YOLO Architecture Explained: From YOLOv3 to YOLO26

Every Ultralytics YOLO model is built from three stages: a backbone that extracts features, a neck that fuses them across scales, and a head that predicts boxes and classes. This guide documents the modules that make up each stage and how they changed from YOLOv3 to YOLO26, tracing every component to its definition in the configuration files under ultralytics/cfg/models/ and the module classes in ultralytics/nn/modules/.

Each model is defined declaratively in a YAML file as an ordered list of layers, where every layer follows the [from, repeats, module, args] format: which layer(s) feed it, how many times the module repeats, the layer class (Conv, C3k2, SPPF, Detect, …), and its constructor arguments. The Model YAML Configuration Guide documents this format — including how repeats and args scale with the variant's depth and width multiples — along with the module-resolution system in full. This guide focuses on the modules themselves and how they changed from version to version.

The Three Stages

Every Ultralytics YOLO model routes the image through three sequential stages, each with a distinct job:

The fundamental unit is the Conv block (defined in conv.py): a 2D convolution, batch normalization, and a SiLU activation, applied in sequence. Every larger module below is built by composing Conv blocks.

Architecture Diagrams

Each version keeps the same backbone → neck → head skeleton and changes specific stages. The tabs below show the per-version structure: the backbone and neck stages follow the configs in ultralytics/cfg/models/, while the YOLOv3 and YOLOv5 heads are drawn in their original anchor-based form rather than the anchor-free u-variant head their package configs actually ship. Stepping through the tabs shows what each generation added. In short, the progression is: YOLOv3 is an FPN-only, anchor-based detector; YOLOv5 adds the bottom-up PAN path and SPPF; YOLOv8 switches to the C2f block with an anchor-free, DFL head; YOLO11 inserts C2PSA attention and the C3k2 block; and YOLO26 adds an SPPF residual and makes the head NMS-free and DFL-free. Node colors follow the documentation diagram convention: green input, blue backbone, slate spatial pooling and attention, orange neck, purple head and output.

=== "YOLOv3"

```mermaid
flowchart TD
    IN[Input 640x640]:::start --> ST[Conv stem

5x stride-2 down to P1-P5]:::proc ST --> BB[Darknet-53 backbone stacked Bottleneck]:::proc BB --> FPN[Neck FPN only top-down Upsample + Concat]:::decide FPN --> HD[Detect head 3 scales, anchor-based]:::out HD --> O[Predictions + NMS]:::out classDef start fill:#4CAF50,color:#fff classDef proc fill:#2196F3,color:#fff classDef decide fill:#FF9800,color:#fff classDef out fill:#9C27B0,color:#fff ```

=== "YOLOv5"

```mermaid
flowchart TD
    IN[Input 640x640]:::start --> BB[CSP backbone

C3 blocks]:::proc BB --> SP[SPPF]:::extern SP --> FPN[Neck FPN top-down Upsample + Concat]:::decide FPN --> PAN[Neck PAN bottom-up Conv + Concat]:::decide PAN --> HD[Detect head anchor-based]:::out HD --> O[Predictions + NMS]:::out classDef start fill:#4CAF50,color:#fff classDef proc fill:#2196F3,color:#fff classDef decide fill:#FF9800,color:#fff classDef out fill:#9C27B0,color:#fff classDef extern fill:#607D8B,color:#fff ```

=== "YOLOv8"

```mermaid
flowchart TD
    IN[Input 640x640]:::start --> BB[CSP backbone

C2f blocks]:::proc BB --> SP[SPPF]:::extern SP --> FPN[Neck FPN top-down Upsample + Concat]:::decide FPN --> PAN[Neck PAN bottom-up Conv + Concat]:::decide PAN --> HD[Detect head anchor-free, decoupled, DFL reg_max 16]:::out HD --> O[Predictions + NMS]:::out classDef start fill:#4CAF50,color:#fff classDef proc fill:#2196F3,color:#fff classDef decide fill:#FF9800,color:#fff classDef out fill:#9C27B0,color:#fff classDef extern fill:#607D8B,color:#fff ```

=== "YOLO11"

```mermaid
flowchart TD
    IN[Input 640x640]:::start --> BB[CSP backbone

C3k2 blocks]:::proc BB --> SP[SPPF]:::extern SP --> PSA[C2PSA attention]:::extern PSA --> FPN[Neck FPN top-down Upsample + Concat]:::decide FPN --> PAN[Neck PAN bottom-up Conv + Concat]:::decide PAN --> HD[Detect head anchor-free, DFL reg_max 16]:::out HD --> O[Predictions + NMS]:::out classDef start fill:#4CAF50,color:#fff classDef proc fill:#2196F3,color:#fff classDef decide fill:#FF9800,color:#fff classDef out fill:#9C27B0,color:#fff classDef extern fill:#607D8B,color:#fff ```

