---
comments: true
description: Learn to freeze YOLOv5 layers for efficient transfer learning, reducing resources and speeding up training while maintaining accuracy.
keywords: YOLOv5, transfer learning, freeze layers, machine learning, deep learning, model training, PyTorch, Ultralytics
---

📚 This guide explains how to **freeze** YOLOv5 🚀 layers when **transfer learning**. Transfer learning is a useful way to quickly retrain a model on new data without having to retrain the entire network. Instead, part of the initial weights are frozen in place, and the rest of the weights are used to compute loss and are updated by the optimizer. This requires less resources than normal training and allows for faster training times, though it may also result in reductions to final trained accuracy.

## Before You Start

Clone repo and install [requirements.txt](https://github.com/ultralytics/yolov5/blob/master/requirements.txt) in a [**Python>=3.8.0**](https://www.python.org/) environment, including [**PyTorch>=1.8**](https://pytorch.org/get-started/locally/). [Models](https://github.com/ultralytics/yolov5/tree/master/models) and [datasets](https://github.com/ultralytics/yolov5/tree/master/data) download automatically from the latest YOLOv5 [release](https://github.com/ultralytics/yolov5/releases).

```bash
git clone https://github.com/ultralytics/yolov5  # clone
cd yolov5
pip install -r requirements.txt  # install
```

## Freeze Backbone

All layers that match the train.py `freeze` list in train.py will be frozen by setting their gradients to zero before training starts.

```python
# Freeze
freeze = [f"model.{x}." for x in range(freeze)]  # layers to freeze
for k, v in model.named_parameters():
    v.requires_grad = True  # train all layers
    if any(x in k for x in freeze):
        print(f"freezing {k}")
        v.requires_grad = False
```

To see a list of module names:

```python
for k, v in model.named_parameters():
    print(k)

"""Output:
model.0.conv.conv.weight
model.0.conv.bn.weight
model.0.conv.bn.bias
model.1.conv.weight
model.1.bn.weight
model.1.bn.bias
model.2.cv1.conv.weight
model.2.cv1.bn.weight
...
model.23.m.0.cv2.bn.weight
model.23.m.0.cv2.bn.bias
model.24.m.0.weight
model.24.m.0.bias
model.24.m.1.weight
model.24.m.1.bias
model.24.m.2.weight
model.24.m.2.bias
"""
```

Looking at the model architecture we can see that the model backbone is layers 0-9:

```yaml
# YOLOv5 v6.0 backbone
backbone:
    # [from, number, module, args]
    - [-1, 1, Conv, [64, 6, 2, 2]] # 0-P1/2
    - [-1, 1, Conv, [128, 3, 2]] # 1-P2/4
    - [-1, 3, C3, [128]]
    - [-1, 1, Conv, [256, 3, 2]] # 3-P3/8
    - [-1, 6, C3, [256]]
    - [-1, 1, Conv, [512, 3, 2]] # 5-P4/16
    - [-1, 9, C3, [512]]
    - [-1, 1, Conv, [1024, 3, 2]] # 7-P5/32
    - [-1, 3, C3, [1024]]
    - [-1, 1, SPPF, [1024, 5]] # 9

# YOLOv5 v6.0 head
head:
    - [-1, 1, Conv, [512, 1, 1]]
    - [-1, 1, nn.Upsample, [None, 2, "nearest"]]
    - [[-1, 6], 1, Concat, [1]] # cat backbone P4
    - [-1, 3, C3, [512, False]] # 13

    - [-1, 1, Conv, [256, 1, 1]]
    - [-1, 1, nn.Upsample, [None, 2, "nearest"]]
    - [[-1, 4], 1, Concat, [1]] # cat backbone P3
    - [-1, 3, C3, [256, False]] # 17 (P3/8-small)

    - [-1, 1, