What Are PCI Express Slots Used For? Differences Between x1 to x16 and Practical Examples [Custom PC]

PCI Express is an important standard that enables high-speed data transfer inside a computer.

It is widely used especially to connect graphics cards, NVMe SSDs, and other expansion cards.

However, choosing parts without understanding PCI Express standards and lane counts can lead to cases where a card cannot be inserted physically, or performance cannot be maximized due to version differences.

This article explains the basic role and versions of PCI Express, use cases by lane count, and also introduces points to note when selecting expansion cards.

Standards for motherboards, names of each part, and selection from performance and compatibility viewpoints are also explained.

≫ Related article: How to Choose a Motherboard for a Custom PC [Performance / Features / Compatibility]

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About PCI Express

Let’s cover the basic role of PCI Express.

What is PCI Express?

PCI Express (Peripheral Component Interconnect Express, abbreviated PCIe) is an interface standard, or bus standard, that enables high-speed data transfer.

It is mainly mounted on motherboards and is used to connect various expansion cards such as graphics cards, NVMe SSDs, and network cards.

When PCIe is mentioned, the first thing that comes to mind is the PCI Express slot on a motherboard.

This slot is for connecting various expansion cards including graphics cards.

But PCIe is also used in the M.2 slot that mounts NVMe SSDs.

It may seem strange that “PCIe slots” are not the only places where PCIe is used.

It is a bit confusing, but even though PCIe slots and M.2 slots have different shapes, both use the PCIe bus to exchange data.

In other words, regardless of slot shape, the PCIe standard is a communication standard that provides the same high-speed data transfer.

Difference between interface standards and bus standards

An interface standard defines the “connection method” and “communication protocol” used when parts and devices communicate, including data format, speed, connector shape, and communication procedures.

USB, HDMI, and SATA are examples.

PCIe is also an interface standard, characterized by serial connections between expansion cards that allow high-speed communication.

A bus standard defines the “communication path (bus)” that allows multiple internal parts and devices to share data and communicate inside a computer.

A bus is a common pathway for exchanging data and signals; the CPU, memory, storage, and expansion cards exchange data via this bus.

Examples include:

• System bus: bus for data communication between CPU and memory
• Data bus: bus for data exchange between parts
• Address bus: bus that carries addresses to access memory and I/O devices
• PCIe: high-speed serial bus connecting expansion cards and the motherboard

PCIe is also a bus standard that performs high-speed serial data transfer between expansion cards and devices.

Therefore, even with different slot shapes such as PCIe slots and M.2 slots, the same PCIe bus can be used to connect devices.

Flexibility of PCIe

PCIe is a flexible standard that does not depend on shape or slot.

A PCIe slot functions as an interface standard to physically connect devices, and an M.2 slot uses the same bus standard to connect devices.

Thanks to this flexibility, it supports not only expansion cards but also various devices such as NVMe SSDs.

In short, PCIe enables high-speed data transfer regardless of slot shape, and even with different slot types, the same technology is used.

About other expansion cards

There are many kinds of expansion cards that fit PCIe slots besides graphics cards.

For example, here are typical PCIe expansion cards:

A card that benefits most users is the Wi-Fi/Bluetooth card.

Laptops include Wi-Fi/Bluetooth by default, while desktop PCs are typically wired by default.

That means a wired LAN must be run from the router, and wired mice and keyboards can leave cables on the desk.

A Wi-Fi/Bluetooth card solves these issues and is highly recommended.

Features of each expansion card are explained in detail here:

≫ Related article: How to Choose Expansion Cards for a Custom PC [Types / Compatibility]

≫ Related article: PC Parts to Add Wi-Fi/Bluetooth to a Custom PC

About PCI Express Lane Counts

This section explains lane counts, the features and uses of each configuration, and compatibility.

PCI Express lane counts (use cases by lane count)

PCIe performance is mainly determined by lane count and bandwidth.

A lane is an independent pair of signal lines for sending and receiving data; one lane consists of one transmit pair and one receive pair.

Typical lane counts are x1, x4, x8, and x16. More lanes provide higher data transfer speed.

For example, a PCIe x16 slot provides 16 times the bandwidth of x1.

Bandwidth is the amount of data transferred per second and varies by PCIe version.

Specifications by version are described later.

For example, the latest PCIe 5.0 provides about 32 GT/s (gigatransfers per second) per lane, and an x16 link provides a theoretical maximum bandwidth of about 512 GB/s.

This allows high-performance devices such as graphics cards and SSDs to process data faster.

Also, the physical size of PCIe slots differs by lane count.

Therefore, when installing a graphics card or another expansion card, make sure it matches the size of the motherboard’s PCIe slot.

Motherboards have x1, x4, x8, and x16 PCIe slots, but x16 and x1 are most commonly implemented.

PCIe x16 slot

The PCIe x16 slot has the largest bandwidth and supports up to 16 lanes of data transfer.

It is most commonly used for connecting high-performance graphics cards.

Graphics cards require high bandwidth because they process large amounts of data for gaming, 3D rendering, video editing, and AI training.

