DC Barrel Jack Sizes: 5.5mm × 2.1mm vs 2.5mm And More

May 26, 2026

You've lost the power adapter for some electronic device — a router, a small speaker, an old guitar pedal, a security camera. The device has a round power input jack, but you have no idea which specific power supply it needs. There are dozens of similar-looking barrel connectors in different sizes, voltages, and polarities. Pick the wrong one and you can damage the device.

Here's how to identify the right DC barrel jack for any device.

What is a DC barrel jack?

A DC barrel jack (also called a barrel connector, coaxial power connector, or sometimes a "Tip & Sleeve" connector) is the cylindrical metal connector found on most low-power DC adapters. It consists of two concentric conductors:

Inner conductor (pin/tip) — Usually carries the positive DC voltage
Outer conductor (sleeve) — Usually carries the negative (ground/return)

The plug slides over a matching pin on the device's power input jack. When fully inserted, both conductors make contact and the device gets power.

This connector design has been used since the 1970s for everything from radio equipment to modern routers, LED lighting, and IoT devices. It's simple, cheap to manufacture, and reasonably reliable.

The three measurements that matter

A barrel jack is described by three numbers, usually written like "5.5mm × 2.1mm × 12mm":

Outer diameter (OD) — The outside diameter of the plug. Common sizes: 5.5mm, 4.0mm, 3.5mm, 3.0mm, 2.5mm.

Inner diameter (ID) — The diameter of the hole in the plug (which fits over the pin in the device's jack). Common sizes: 2.1mm, 2.5mm, 1.7mm, 1.35mm.

Length — The depth the plug extends into the jack. Typically 9.5mm to 14mm. Less critical than OD and ID since most jacks accommodate a range.

The combination of OD and ID determines compatibility. A plug with the wrong inner diameter either won't make contact (if too large) or will be loose/insecure (if too small).

The most common size: 5.5mm × 2.1mm

This is by far the most common DC barrel jack size in consumer electronics. You'll find it on:

Most routers and modems
LED strip light power adapters
Small monitor power adapters
Older external hard drives
Most Raspberry Pi alternatives (though Pi 4 uses USB-C)
Many guitar effects pedals
Surveillance cameras
WiFi access points

If you find a barrel jack and don't know the size, 5.5mm × 2.1mm is your best initial guess — it's probably right more often than any other size.

The second most common: 5.5mm × 2.5mm

Visually identical to 5.5mm × 2.1mm but with a slightly larger inner hole. Found on:

Some laptop chargers (especially older ThinkPads, some Asus)
Some monitor power supplies
Some industrial equipment
Some older guitar amplifiers

The slight size difference creates a real compatibility problem: a 2.5mm device with a 2.1mm plug may seem to fit but won't make solid electrical contact, leading to intermittent power. A 2.1mm device with a 2.5mm plug won't fit at all.

How to measure if you have the plug

You need digital calipers for precise measurement. With the plug in hand:

Outer diameter: Measure the widest part of the metal cylinder
Inner diameter: Measure the hole in the center of the plug
Length: Measure how far the metal extends from the plastic housing

If you don't have calipers, the easy comparison test:

Compare to a known plug (most 5.5×2.1mm power adapters in your house are this size)
Look at the device manual or printed label — many devices list "5.5×2.1mm" or similar
Use a smartphone macro photo and measure on screen with a ruler

Polarity matters even more than size

Beyond size, you must match the polarity — which conductor carries positive voltage.

Center positive (most common) — The inner pin is +, the outer sleeve is −. Most modern consumer electronics use this.

Center negative — The inner pin is −, the outer sleeve is +. Common in some guitar effects pedals, older audio equipment, and a few specialty devices.

Look for the polarity symbol on the device near the power input. It looks like:

⊕ ───┤├─── ⊖ with a "C" or "P" symbol shape
A small diagram showing which side gets the plus or minus sign

Connecting reversed polarity can permanently damage your device. Modern equipment often includes reverse polarity protection, but never count on it. Older equipment usually has no protection and will fail immediately.

Voltage and current ratings

The connector itself doesn't determine voltage — that's set by the power adapter (transformer) feeding it. Common voltages for barrel jack devices:

5V — USB-era voltage, used for low-power devices
9V — Common for guitar pedals, some routers, older synthesizers
12V — Most common: LED strips, routers, security cameras, monitors
15V — Some laptop chargers, professional audio
19V — Many laptop chargers
24V — Industrial equipment, some PoE injectors

Voltage must match exactly. A device needing 12V won't work properly with a 9V adapter (often won't power on) and will burn out with a 19V adapter (over-voltage damage is permanent).

Current (Amperage) is more flexible. The adapter must supply at least the current the device needs. A device needing 2A works fine with a 3A or 5A adapter — it only draws what it needs. But a 1A adapter for a 2A device will overheat and fail.

Example: a "12V 2A center positive 5.5×2.1mm" adapter can be replaced with a "12V 5A center positive 5.5×2.1mm" without issue — the higher amperage just means more headroom.

Universal multi-tip adapters

Universal power adapters come with a single adapter and multiple interchangeable tips. They handle the variety of barrel sizes by including 6-10 different tips. The adapter has a voltage selector for choosing the right output voltage.

These are useful for finding what works with an orphaned device — try different tips and voltages until the device powers on correctly. Always check polarity first!

For a permanent solution, dedicated single-purpose adapters are generally more reliable than universal ones.