=== "YOLO26"

```mermaid
flowchart TD
    IN[Input 640x640]:::start --> BB[CSP backbone

C3k2 blocks]:::proc BB --> SP[SPPF + shortcut]:::extern SP --> PSA[C2PSA attention]:::extern PSA --> FPN[Neck FPN top-down Upsample + Concat]:::decide FPN --> PAN[Neck PAN bottom-up Conv + Concat]:::decide PAN --> HD[Detect head anchor-free, reg_max 1, end2end]:::out HD --> O[End-to-end predictions NMS-free]:::out classDef start fill:#4CAF50,color:#fff classDef proc fill:#2196F3,color:#fff classDef decide fill:#FF9800,color:#fff classDef out fill:#9C27B0,color:#fff classDef extern fill:#607D8B,color:#fff ```

The YOLOv3 and YOLOv5 diagrams show the original anchor-based head. The ultralytics package ships the anchor-free YOLOv3u and YOLOv5u configs — the same Darknet-53 and C3 backbones with YOLOv8's Detect head — described under Detection Head.

Backbone Blocks: Bottleneck → C3 → C2f → C3k2

The backbone stacks a repeating CSP (Cross-Stage Partial) block between stride-2 Conv downsampling layers. That repeating block is what changed most across versions. All blocks below live in block.py; c1/c2 are input/output channels and c = 0.5 * c2 is the hidden width.

Bottleneck (YOLOv3)

The base unit is Bottleneck: two Conv layers (default kernels (3, 3)) with an optional residual add when shortcut=True and c1 == c2. YOLOv3's Darknet-53 backbone stacks these directly, with no CSP split, and detects at three scales (strides 8, 16, 32).

C3 (YOLOv5)

YOLOv5's C3 splits the input across two 1x1 convolutions: cv1 feeds n sequential Bottleneck blocks (kernels (1, 1) then (3, 3)), cv2 bypasses them. The two paths are concatenated and fused by a third 1x1 Conv:

def forward(self, x):
    # C3: bottleneck path m(cv1(x)) concatenated with bypass cv2(x), then fused by cv3
    return self.cv3(torch.cat((self.m(self.cv1(x)), self.cv2(x)), 1))

Only the final bottleneck output reaches the fusion conv, so cv3 sees 2 feature maps.

C2f (YOLOv8)

YOLOv8's C2f ("CSP Bottleneck with 2 convolutions, faster") changes which features reach the fusion conv:

1. cv1 = Conv(c1, 2 * c, 1), then chunk(2) splits the output into two c-channel tensors.
2. n Bottleneck(c, c) blocks (kernels (3, 3), (3, 3)) run sequentially, each fed the previous block's output.
3. All n + 2 intermediate tensors are concatenated and fused by cv2 = Conv((2 + n) * c, c2, 1).

Where C3 passes 2 feature maps into its fusion conv, C2f passes n + 2 — every intermediate bottleneck output is reused.

C3k2 (YOLO11 and YOLO26)

YOLO11 and YOLO26 use C3k2, a subclass of C2f that swaps the repeating unit. Each of the n blocks becomes, depending on the constructor flags:

• a plain Bottleneck (default, c3k=False),
• a C3k block (c3k=True) — a C3 variant with a configurable kernel size, or
• a Bottleneck + PSABlock pair (attn=True).

The second YAML arg sets c3k; for example [-1, 2, C3k2, [512, True]] builds one C3k2 module at 512 output channels whose internal blocks are C3k (since c3k=True). For CSP modules, the repeats field — here 2, before it is scaled by the variant's depth multiple — becomes the block's internal repeat count rather than stacking separate modules.

Spatial Pooling: SPP → SPPF

At the end of the backbone, a spatial-pyramid-pooling block widens the receptive field. YOLOv5 replaced the original multi-kernel SPP with SPPF (Spatial Pyramid Pooling - Fast): a single MaxPool2d(kernel_size=5, stride=1, padding=2) applied n = 3 times in sequence, with the input and all three pooled outputs concatenated and fused by a 1x1 Conv. This is mathematically equivalent to SPP(k=(5, 9, 13)) but cheaper, because the chained 5x5 pools cover the larger kernels' receptive fields.

YOLO26 passes a shortcut flag (SPPF, [1024, 5, 3, True]); since c1 == c2 == 1024 at the deepest layer, SPPF adds a residual connection (return y + x).

Spatial Attention: C2PSA (YOLO11+)

YOLO11 added C2PSA after SPPF. It is a CSP block whose active branch is a stack of n PSABlock (Position-Sensitive Attention) modules: cv1 = Conv(c1, 2 * c, 1) splits the features, one half passes through the PSABlock stack, and cv2 = Conv(2 * c, c1, 1) fuses the concatenation. Each PSABlock applies multi-head attention followed by a two-layer feed-forward network (Conv(c, 2 * c, 1) → Conv(2 * c, c, 1)), each with a residual connection. YOLO26 keeps the same C3k2 + C2PSA backbone.

