Choosing the Right PSU Form Factor: ATX, SFX, and More
Introduction
Choosing the right power supply unit (PSU) form factor is one of the most important decisions when building or upgrading a PC. While wattage and efficiency often get the most attention, the physical size and layout of the PSU can significantly affect case compatibility, airflow, cable management, upgrade flexibility, noise levels, and overall build quality. Whether you are assembling a compact small form factor gaming system, a silent workstation, or a high-performance ATX tower, understanding PSU form factors ensures you select a unit that fits your needs without compromise.
This guide explains the most common PSU form factors—ATX, SFX, SFX-L, TFX, Flex ATX, and others—along with their benefits, limitations, and best use cases. By the end, you will have a comprehensive understanding of how to choose the right PSU form factor for your next PC build.
What Is a PSU Form Factor?
A PSU form factor defines the physical dimensions, mounting hole placement, connector locations, airflow pattern, and sometimes internal layout of a power supply. These specifications ensure the PSU fits correctly inside compatible PC cases and aligns with standardized mounting brackets. Choosing an incompatible form factor can prevent installation entirely or cause airflow and cable routing issues.
Although electrical performance such as wattage and efficiency does not directly depend on form factor, smaller PSUs typically have limitations due to reduced space for cooling, circuitry, and protective components. Larger PSUs can house more robust hardware, enabling higher wattages and quieter cooling solutions.
The Most Common PSU Form Factors
1. ATX Power Supplies
The ATX form factor is the most widely used PSU type in consumer and enthusiast desktop builds. ATX PSUs are designed for mid-tower, full-tower, and many micro-ATX cases. Their relatively large size allows them to deliver high wattage and maintain lower fan noise under load.
- Typical dimensions: 150 × 86 × 140–180 mm
- Power output range: 450W to 1600W+
- Best use cases: Gaming PCs, professional workstations, high-performance rigs
Because ATX PSUs come in a wide range of wattages and efficiency levels, they are suitable for nearly every mainstream PC build. They often include fully modular cabling, larger cooling fans, and higher-quality components. If your case supports ATX, it is usually the easiest and most flexible option.
2. SFX Power Supplies
Small Form Factor (SFX) PSUs are designed for compact and mini-ITX cases where space is limited. An SFX PSU is smaller in every dimension than an ATX PSU, making it ideal for ultra-compact gaming builds.
- Typical dimensions: 100 × 125 × 63.5 mm
- Power output range: 300W to 850W+
- Best use cases: Mini-ITX systems, compact gaming PCs, portable LAN rigs
Despite their compact size, SFX PSUs can deliver impressive performance. High-end SFX units often include premium components and can achieve 80 Plus Gold or even Platinum efficiency ratings. However, due to limited space, SFX PSUs usually feature smaller, faster-spinning fans, which can generate more noise.
3. SFX-L Power Supplies
SFX-L power supplies are an extended version of traditional SFX units. They maintain the same height and width but add extra length to allow for more robust cooling hardware, such as 120mm fans instead of 80mm.
- Typical dimensions: 100 × 125 × 100–130 mm
- Power output range: 600W to 1000W+
- Best use cases: High-performance SFF builds needing quieter cooling
SFX-L PSUs strike an excellent balance between compact size and quiet performance. Many small form factor cases are specifically designed to accommodate SFX-L units due to their reduced noise and higher wattage capabilities.
4. TFX Power Supplies
TFX power supplies are long, narrow PSU designs most commonly used in slim desktops, home theater PCs, and low-profile office machines. Their shape allows PC cases to remain very thin while still offering adequate power delivery.
- Typical dimensions: 85 × 65 × 175 mm
- Power output range: 200W to 400W
- Best use cases: Slim desktops, media centers, OEM-style systems
TFX PSUs are not designed for gaming or workstation applications but are ideal for low-wattage systems with limited vertical space. They typically use smaller fans and simpler circuitry due to power restrictions.
