How to Check If Your PC Parts Are Compatible (UK Guide 2026)

Building your own PC is one of the most satisfying things you can do in tech — but the fear of buying parts that don't work together stops a lot of first-time builders before they even start. Will this CPU fit that motherboard? Is my RAM the right type? Will my GPU physically fit in the case? This guide walks you through every compatibility check you need to make, step by step, so you can buy with confidence.

Whether you're building from new parts, sourcing used components from the UK second-hand market, or mixing both, the compatibility rules are the same. Get these checks right and everything will slot together without drama. Get them wrong and you're looking at return postage, wasted time, and possibly damaged parts.

This guide is written for the UK market in 2026 and covers every current platform — AMD AM4, AM5, Intel LGA 1700, and LGA 1851. We'll go through each component in the order you should choose them, explain exactly what to check, and show you the quickest way to verify everything lines up.

Step 1: Start with Your CPU

Your CPU choice is the single most important compatibility decision because it locks you into a specific platform. Every other component choice flows from this one. The CPU determines your socket type, which determines your motherboard, which determines your RAM type, which influences everything else.

In 2026, here are the main platforms available in the UK:

The key thing to note: you cannot mix sockets. An AM5 CPU will not fit an AM4 motherboard. An LGA 1700 chip will not fit an LGA 1851 board. The socket is a hard physical constraint — there is no adapter, no workaround.

If you're building on a budget with used parts, AM4 is currently the sweet spot in the UK. The used CPU market is flooded with excellent Ryzen 5000 chips at very low prices, and B550 motherboards are abundant. If you're building new for longevity, AM5 gives you a platform with years of support ahead.

Step 2: Match Your Motherboard

Your motherboard must match your CPU's socket. But socket compatibility alone isn't enough — you also need to check the chipset and form factor. Here's what matters:

Socket compatibility

This is binary: it either matches or it doesn't. An AMD Ryzen 7 7800X3D requires an AM5 motherboard. An Intel Core i5-12400F requires an LGA 1700 board. No exceptions.

Chipset

Within each socket, there are multiple chipsets at different price points. The chipset determines features like the number of USB ports, PCIe lanes, overclocking support, and built-in Wi-Fi. For most builders:

• AMD AM5: B650 is the sweet spot. X670/X670E for enthusiasts who need more USB/PCIe lanes.
• AMD AM4: B550 is ideal. B450 works but may need a BIOS update for Ryzen 5000 — check the box for a "Ryzen 5000 Ready" sticker or ask the seller.
• Intel LGA 1700: B660/B760 for most people. Z690/Z790 only if you're overclocking a K-series CPU.
• Intel LGA 1851: B860 for mainstream builds. Z890 for overclocking.

Form factor

Motherboards come in three main sizes: ATX, Micro-ATX (mATX), and Mini-ITX. Your motherboard form factor must be supported by your case. ATX is the most common and gives you the most expansion slots. mATX is smaller but still has 2–3 PCIe slots. Mini-ITX is compact and typically has just one PCIe slot.

A smaller motherboard will fit in a larger case (an mATX board works in an ATX case), but not the other way around. An ATX motherboard will not fit in a Mini-ITX case.

BIOS version

This catches a lot of first-time builders. Even if the socket is correct, some motherboards need a BIOS update to support newer CPUs. The most common example in 2026: buying a B450 board with a Ryzen 5 5600 and discovering the board shipped with a BIOS that only supports up to Ryzen 3000 series.

How to check: visit the motherboard manufacturer's website, find your exact board model, go to the CPU compatibility or support page, and check which BIOS version is needed for your CPU. If you're buying used, ask the seller what BIOS version is installed.

Step 3: Check Your RAM Type

RAM compatibility is where a lot of first-time builders trip up because it's not always obvious. The two critical factors are the DDR generation and the speed your motherboard supports.

DDR4 vs DDR5

DDR4 and DDR5 are physically different — the notch on the stick is in a different position, so you literally cannot plug DDR5 into a DDR4 slot or vice versa. This is a hard physical incompatibility.

• AM4: DDR4 only
• AM5: DDR5 only
• LGA 1851: DDR5 only
• LGA 1700: Depends on the motherboard — some boards are DDR4, some are DDR5. Check the motherboard specs carefully. This is easily the most common RAM compatibility mistake people make in 2026.