Neck: FPN + PAN

The neck fuses the backbone's P3/P4/P5 feature maps with a top-down Feature Pyramid Network (FPN) followed by a bottom-up Path Aggregation Network (PAN). In the YAML head section, FPN is nn.Upsample + Concat (carrying semantic information down to higher resolutions) and PAN is stride-2 Conv + Concat (carrying localization information back up):

# YOLO11 head (FPN top-down, then PAN bottom-up)
- [-1, 1, nn.Upsample, [None, 2, "nearest"]]
- [[-1, 6], 1, Concat, [1]] # cat backbone P4
- [-1, 2, C3k2, [512, False]] # 13
# ... second upsample + concat to P3 ...
- [-1, 1, Conv, [256, 3, 2]]
- [[-1, 13], 1, Concat, [1]] # cat head P4 (PAN)
- [-1, 2, C3k2, [512, False]] # 19

The neck reuses the backbone block of its generation — C3 in YOLOv5, C2f in YOLOv8, C3k2 in YOLO11 and YOLO26 — so each merge point runs the same module the backbone uses. The three fused outputs feed the head. YOLOv3 is the exception: its neck is top-down FPN only (its YAML head has no stride-2 downsampling), without the bottom-up PAN path that YOLOv5 introduced.

Detection Head: Anchor-Based → Anchor-Free → NMS-Free

The head turns the three fused feature maps into predictions for the detection task. Its design has changed across versions, from anchor-based to anchor-free to NMS-free.

Anchor-free, decoupled Detect

The original YOLOv3 and YOLOv5 used an anchor-based, coupled head: predefined anchor boxes and a shared branch for box and class predictions. The standalone ultralytics/yolov3 and ultralytics/yolov5 repositories keep that anchor-based design. The main ultralytics package instead ships the anchor-free YOLOv3u and YOLOv5u variants — the same Darknet-53 and C3 backbones with YOLOv8's anchor-free Detect head — and the yolov3.yaml and yolov5.yaml configs documented here are these u variants, not the historical design.

The Detect head (head.py) is anchor-free and decoupled: per pyramid level it runs two parallel branches, and predicts directly on grid points rather than against anchor boxes.

• Box branch (cv2): Conv(x, c2, 3) → Conv(c2, c2, 3) → Conv2d(c2, 4 * reg_max, 1).
• Class branch (cv3): in YOLO11 and YOLO26, two depthwise-separable blocks (DWConv + 1x1 Conv) → Conv2d(c3, nc, 1); YOLOv8 uses the legacy variant, two 3x3 Conv layers → Conv2d(c3, nc, 1).

Each anchor point therefore emits no = nc + 4 * reg_max outputs. Removing predefined anchors drops the anchor box sizes and aspect ratios from the hyperparameters that must be tuned.

Distribution Focal Loss (DFL)

YOLOv8 and YOLO11 regress each of the 4 box coordinates as a distribution over reg_max = 16 bins rather than a single scalar (the integral form from Generalized Focal Loss). The DFL module reshapes the 4 * reg_max box channels to (4, reg_max), applies a softmax over the reg_max bins, and takes the expected bin index — each bin index weighted by its softmax probability, then summed — as the predicted coordinate. This is implemented as a fixed 1x1 convolution whose weights are the bin indices arange(reg_max), so the weighted sum is a single dot product.

YOLO26: NMS-free, DFL-free

YOLO26 sets two YAML parameters that the head reads directly:

• end2end: True — Detect deep-copies its branches into a one-to-one head (one2one_cv2/one2one_cv3) that produces a single prediction per object, removing the Non-Maximum Suppression (NMS) post-processing step. See the End-to-End Detection guide for export and migration details.
• reg_max: 1 — with one bin, self.dfl becomes nn.Identity() and no = nc + 4; the head regresses coordinates directly and no DFL operation appears in the exported ONNX graph.

Across its five model sizes (n/s/m/l/x), YOLO26 reaches 40.9-57.5 mAP on COCO at 1.7-11.8 ms T4 TensorRT latency, as reported in the YOLO26 paper.

Version-by-Version Summary

For per-model details, performance tables, and usage examples, see the individual pages for YOLOv3, YOLOv5, YOLOv8, YOLO11, and YOLO26.

Inspect the Architecture Yourself

The model.info() method prints a layer, parameter, and FLOPs summary, and the parsed module list is available on model.model.model.

!!! example "Inspect a YOLO model's architecture"