5. Flex ATX Power Supplies
Flex ATX PSUs are extremely compact units used in miniature systems such as embedded PCs, networking devices, and highly compact OEM builds. They are among the smallest PSU form factors that still support standardized ATX connectors.
- Typical dimensions: 150 × 81.5 × 40.5 mm
- Power output range: 150W to 300W
- Best use cases: Ultra-compact PCs, embedded systems, networking equipment
While useful for very small systems, Flex ATX PSUs are generally louder due to their tiny, high-RPM fans. They are not intended for consumer desktop builds unless specifically required by a case.
6. Proprietary PSU Form Factors
Some OEM manufacturers, especially those designing business and all-in-one PCs, use proprietary PSU sizes. These power supplies rarely fit standard consumer cases, and replacement parts often must be purchased directly from the original manufacturer.
- Used by: Dell, HP, Lenovo, and other OEMs
- Best use cases: Only for repairing or upgrading OEM systems
If you own an OEM PC, always check PSU dimensions carefully before attempting an upgrade. In many cases, an adapter bracket may allow the use of a standard SFX or ATX PSU, though space limitations may still apply.
Comparison Table: ATX vs. SFX vs. SFX-L vs. TFX
| Form Factor | Standard Dimensions | Typical Wattage | Ideal Use Case |
| ATX | 150 × 86 × 140–180 mm | 450W–1600W+ | Gaming towers, workstations |
| SFX | 100 × 125 × 63.5 mm | 300W–850W+ | Small form factor gaming builds |
| SFX-L | 100 × 125 × 100–130 mm | 600W–1000W+ | Quiet high-performance SFF systems |
| TFX | 85 × 65 × 175 mm | 200W–400W | Slim desktops, HTPCs |
How to Choose the Right PSU Form Factor
1. Check Your Case Compatibility
Your PC case determines which PSU form factors will fit. Always start by reading the case specifications. Many SFF cases support both SFX and SFX-L, while larger ATX cases often allow SFX units with a bracket (sometimes included).
2. Determine Your Power Requirements
A small case does not always mean low power needs. Modern GPUs such as the NVIDIA RTX 4070 Super can be paired with SFX-L PSUs effectively. However, for extreme performance systems with high-core-count CPUs and powerful GPUs, an ATX PSU may be necessary for wattage and thermal headroom.
3. Consider Noise Levels
Smaller PSUs have smaller fans that spin faster. If silence is important, SFX-L or ATX models usually offer quieter cooling. SFX PSUs can still be quiet under light loads, but expect more noise during gaming or heavy workloads.
4. Think About Future Upgrades
A compact PSU limits upgrade flexibility. ATX PSUs offer the best long-term expandability because they support more peripheral connectors, higher wattage, and larger cooling solutions.
5. Check Cable Length and Modularity
Shorter cables are ideal for SFF builds to reduce clutter, while ATX systems benefit from longer cables to reach components across large cases. Modular cables make cable management easier regardless of form factor.
Recommended PSU Options (with Affiliate Links)
- ATX: High-Wattage Modular PSU – View on Store
- SFX: Compact 80 Plus Gold PSU – View on Store
- SFX-L: Quiet Small Form Factor PSU – View on Store
- TFX: Slim Desktop PSU – View on Store
Internal Resource
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Frequently Asked Questions
What PSU form factor do I need for a mini-ITX case?
Most mini-ITX cases use SFX or SFX-L PSUs. Check your case specifications to confirm compatibility.
Can I use an SFX PSU in an ATX case?
Yes. Many ATX cases support SFX PSUs when used with an inexpensive mounting bracket.
Is an SFX PSU powerful enough for gaming?
Absolutely. Modern SFX and SFX-L units can deliver up to 1000W, more than enough for high-performance gaming systems.
Are proprietary PSUs replaceable with standard models?
Usually no, but some OEM PCs support adapters or have cases large enough to accommodate standard SFX or ATX PSUs with modifications.
Does PSU size affect noise levels?
Yes. Larger PSUs generally allow for larger, quieter fans. Smaller PSUs often use smaller fans that spin faster, producing more noise.