Speed and capacity

Your motherboard has a maximum supported RAM speed and total capacity. For gaming in 2026:

• DDR4: 16GB (2×8GB) at 3200–3600MHz is the sweet spot. 32GB if you multitask heavily.
• DDR5: 32GB (2×16GB) at 5600–6000MHz is mainstream. DDR5 kits below 5200MHz aren't worth buying in 2026.

Number of sticks and channels

Always use two sticks in dual-channel mode (matching slots, usually A2 and B2 — check your motherboard manual). A single stick of RAM runs in single-channel mode and halves your memory bandwidth, which noticeably hurts gaming performance, especially on AMD Ryzen CPUs.

Step 4: GPU — Will It Fit?

Graphics cards are the easiest component to get right electrically — virtually every modern GPU uses PCIe x16 and every motherboard has at least one PCIe x16 slot. The real compatibility issues with GPUs are physical.

PCIe slot compatibility

All modern GPUs use a PCIe x16 slot. PCIe is backwards and forwards compatible: a PCIe 4.0 card works fine in a PCIe 3.0 slot (just at 3.0 speeds), and a PCIe 3.0 card works in a 5.0 slot. You do not need to match PCIe generations.

Physical length

This is where things go wrong. Modern GPUs — especially high-end cards like the RTX 4080/4090 — can be 330–350mm long. Many cases, particularly budget mid-towers and small form factor builds, have limited GPU clearance. Before buying:

1. Check the GPU's length in its spec sheet (measured in mm).
2. Check your case's maximum GPU clearance (also in mm, listed in the case specs).
3. Leave at least 10mm of breathing room — some cases have cables or fan mounts that eat into the stated clearance.

Thickness (slot width)

Many modern GPUs are 2.5-slot or even 3.5-slot designs. This matters if you're using a small case or plan to use the PCIe slots below the GPU for other cards (like a sound card or capture card). Check the GPU's slot width in its specs.

Power connectors

Your GPU needs power cables from the PSU. Most mid-range GPUs need one or two 8-pin (6+2) PCIe power connectors. High-end cards may need three, or use the newer 12VHPWR / 12V-2x6 connector. Check what your GPU needs and confirm your PSU has matching cables.

If you're sourcing a used GPU in the UK, check that the seller includes any adapter cables that came with the card originally.

Step 5: PSU — Power Delivery

Your power supply unit needs to deliver enough wattage for your entire system and have the right cables for every component. This is the part that protects everything else — a bad PSU choice can mean instability, shutdowns, or in the worst case, damaged components.

Calculating wattage

Add up the TDP (thermal design power) of your CPU and GPU, then add roughly 100–150W for everything else (motherboard, RAM, storage, fans). Then add a 20% headroom buffer. Here are rough guidelines for 2026 UK builds:

80 PLUS rating

The 80 PLUS certification tells you how efficiently the PSU converts AC mains power to DC. For UK builds, look for at least 80+ Bronze. Gold is the sweet spot for price-to-efficiency. Platinum and Titanium exist but the premium rarely pays for itself on UK electricity rates.

Modular vs non-modular

Modular PSUs let you use only the cables you need, making cable management much easier. Semi-modular is a good middle ground: the essential cables (24-pin motherboard, 8-pin CPU) are fixed, and everything else is detachable.

Cable compatibility warning

Never mix PSU cables between different PSU models or brands. Modular PSU cables are not standardised — the pinouts at the PSU end vary between manufacturers. Using the wrong cables can short-circuit and destroy components. If you buy a used modular PSU, make sure it comes with its original cables.

Step 6: Case — Form Factor and Clearances

Your case is the container everything fits into, so it needs to support your motherboard size and have enough room for your GPU, CPU cooler, and PSU. These are the clearance numbers you need to check:

A common first-time builder mistake in the UK: buying a compact case because it looks great on Scan or Overclockers, then discovering the RTX 4070 Ti is 10mm too long for it. Always check the numbers before ordering.

Step 7: CPU Cooler

Your CPU cooler needs to be compatible with your CPU's socket and physically fit inside your case. There are two types: air coolers (tower heatsinks with fans) and AIO liquid coolers (radiator + pump unit). Both work well — the choice depends on your budget, case size, and noise preference.

Socket compatibility

Every cooler lists which CPU sockets it supports. Most aftermarket coolers from major brands (Noctua, be quiet!, Arctic, Thermalright, DeepCool) include mounting hardware for multiple sockets. But you must verify. Key things to check:

• AM5: Uses the same mounting holes as AM4. Most AM4-compatible coolers work with AM5 with little or no adapter.
• LGA 1700 / LGA 1851: Requires specific LGA 1700 mounting hardware. Older coolers designed for LGA 1200/1151 will not fit without an adapter bracket (many manufacturers offer free upgrade kits).

