USB Speed & Power Chart (USB 2.0 to USB4, PD)

Understanding the USB speed and power chart — concept and principle

USB names have changed several times, so a cable or port labelled "USB 3.2" or "USB-C" tells you surprisingly little about how fast it is or how much power it can deliver. This chart links each USB standard to its marketing name, maximum data rate and default power without USB Power Delivery (PD). Use it when buying an SSD, dock, charger or cable, or when a transfer seems slow.

How it is calculated

This is a lookup chart, not a formula: pick a standard and the page shows its row from the USB-IF specifications. Two simple calculations help you use it:

• theoretical transfer time = file size (bits) ÷ data rate; bytes = bits ÷ 8
• USB PD power = voltage × current, for example 20 V × 5 A = 100 W and 48 V × 5 A = 240 W

The principle

Data rates are signalling rates on the wire. Part of every link is spent on encoding and protocol overhead: USB 3.2 Gen 1 uses 8b/10b encoding, so only 4 of its 5 Gbps carry data, while newer modes use the far more efficient 128b/132b scheme. Faster modes also need better cables and, for 20 Gbps and above, the USB-C connector. Power is separate from data. Without PD, a USB 2.0 port supplies 5 V at 0.5 A (2.5 W), a USB 3.x port 5 V at 0.9 A (4.5 W), and a USB-C port up to 5 V at 3 A (15 W). With PD, the charger and device negotiate a higher voltage: Standard Power Range (SPR) reaches 20 V and 100 W, and Extended Power Range (EPR, PD 3.1) adds 28, 36 and 48 V for up to 240 W. Currents above 3 A require an electronically marked 5 A cable.

Worked example

Default selection: USB4 (v1).

1. Marketing name: USB4 40Gbps; maximum data rate 40 Gbps.
2. In bytes: 40 ÷ 8 = 5 GB/s theoretical.
3. A 50 GB video folder would take at best 50 × 8 ÷ 40 = 10 seconds; real drives and overhead make it noticeably slower.
4. Default power: 15 W; PD required. A USB-C port gives up to 5 V × 3 A = 15 W without PD; USB4 ports must support PD, which can raise charging to 100 W (SPR) or 240 W (48 V × 5 A, EPR) if charger, device and cable all support it.

How to read the result

Your actual speed is limited by the slowest link: port, cable, hub, enclosure and the storage itself. A 40 Gbps port with a 480 Mbps charging cable runs at USB 2.0 speed. For charging, the wattage printed on the charger is only reached when the device requests it and the cable is rated for it.

Common mistakes

• Assuming every USB-C cable supports high speed; many are USB 2.0 only.
• Confusing Gbps (gigabits) with GB/s (gigabytes); divide by 8.
• Expecting 240 W from any USB4 port; EPR is optional and needs matching devices.

Frequently asked questions

How fast is USB 3.2 Gen 2?

10 Gbps signalling rate, also marketed as USB 10Gbps.

Is Thunderbolt 4 the same as USB4?

Both run at 40 Gbps over USB-C and are largely compatible, but Thunderbolt 4 requires a stricter set of features.

What is the maximum USB-C charging power?

240 W with USB PD 3.1 EPR and a cable rated for 240 W.

Sources: USB Implementers Forum (USB-IF), Universal Serial Bus 3.2 Specification · USB-IF, USB4 Specification (versions 1.0 and 2.0) · USB-IF, USB Power Delivery Specification, Revision 3.1