Can I Use an x4 or x8 Card in an x16 PCIe Slot?
You can use PCIe cards of a lower lane width (e.g., x2, x4, or x8) in a higher lane width slot (e.g., x16).
PCI-Express will negotiate how many lanes will be used, and your card will work at its full speed, assuming the PCIe slot has the same or higher PCIe version.
To be thorough, these non-matching PCIe card and slot sizes all work:
- x1 PCIe card in an x2 PCIe slot
- x1 PCIe card in an x4 PCIe slot
- x1 PCIe card in an x8 PCIe slot
- x1 PCIe card in an x16 PCIe slot
- x2 PCIe card in an x4 PCIe slot
- x2 PCIe card in an x8 PCIe slot
- x2 PCIe card in an x16 PCIe slot
- x4 PCIe card in an x8 PCIe slot
- x4 PCIe card in an x16 PCIe slot
- x8 PCIe card in an x16 PCIe slot
What Are PCIe Lanes?
PCIe lanes are connections between a PCI-Express
expansion card or device and the CPU. PCIe lanes often communicate with the CPU via chipsets on the motherboard. Each PCIe lane is composed of 4 wires (two differential pairs).
With naming similar to a road, the number of lanes is referred to as the lane size, or how many lanes wide a link or port uses. A single lane is referred to as x1 or "one lane wide."
The maximum throughput (speed) is multiplied by the number of lanes. An x8 PCIe port has twice the throughput of an x4 port.
Some CPUs and motherboards provide PCI-Express lanes at multiple PCIe versions. In these cases, you can choose which devices need the most bandwidth to decide which should be connected to the highest version PCIe lanes. While laying out your system, keep in mind that PCIe speeds will be based on the lowest PCIe version between the slot/port and the device.
You can insert a PCIe add-in card (AIC) into a slot that supports a higher number of lanes. In this case, it would use up to the number of PCIe lanes that the card has. For example, you could insert an x4 PCIe network card into an x16 PCIe AIC slot, and it would run at full x4 speed.
In various scenarios, a PCIe device may not use the maximum number of lanes for which the device was designed. For example, some motherboards have x8 PCIe ports that are only electrically wired for x4 lanes. In another case, a system may have limited lanes provided by the CPU, distributed based on availability or configuration.
Devices will negotiate the number of lanes to use, based on system availability, and should still perform fine at a reduced overall bandwidth in most cases. Refer to your motherboard's documentation and CPU specifications to determine PCIe lane quantities and allocations.
Do I Need an Adapter to Put a Shorter PCIe Card in a Longer Slot?
No adapter is needed for inserting a shorter PCIe card into a larger PCIe slot. PCI-Express will negotiate how many lanes will be used, and your card will work at its full speed, assuming the PCIe slot has the same or higher PCIe version.
Are PCIe Versions Backward Compatible?
PCI-Express versions are backward compatible, meaning that you can use a PCIe 4.0 graphics card or storage device on a PCIe 3.0 or PCIe 2.0 system. However, PCI-Express will use speeds based on the lowest of the two versions for communication. For example, if you use a PCIe 3.0 NVMe SSD with a PCIe 4.0 system, the SSD would be running at PCIe 3.0 speeds.
Reducing the PCIe version would halve the max potential bandwidth. However, you may not experience a performance impact if your device is not using the available bandwidth.
You can refer to the following table to determine the bandwidth limit and decide whether that is adequate for your needs.
| x1 Bandwidth | x2 Bandwidth | x4 Bandwidth | x8 Bandwidth | x16 Bandwidth | |
|---|---|---|---|---|---|
| PCIe 1.0 | 250 MB/s | 500 MB/s | 750 MB/s | 2 GB/s | 4 GB/s |
| PCIe 2.0 | 500 MB/s | 1000 MB/s | 2 GB/s | 4 GB/s | 8 GB/s |
| PCIe 3.0 | 1 GB/s | 2 GB/s | 4 GB/s | 8 GB/s | 16 GB/s |
| PCIe 4.0 | 2 GB/s | 4 GB/s | 8 GB/s | 16 GB/s | 32 GB/s |
| PCIe 5.0 | 4 GB/s | 8 GB/s | 16 GB/s | 32 GB/s | 63 GB/s |
| PCIe 6.0 | 8 GB/s | 16 GB/s | 32 GB/s | 63 GB/s | 126 GB/s |
Learn more about PCIe's backward compatibility in Is PCIe Backward Compatible?.
Other Considerations When Building a PC
Want to brush up on other new technologies to consider when building a computer? Check out these articles:
- Cases:
- CPUs:
- Which Intel and AMD CPUs Support PCIe 5.0?
- Which Intel and AMD CPUs Support PCIe 4.0?
- LGA 1700 CPU List
- LGA 1200 CPU List
- Look up an Intel or AMD CPU on TechReviewer for related recommendations:
- Which Intel and AMD CPUs Support PCIe 5.0?
- CPU Coolers:
- Storage:
- Memory:
- PCI-Express:
- Motherboards:
- Graphics Cards:
- Power Supplies:
- Keyboards:
- Monitors:
Learn More About PCI-Express
Want to brush up on the latest PCIe products, versions, and features? Check out the articles in this PCI-Express series:
- What is PCIe?, What is PCIe 5.0?, and What is PCIe 4.0?
- Is PCIe 5.0 Worth It? and Is PCIe 4.0 Worth It?
- Which Intel and AMD CPUs Support PCIe 5.0?
- Which Intel and AMD CPUs Support PCIe 4.0?
- Which Motherboards Support PCIe 5.0?
- Which Graphics Cards Support PCIe 4.0?
- How Fast is PCIe 5.0? and How Fast is PCIe 4.0?
- Is PCIe Backward Compatible?
- Can I Put a PCIe x4 or x8 Card in an x16 Slot?
- Can I Use a PCIe 4.0 NVMe SSD in a PCIe 3.0 Motherboard Slot?
- Can I Use a PCIe 3.0 NVMe SSD in a PCIe 4.0 Motherboard Slot?
- Can I Use a PCIe 4.0 Graphics Card in a PCIe 3.0 Slot?
- Can I Use a PCIe 3.0 Graphics Card in a PCIe 4.0 Slot?
- What is the Latest Version of PCIe?
- PCI vs. AGP vs. PCIe? Time to Upgrade!