GPU accelerators and computers for AI/machine learning (ML) also make heavy use of x16 slots.

PCIe x8 slot

A PCIe x8 slot is used mainly for the second graphics card in dual-GPU setups (such as SLI or CrossFire), and for high-performance network cards (10GbE, 40GbE) and storage controllers.

In servers and high-performance workstations with GPU accelerators or multiple NVMe SSDs, x8 slots are frequently used.

As a physical slot shape, x8 is sometimes implemented on server motherboards, but it is rarely seen on consumer motherboards.

However, it is common to see a slot shaped like PCIe x16 but electrically recognized as PCIe x8.

PCIe x4 slot

PCIe x4 is most often used for M.2 slots that host NVMe SSDs.

NVMe SSDs are far faster than SATA SSDs, and x4 bandwidth allows their full performance.

It is also used for storage controllers and other expansion cards (such as RAID controllers), but motherboards that implement a physical PCIe x4 slot are uncommon.

PCIe x1 slot

The PCIe x1 slot is the smallest slot and provides one lane of data transfer.

It is used for expansion cards that need little bandwidth, such as network cards (Gigabit Ethernet), sound cards, USB expansion cards, and Wi-Fi/Bluetooth cards.

Compatibility of PCI Express slot sizes

PCIe slot sizes are intercompatible, so a smaller card can be installed in a larger slot.

For example, a PCIe x16 slot can accept x8, x4, or x1 expansion cards.

Conversely, a larger card cannot be installed in a smaller slot because the physical size does not match.

Therefore, when installing many expansion cards, check PCIe slot availability considering this compatibility.

Physical size vs recognized lanes for PCI Express slots

In some cases, a PCIe slot’s physical lane size and its recognized lane count differ.

A common case is a slot that has the physical shape of x16 but is recognized as x8.

Even if the slot is physically x16, the motherboard design may wire only x8 lanes.

In that case, although the slot looks like x16, the device operates with x8 bandwidth.

However, the first PCIe x16 slot is almost always wired and recognized as x16 because it is intended for a graphics card.

For the second and later PCIe x16 slots, it is common to have an x16-shaped slot recognized as x8.

About PCI Express Versions

Let’s look at PCIe versions.

PCIe is backward compatible, so in principle different versions still work, but in some cases, parts must be chosen with version in mind to avoid limiting maximum performance.

Specifications by PCI Express version

First, check specifications and characteristics by version.

The recent mainstream is PCIe 4.0 and 5.0. Motherboard PCIe slots and M.2 slots often support these two, and graphics cards and NVMe SSDs commonly support them as well.

PCIe 6.0 has been released too, but it is used for industrial areas such as data centers, HPC (supercomputers and research), AI and machine learning, telecom infrastructure, autonomous driving and ADAS, and cloud.

PCIe 1.0

PCIe 1.0, introduced in 2003, was designed as a new interface standard to replace legacy PCI and AGP (Accelerated Graphics Port).

It dropped the parallel bus and adopted serial transmission to achieve higher bandwidth and transfer speed.

PCIe 1.0 offers 250 MB/s per lane, and up to 4 GB/s using an x16 slot.

It helped improve graphics and other expansion card performance and laid the foundation for later versions.

PCIe 2.0

Released in 2007, PCIe 2.0 doubled the transfer speed of PCIe 1.0, offering 500 MB/s per lane.

An x16 slot provides up to 8 GB/s of bandwidth, enabling connections to higher-performance devices.

PCIe 2.0 kept backward compatibility, so PCIe 1.0 devices could be used in the same slots.

This version was used especially for graphics cards and fast storage to handle more data efficiently.

PCIe 3.0

PCIe 3.0, released in 2010, brought a major performance increase to 1 GB/s per lane and up to 16 GB/s for x16 slots.

A major change was the encoding scheme: 128b/130b replaced 8b/10b, reducing overhead.

This improved transfer efficiency and optimized bandwidth use.

PCIe 4.0

Introduced in 2017, PCIe 4.0 doubled PCIe 3.0’s transfer speed.

Per-lane speed reached 2 GB/s, and an x16 slot provides up to 32 GB/s.

This supports newer graphics cards and high-performance NVMe SSDs that process even more data.

It is widely used in gaming PCs and professional workstations to remove data transfer bottlenecks and remains common today.

PCIe 5.0

PCIe 5.0 was released in 2019, doubling per-lane speed to 4 GB/s.

An x16 slot provides up to 64 GB/s, enabling unprecedented data transfer rates.

PCIe 5.0 is increasingly being adopted in fields that require large-scale data processing, such as artificial intelligence (AI), machine learning (ML), and high-performance computing (HPC).

In systems that perform these advanced computational tasks, the high bandwidth of PCIe 5.0 delivers significant benefits, serving as a key technology to eliminate data transfer bottlenecks.

For general consumers, its advantages can be experienced through NVMe SSDs, which enable even faster data transfers compared to PCIe 4.0 SSDs.

This makes PCIe 5.0–based NVMe SSDs a next-generation storage solution, especially in applications that demand high-speed access to large volumes of data, such as gaming and video editing.