If you're buying a used cooler, always confirm the seller has the correct mounting hardware for your socket. Missing brackets are the number one problem with used cooler purchases.

Height clearance (air coolers)

Tower coolers can be surprisingly tall. The popular Noctua NH-D15 is 165mm — check your case's maximum CPU cooler height and make sure you have room. Budget coolers like the Thermalright Peerless Assassin 120 are 155mm, which fits most mid-tower cases comfortably.

Radiator size (AIO coolers)

If you're using an all-in-one liquid cooler, your case needs to have a compatible radiator mounting position. Check for 240mm, 280mm, or 360mm radiator support at the top, front, or side of your case. Also check tube length — some case/radiator combinations require awkward tube routing.

RAM clearance

Large tower coolers can overhang the first RAM slot on some motherboards. If you have tall RAM with decorative heatspreaders, check whether your cooler's fan position conflicts. Low-profile RAM kits avoid this issue entirely.

Step 8: Storage — M.2 vs SATA

Storage compatibility is straightforward once you understand the two main interfaces: M.2 NVMe and SATA. Both connect to your motherboard, but they're different physically and in speed.

M.2 NVMe drives

M.2 NVMe is the standard for primary drives in 2026. These are small cards that plug directly into M.2 slots on your motherboard. There are different NVMe generations:

• Gen 3 NVMe: Up to ~3,500 MB/s. Still perfectly fast for gaming and general use. Cheap and abundant used.
• Gen 4 NVMe: Up to ~7,000 MB/s. The current sweet spot for price vs performance.
• Gen 5 NVMe: Up to ~12,000 MB/s. Only supported on newer boards (B650E, X670E, Z790, B860, Z890). Overkill for gaming — diminishing returns beyond Gen 4 for most UK users.

Your motherboard's M.2 slots have specific supported generations. A Gen 4 drive in a Gen 3 slot will work but at Gen 3 speeds. Check how many M.2 slots your motherboard has — budget boards may have just one; mid-range boards typically have two or three.

M.2 physical sizes

The vast majority of consumer NVMe drives are 2280 (22mm wide, 80mm long). Some boards support 2230 or 2242 sizes too. Unless you're building in a very small form factor, 2280 is what you want.

SATA drives

SATA SSDs (2.5") and SATA HDDs (3.5") connect via SATA cables to your motherboard's SATA ports. These are fine for secondary storage but slower than NVMe. One thing to watch: on some motherboards, using certain M.2 slots disables specific SATA ports. Your motherboard manual will tell you which ones.

The Easy Way: Use Koukan's Free Compatibility Checker

If all of that feels like a lot to keep track of — it is. That's why we built the Koukan PC Builder. It's a free tool that automatically checks compatibility as you select components. Pick a CPU, and it only shows you motherboards with the right socket. Pick a motherboard, and it filters RAM to the correct DDR type. It catches the mistakes before you make them.

It also pulls live prices from UK sellers so you can compare costs as you build your parts list. If you want to source used parts, you can browse Koukan's verified listings from UK sellers with buyer protection.

Common Compatibility Mistakes and How to Avoid Them

After years of helping UK builders on Koukan, these are the mistakes we see over and over again. Every one of them is avoidable with a 30-second check.

Most of these mistakes happen when people rush the buying process or trust a product listing description instead of checking the manufacturer's official spec page. Take five extra minutes to cross-reference specs and you'll avoid all of them. Or use the Koukan PC Builder and let it catch them for you.

Quick Compatibility Checklist

Before you order anything, run through this checklist. If every line checks out, your parts will work together.

If you're building with used parts, our guide to buying used PC parts in the UK covers the additional checks you should make for each component type — condition, testing, and what to avoid. For a deeper dive into how compatibility works across the whole system, read our complete PC parts compatibility guide for UK builders.

FAQ: PC Parts Compatibility

Further Reading

• PC Parts Compatibility Guide for UK Builders — deeper technical reference covering edge cases and niche platforms.
• Build a Cheap Gaming PC with Used Parts (UK 2026) — three complete builds from £200 to £500 with full parts lists.
• How to Buy Used PC Parts Safely in the UK — what to check, where to buy, and how to protect yourself.
• Buying Used CPUs Online in the UK — CPU-specific buying advice, testing, and what to avoid.
• Buying Used GPUs in the UK — GPU-specific checks, benchmarks to request, and pricing.