Can PCIe X4 Card Fit in X16 Slot? – Can it Work?

Simply put, a PCIe x4 card CAN fit in a PCIe x16 slot. The main reason this is possible lies within the fact that the PCIe standard is cross-compatible as well as backward-compatible.

Meaning PCIe devices can work in different slots as long as they are provided with the required PCIe lanes. They can also work across different generations, provided that they get enough bandwidth.

So Can a PCIe x4 Card Fit in an x16 slot? Yes, it can. However, we explore further in this article whether this is a good idea or not.

The critical point to note here is that the PCIe device and slot work at the speeds of the slowest component. So putting the x4 device in an x16 slot will not result in any performance boost.

Let’s explore this topic a bit in depth below.

What are PCIe x4 and x16 Slots?

 

Image: NVMe SSD Expansion Cards Require x4 PCIe Lanes

Before diving deeper into the subject, let’s get acquainted with the two PCIe configurations we will discuss.

For starters, different PCIe cards have additional PCIe lane requirements. An NVMe SSD Expansion Card, for instance, requires four PCIe lanes. The more demanding a card and the more data it generates, the more PCIe lanes it will require.

A PCIe x4 slot comes with 4 PCIe lanes. This means the slot has four channels through which data is transmitted between the PCIe device and the computer’s processor.

PCIe lanes are scalable. If a single PCIe lane has speeds of 1.0 GB/s, 4 PCIe lanes will have four times the speed.

An actual x16 slot is the largest PCIe slot on most motherboards in terms of connectors and the number of lanes it has to transport the data.

It has quadruple lanes compared to an x4 slot and is ideal for demanding expansion cards like graphics cards.

Also Read:

• What are PCIe Lanes? – Beginners Guide
• What are PCIe Slots? – Beginners Guide

x4 Slots Look the Same as x16 slots

In most cases, on modern motherboards, an x4 slot has the physical look of an x16 slot.

 

Image: Gigabyte GA-P67A-UD3 has two x16 slots (Blue), but the bottom one only has four lanes. Hence it is an x4 slot in terms of performance.

Gigabyte GA-P67A-UD3 above has an x16 slot but is connected only to 4 PCIe lanes.

Hence, when looking at the PCIe slots, particularly the x16 slots, you must consult your motherboard specifications to learn about its actual PCIe lane count.

Also Read: Does it Matter Which PCIe x16 Slot I Use?

PCIe Version Matters

Another critical point to note is that every new generation of PCIe doubles the lanes’ throughput rate (speed).

Version	x1 (GB/s)	x2 (GB/s)	x4 (GB/s)	x8 (GB/s)	x16 (GB/s)
1.0	0.250	0.500	1.000	2.000	4.000
2.0	0.500	1.000	2.000	4.000	8.000
3.0	0.985	1.969	3.938	7.877	15.754
4.0	1.969	3.938	7.877	15.754	31.508
5.0	3.938	7.877	15.754	31.508	63.015
6.0	7.877	15.754	31.508	63.015	126.031

The most current PCIe generation available in the market is PCIe v4.0. This generation promises 1.969 GB/s transfer speed per lane.

Therefore, an x4 slot here has a throughput rate of 7.877 GB/s, and an actual x16 slot has a rate of 31.508 GB/s.

This is important to note here because if you were to install a PCIe 3.0 device in a PCIe 2.0 slot, it would only conform to the speeds of PCIe 2.0.

Also Read: PCIe 3.0 vs. 2.0

So Can a PCIe x4 Card Fit and Work in an x16 Slot?

Although the short answer to the question is yes, there’s a lot to the compatibility between different cards and slot sizes.

Smaller x4 cards WILL fit in larger x16 slots because the slots have the required connections and the size to accommodate the cards.

This is possible because the PCIe interface is Cross and Backward Compatible. Meaning devices can work across different slots as well as across different generations.

Therefore, if you have a PCIe v3.0 device, you can install it in PCIe v4.0 slots and vice versa.

Similarly, if you have a PCIe x4 card, you can fit it on an x16 slot. The reverse is also possible. For instance, you can put an AMD graphics card that requires 16 lanes, ideally, on an x4 slot; however, the graphics card’s performance will be significantly reduced.

Should You Plug a PCIe x4 Card in an x16 Slot?

No, in the ideal case, this arrangement must be avoided if possible because of the following reasons:

1. You Have Limited True x16 Slots – Don’t Waste Them

 

Image: Top PCIe x16 has 16 lanes. Use it for graphics cards or more demanding devices.

As mentioned earlier, sometimes, x4 slots are disguised as x16 slots. A slot may have the full x16 size but only have 4 PCIe lanes connected.

So there is a difference between TRUE x16 slots with full 16 lanes and x16(x4) slots.

Genuine x16 slots are minimal. A commercial motherboard only has a SINGLE proper x16 slot, mainly intended for the graphics card.

As a rule of thumb, if your motherboard has two or more x16 slots, the top one is always the TRUE x16 slot. Hence, if you have a high-speed device like a graphics card, always occupy it on the top x16 slot.

The bottom x16 slot can install x4 cards like M.2 NVMe expansion card, 4k video capture card, etc.

The point to note here is that while you can still put the x4 card in the actual x16 slot, you will have wasted the latter’s potential.

2. You Don’t Get Any Performance Boost

Putting an x4 card in an x16 slot does not give any performance boost to the installed card.

Again, you waste the potential of the larger slot here.

PCIe cards and slots operate on the throughput rate of the weakest of the two. This applies to both lane count as well as the PCIe version.

So if you install an x4 PCIe v2.0 device on an x16 PCIe v3.0 slot, the more powerful slot would clock down to 4 lanes and V2.0 speeds.

Exceptional Cases

Of course, there are exceptional cases where this arrangement of putting an x4 device in an x16 slot may be warranted.

For instance, if you want to add an extra NVME SSD expansion card but you have no x4 slot on your motherboard, then an x16 slot would be your only option to go with.

This would mean sacrificing your graphics card, but you wouldn’t need a graphics card if you are not a professional or a gamer.

Also Read:

• Can You Plug A PCIe X1 Into an X16 Slot
• Can PCIe X1 Card Fit in X4 Slot?

Final Words

In this article, we have looked at whether a PCIe X4 card fits in an X16 slot and found it possible. The card will fit and work well in the slot without much hassle.

However, it is not ideal to waste the potential of the x16 slot on a mere x4 device. Because the x4 card has only four data lanes, we find that the full link will work at the speed of the x4 card and not at the speed of the x16 slot.

While you don’t get an increase in bandwidth, you lose the chance to use some of the available lanes, which can cost you somewhere along the road.

The only workaround is to be mindful of your requirements and what devices you want to install when procuring a motherboard.

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