A borrowed or replacement charger is the first thing many people try when a laptop will not charge. Whether it can work depends on three things: the voltage must match, the power must be enough, and the plug — including any identification pin — must be right. This calculator handles the first two from the numbers printed on the labels.
How to use it
- From the charger's label, enter its output voltage and current (the "Output" line, not "Input").
- From the laptop's label or the original charger, enter the laptop's input voltage and current.
- Read both wattages, the headroom and the verdict.
The rules
- Voltage must match within a few percent. 19 V, 19.5 V and 20 V barrel chargers are often interchangeable on the same brand; 12 V or 24 V are not.
- Power must be at least the laptop's rating. A larger adapter is fine: the laptop takes only what it needs.
- A smaller adapter may charge slowly, only while the laptop is off, throttle the processor, or be rejected outright.
What the numbers cannot check
- Plug size and shape — barrel diameters differ by fractions of a millimetre.
- Polarity — almost all laptop chargers are centre-positive, but check the symbol.
- ID signal — many brands use a centre pin or an extra contact to tell the laptop the charger's rating. A charger with the right voltage and power but a missing or wrong ID can still be refused or limited.
Testing a charger with a meter is covered in How to Test a Laptop Charger with a Multimeter; USB-C chargers negotiate their voltage, and USB-C Power Delivery Calculator handles them.
Worked example
A laptop rated 19.5 V 4.62 A (90 W) is tried with a 19.5 V 3.34 A (65 W) charger of the same brand. Voltage matches, power is 72 % of the rating: the verdict is "underpowered". In practice such a laptop may run and charge slowly at idle, but drain the battery under load; some models show a warning at boot. The reverse — a 90 W charger on a 65 W laptop — is perfectly fine.
Where it matters
- Choosing a replacement charger.
- Explaining "plugged in, not charging" or slow-charging complaints.
- Ruling out the charger before board-level diagnosis, as in Laptop Not Charging: Charging Circuit Diagnosis.
FAQ
Can a higher-wattage charger damage my laptop?
No, if the voltage and plug match. The laptop draws only the current it needs.
Is 19 V the same as 19.5 V or 20 V?
Close enough for most laptops of the same brand. Large differences, such as 12 V vs 19 V, are not.
Why does my laptop say the adapter is unknown?
The ID signal is missing or does not match — often a broken centre pin or a third-party charger without the ID chip.