PCIe 6.0

PCIe 6.0, released in 2022, greatly advanced the standard.

It adopts PAM4 (4-level pulse amplitude modulation), reaching 8 GB/s per lane and up to 128 GB/s for x16.

PCIe 6.0 is designed for massive, efficient data handling in data centers, cloud computing, and next-gen telecom.

It is ready for future high-performance device needs.

However, it is not yet common for consumers, and parts using this version are not expected on the market.

Therefore, PCIe 4.0 and 5.0 are mainstream for consumer PCs.

Backward compatibility allows operation on older versions

Because PCIe versions are backward compatible, parts and motherboard slots work even if versions do not match.

Backward compatibility in PCIe refers to the ability of newer versions of the PCIe standard to work with components and slots from earlier versions.

This means that older PCIe components can be used on newer motherboards, and conversely, newer PCIe components can still operate when inserted into older motherboards.

Thanks to this property, flexible upgrades and system builds are possible even when parts and motherboards differ in version.

For example, a PCIe 4.0 part can be used in a PCIe 3.0 slot.

Conversely, a PCIe 3.0 part inserted in a 4.0 slot operates as 3.0.

In such cases, the lower version between the part and the slot is used, which may reduce performance.

Check versions when using the latest parts

With PCIe’s backward compatibility, parts basically operate regardless of version.

However, because operation falls back to the lower version, care is needed when a part requires higher version speed to show full performance.

This is especially important for storage, where transfer speed matters.

For example, using a PCIe 5.0 NVMe SSD in a PCIe 4.0 M.2 slot forces the SSD to run at PCIe 4.0 speeds, limiting its peak transfer rate.

PCIe 5.0 NVMe SSDs are about 9,500–12,400 MB/s, while PCIe 4.0 ones are about 3,500–7,500 MB/s.

Thus, even after buying a fast, latest NVMe SSD, a lower-version slot can prevent full speed.

To avoid this, carefully check the PCIe version of both the part and the motherboard slot.

Notes When Choosing Expansion Cards

Here are several notes when selecting expansion cards.

There is some overlap with earlier sections, but the focus here is on selection points.

PCI Express lane count

PCIe lane counts come in four patterns: x1, x4, x8, and x16, each with a different physical slot size.

Choosing the wrong lane count means the part cannot be installed physically.

A smaller-lane card can be installed in a larger slot.

For example, a PCIe x1 expansion card can be installed in a PCIe x16 slot.

Conversely, a larger-lane card cannot fit a smaller slot. Confirm that the motherboard has an appropriate PCIe slot for the expansion card.

Number of occupied slots (pay special attention to GPUs)

Depending on how many slots an expansion card (mainly a GPU) occupies, other motherboard PCIe slots may become unusable.

For example, recent high-performance graphics cards commonly occupy two slots, and some occupy 2.5 or 3 slots.

When multiple slots are occupied, the PCIe slot directly below the graphics card overlaps with it and cannot accept another card.

Therefore, when installing other expansion cards along with a large GPU, check remaining free slots including the slot occupancy.

Most expansion cards other than graphics cards occupy one slot, so in those cases all PCIe slots can usually be used.

When a build includes a graphics card and other expansion cards, consider slot occupancy and check for free slots.

PCI Express version

Because PCI Express is backward compatible, different versions still work.

Therefore, the PCIe version typically needs little attention.

However, for parts where transfer speed matters—such as NVMe SSDs—and for using the latest models, version checks are needed.

There are many NVMe SSDs that support the newest PCIe versions to meet demand for high storage speed.

If a part and a motherboard slot differ in version, operation falls back to the lower version.

Thus, a lower motherboard slot version can limit an NVMe SSD’s top speed. In that case, match versions in advance.

Physical slot size and recognized lanes can differ

Sometimes a PCIe slot’s physical lane size and recognized lane count differ.

A common case is a slot that looks like x16 but is recognized as x8.

However, the first PCIe x16 slot is almost always recognized as x16 because it is intended for a graphics card.

For the second and later PCIe x16 slots, it is common to have x16 shape but x8 recognition.

This is usually written in the motherboard specifications, so do not judge PCIe lanes and slot counts by photos alone; check the specs.

The second PCIe slot is not often used, but it is good to know.

Summary: Check lane counts, occupied slots, and recognized lanes

This article covered PCIe basics, lane counts, versions, and notes for selecting expansion cards.

Here are the key points again.

PCIe is an interface that enables high-speed data transfer inside a computer, and performance differs by lane count and version.

When selecting expansion cards—especially when installing the latest PCIe-version NVMe SSDs—confirm that the motherboard’s M.2 slot supports the same latest PCIe version.

Also, when installing multiple expansion cards including a graphics card, check that enough slots remain, keeping the GPU’s slot occupancy in mind.

Standards for motherboards, names of each part, and selection from performance and compatibility viewpoints are also explained.

≫ Related article: How to Choose a Motherboard for a Custom PC [Performance / Features / Compatibility]

あわせて読みたい

PC Parts Estimation & Compatibility Check Tool

Automatically checks each component you need when selecting parts for a custom-built PC—perfect for beginners.