sheet for the example box (3/4" MDF)| Piece | Qty | Size |
|---|
| Front (baffle) | 1 | 32 1/2" × 11" |
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| Back | 1 | 32 1/2" × 11" |
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| Top and bottom | 2 | 32 1/2" × 18 1/2" |
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| Sides | 2 | 9 1/2" × 18 1/2" |
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Same slot port at other tunings
Same slot port at other tunings| Tuning | Slot length |
|---|
| 30 Hz | 37 1/8" (943 mm) |
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| 32 Hz | 32 1/16" (814 mm) |
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| 34 Hz | 27 7/8" (707 mm) |
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| 36 Hz | 24 5/16" (618 mm) |
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Typical net volume for 8" subs
Typical net volume for 8" subs| Subs | Sealed | Ported |
|---|
| 1 | 0.35–0.6 ft³ | 0.6–1 ft³ |
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| 2 | 0.7–1.2 ft³ | 1.2–2 ft³ |
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| 3 | 1.05–1.8 ft³ | 1.8–3 ft³ |
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| 4 | 1.4–2.4 ft³ | 2.4–4 ft³ |
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Before you cut
Take off the sub’s own displacement (in its manual, often 0.05 to 0.2 ft³ per sub) and any bracing: the box calculator does that and redoes the width.
Measure the space twice, including the angle of the seat back in a truck cab. A wedge box often fits where a rectangle doesn’t.
Volume ranges are common starting points for car subs, not a standard. The example uses the middle of the range, 3/4" MDF, and for ported boxes a slot port of 14 sq in per ft³ tuned to 32 Hz along the full inside height.
Other box sizes for 8" subs
Frequently asked questions
What size ported box for 3 8" subs?
3 8" subs usually want 1.8–3 ft³ net (51–85 L) in a ported box. Your sub’s manual wins: if it gives a volume, build to that.
What are the dimensions of a ported box for 3 8" subs?
Example: 2.4 ft³ (68 L) net in 3/4" MDF is a box 32 1/2" × 11" × 20" outside (width × height × depth), with a 3 9/16" × 9 1/2" slot port 32 1/16" long for 32 Hz. The port takes 0.76 ft³ (21.4 L), already counted in that size.
What should a ported box for 3 8" subs be tuned to?
Most daily boxes are tuned between 30 and 36 Hz. Lower goes deeper and needs a longer port; higher hits harder up top. Your sub’s manual may suggest a tuning.