Quick answer
With the default setup, At 20 V and 3 A, a 6 ft cable estimated at 22 AWG drops about 0.701 V, leaving roughly 19.3 V at the device. The estimated drop is 3.5%, and the result is classified as good voltage margin.
- Voltage drop
- 0.701 V
- Voltage at device
- 19.3 V
- Power loss
- 2.1 W
- Round-trip resistance
- 233.7 mOhm
How to use this USB-C cable voltage drop calculator
Start with the USB-C voltage and current you expect the charger and device to use. For many laptop charging setups, that may be 20V at 3A or 5A. For small USB devices, it may be 5V at 1A or 2A.
Enter the one-way cable length and the estimated power conductor gauge. The calculator doubles the cable length internally because current must travel through both the supply and return path. It also adds a connector/contact resistance estimate to make the result more realistic than a copper-only calculation.
If your main question is charger wattage rather than cable drop, use the USB-C Charger Wattage Calculator. For battery time from watt-hours and charger power, use the Battery Charging Time Calculator.
USB-C cable voltage drop formula
AWG diameter mm = 0.127 x 92^((36 - AWG) / 39)wire area mm^2 = pi x (diameter / 2)^2resistance per meter = copper resistivity / wire arearound-trip resistance = resistance per meter x cable length x 2 + connector resistancevoltage drop = current x round-trip resistancevoltage at device = source voltage - voltage droppower loss = current^2 x round-trip resistanceThe calculator uses a copper resistance estimate at room temperature. Real USB-C cables may use multiple conductors, different copper quality, different connector resistance, or internal electronics, so the result should be used as a planning estimate rather than a certification result.
What this calculator does and does not measure
This tool focuses on one practical question: will cable resistance leave enough voltage at the device side? That is different from checking whether a cable is officially rated for a USB-C Power Delivery mode. A cable can have a power rating and still have measurable voltage drop.
- It estimates copper conductor resistance from AWG, length, and current.
- It includes a simple connector/contact resistance input because real cables are not copper-only.
- It reports voltage drop, device-side voltage, power loss, and a recommended AWG target.
- It does not verify cable e-marker data, insulation, heat rise, product safety, charger negotiation, or USB-IF certification.
Example calculations
USB-C laptop cable voltage drop example
A common laptop setup is 20 V at 3 A through a 6 ft cable. With a 22 AWG conductor estimate and a small connector/contact resistance allowance, the default result drops about 0.701 V, leaving roughly 19.3 V at the device.
If the same power is carried by a longer or thinner cable, the voltage drop rises. At 5V, even a small absolute drop can become a large percentage of the supply voltage, which is why some long phone or accessory cables charge slowly or behave unreliably.
Common USB-C cable voltage drop examples
The examples below show why 5V cables are more sensitive to voltage drop, while higher-voltage USB-C PD profiles can carry the same power with less percentage drop at the same current.
| Setup | Voltage | Current | Length | Gauge | Drop | Device voltage | Status |
|---|---|---|---|---|---|---|---|
| Short phone cable | 5 V | 2 A | 3 ft | 24 AWG | 0.388 V (7.8%) | 4.612 V | Borderline drop |
| Long thin 5V cable | 5 V | 2 A | 10 ft | 28 AWG | 2.716 V (54.3%) | 2.284 V | High voltage drop |
| 60W laptop cable | 20 V | 3 A | 6 ft | 22 AWG | 0.701 V (3.5%) | 19.3 V | Good voltage margin |
| 100W laptop cable | 20 V | 5 A | 6 ft | 20 AWG | 0.809 V (4.1%) | 19.19 V | Good voltage margin |
| 240W EPR-style setup | 48 V | 5 A | 6 ft | 20 AWG | 0.809 V (1.7%) | 47.19 V | Good voltage margin |
USB-C 3A, 5A, 60W, 100W, and 240W cable labels
USB-C charging labels usually combine voltage and current. For example, 60W can be 20V at 3A, while 100W uses 20V at 5A. Newer high-power USB-C setups can use higher voltage profiles such as 28V, 36V, or 48V at up to 5A. The cable, charger, and device all need to support the same power level before that profile is useful.
Voltage drop is a separate practical check. A cable may be rated for a power class but still lose voltage if it is long, thin, worn, warm, or poorly made. Use this calculator alongside the USB-C Charger Wattage Calculator when you are planning a laptop, dock, power bank, handheld, or travel charger setup.
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FAQ
How much voltage drop is acceptable for USB-C?
For practical planning, 3-5% is a conservative target and 10% is often a warning sign, especially at 5V. The exact limit depends on the charger, device, negotiated USB PD profile, and how much voltage margin the device can tolerate.
Why does a long USB-C cable charge slower?
A long cable has more copper resistance. At the same current, that resistance creates voltage drop and heat loss. The charger may also negotiate a lower current or the device may reduce charging speed if the voltage at the device side is too low.
Does a 100W or 240W USB-C cable still have voltage drop?
Yes. A higher cable power rating means the cable is designed for a certain USB-C current and voltage class, but it does not remove resistance. Length, conductor size, connector resistance, current, and temperature still affect voltage at the device.
Is 5V USB more sensitive to cable voltage drop than 20V USB-C PD?
Yes. The same 0.5V drop is 10% of a 5V supply but only 2.5% of a 20V supply. That is why low-voltage, high-current USB devices can be more sensitive to cable length and thin conductors.
What AWG is good for a USB-C charging cable?
It depends on length and current. Lower AWG numbers mean thicker copper and less resistance. For higher-current USB-C laptop charging, shorter cables and thicker power conductors are usually better. The calculator estimates whether the selected AWG meets your voltage-drop target.
Why does the calculator ask for connector resistance?
Real cables lose voltage not only in the copper wire but also at connectors and contacts. The default is only an estimate. If you do not know the value, leave the default and treat the result as a planning estimate rather than a lab measurement.
Can this calculator prove a cable is safe?
No. It estimates voltage drop and power loss from resistance. It does not certify insulation, temperature rise, e-marker behavior, USB-IF compliance, connector quality, or product safety.
Should I use a shorter cable or a thicker cable?
Both reduce voltage drop. A shorter cable reduces resistance directly, while a thicker conductor lowers resistance per meter. If the drop is high, the most practical fix is usually a shorter certified cable with an appropriate USB-C power rating.