```python
from ultralytics import YOLO

# Load a pretrained model
model = YOLO("yolo11n.pt")

# Fuse Conv + BatchNorm layers so counts match the published specs
model.fuse()

# Print a summary: layers, parameters, gradients, GFLOPs
model.info()

# Inspect the detection head (the last module in the network)
head = model.model.model[-1]
print(type(head).__name__, "| reg_max:", head.reg_max, "| end2end:", head.end2end)
```

Running the snippet across three generations shows the changes numerically. These are real fused-model outputs from the ultralytics package, matching the parameter and FLOPs counts published on each model page:

YOLO26n reports reg_max=1, end2end=True, and an Identity DFL layer — the architectural signature of its NMS-free, DFL-free head.

!!! note "Fused vs unfused counts"

Parameter and FLOPs values are reported for the **fused** model (`model.fuse()`), which merges each `Conv` and its [batch normalization](https://www.ultralytics.com/glossary/batch-normalization) layer. This matches the published specifications; a freshly loaded checkpoint reports slightly higher counts before fusing.

Conclusion

Across versions, the YOLO architecture changed one stage at a time: the backbone moved from Darknet-53 to CSP-based C3, C2f, and C3k2 blocks with C2PSA attention; the neck kept its FPN + PAN structure while SPP became SPPF; and the head moved from anchor-based to anchor-free, then to YOLO26's NMS-free, DFL-free end-to-end design.

To define custom architectures, see the Model YAML Configuration Guide, or compare models on the model pages. For questions, reach out on GitHub or Discord.

FAQ

What are the three stages of a YOLO architecture?

A YOLO model has a backbone that extracts features from the image at strides 8, 16, and 32, a neck that fuses those features across scales with FPN and PAN, and a head that predicts bounding boxes and class scores. Every Ultralytics YOLO model from YOLOv3 to YOLO26 follows this three-stage design.

What is the difference between the C2f and C3k2 blocks?

C2f (YOLOv8) is a CSP block that concatenates the outputs of every internal Bottleneck — n + 2 feature maps — before its fusion convolution, where the older C3 passes only 2. C3k2 (YOLO11 and YOLO26) is a subclass of C2f that can replace each Bottleneck with a C3k block (a C3 variant with a configurable kernel size) when its c3k flag is set. Both are defined in block.py.

What changed in the architecture between YOLOv8 and YOLO11?

YOLO11 makes three structural changes to YOLOv8: it replaces the C2f backbone and neck block with C3k2, inserts a C2PSA self-attention block after SPPF, and switches the head's classification branch to lighter depthwise-separable convolutions. Both keep the same anchor-free, decoupled Detect head with reg_max=16 DFL regression, so the changes lower parameter and FLOPs counts while raising accuracy rather than redesigning the detection interface.

Is YOLO anchor-free?

Modern Ultralytics YOLO models are anchor-free. YOLOv8, YOLO11, and YOLO26 use an anchor-free, decoupled Detect head with separate branches for box regression and classification. The original YOLOv3 and YOLOv5 were anchor-based, but Ultralytics ships them as the YOLOv3u and YOLOv5u variants, whose configs use the same anchor-free head as YOLOv8.

Did YOLO26 remove NMS?

Yes — YOLO26 sets end2end=True, which gives Detect a one-to-one head that produces a single prediction per object and removes the Non-Maximum Suppression post-processing step required by earlier models. See the End-to-End Detection guide for details.

What is Distribution Focal Loss (DFL) and why did YOLO26 remove it?

DFL regresses each box coordinate as a softmax distribution over reg_max bins (16 by default in YOLOv8 and YOLO11) and takes the expected value as the coordinate, rather than predicting a single scalar. YOLO26 sets reg_max=1, so the DFL layer becomes an identity operation, the head regresses coordinates directly, and no DFL op appears in exported ONNX or TensorRT graphs.

How can I see the architecture of a specific YOLO model?

Load the model in Python and call model.info() for a layer, parameter, and GFLOPs summary. The parsed layers are in model.model.model — for example, model.model.model[-1] is the Detect head, exposing attributes like reg_max and end2end. The full architecture is defined in the model's YAML configuration file.