How to read a voltage marking

Every mains device carries the answer printed on it. It is one line, it is unambiguous, and it settles the adapter-or-converter question in about five seconds — once you know which line to look at.

Where it is

On a charger or laptop brick: moulded or printed into the plastic of the brick itself, usually in type small enough to want a phone camera. On an appliance: a label on the underside, or on the back beside the mains inlet. On something with a removable lead, it is on the appliance, not the lead — the lead has its own separate rating and it is not the one that matters.

You are looking for the line that begins INPUT. Ignore OUTPUT entirely; that describes what the device gives to its battery, and it is the same everywhere in the world.

Reading it

INPUT: 100–240V ~ 50/60Hz 0.5A

Taken piece by piece:

• 100–240V — a range. This is the whole answer. The supply anywhere in the world falls inside it, so this device needs a plug adapter and nothing more.
• ~ — alternating current. A straight line, or ⎓, would mean DC, which is the output side.
• 50/60Hz — both frequencies accepted, which is effectively universal for anything with a switch-mode supply.
• 0.5A — input current, quoted at the lowest voltage in the range. Multiply for a rough wattage: it draws about 50 W flat out.

The four things you might see instead

A single number

INPUT: 120V ~ 60Hz

One voltage, no range. This is a single-voltage device. It needs a transformer or it stays at home, and if it heats, it stays at home — see adapter or converter.

A narrow range

INPUT: 220–240V ~ 50Hz

This is the trap. It is a range, so it looks like the reassuring case, but it only spans the European band. Taken to North America or Japan it is a single-voltage device. Read the numbers, not the presence of a dash.

A switch

Some older equipment has a physical 115/230 selector, occasionally a slot that needs a screwdriver rather than a finger. It must be set before the device is plugged in, and getting it wrong in the up direction destroys the device exactly as if there had been no switch at all.

Nothing legible at all

Worn labels are common on older appliances. Treat an unreadable rating as single-voltage, because that is the assumption whose failure mode is a device you did not bring rather than a device you destroyed.

The symbols worth knowing

• Square inside a square — Class II, double insulated. It has no earth pin because it does not need one, and no adapter can take away protection it never relied on. See what earthing protects against.
• An earth symbol, or simply a three-pin plug — Class I. The earth is doing real work and your adapter needs to carry it through.
• A house, or a crossed-out house — indoor use only, or a warning about outdoor use. Not relevant to voltage, frequently misread as though it were.

Traps that catch careful people

• The laptop is dual-voltage; the dock might not be. Every brick gets read separately. So does the monitor.
• A kettle and its base are one appliance. The rating is on the base, and cordless kettles are essentially never dual-voltage.
• Retail copy is not a rating plate. "Dual voltage" on a listing has been wrong often enough that it is worth thirty seconds with the actual device.
• A USB-C cable carries no rating that matters here. The question is always about whatever converts mains to low voltage, which is the brick or the charger built into the appliance.

Once you know which of your things are dual-voltage, the tool will tell you what each of them needs for a specific trip — including whether the adapter keeps the earth.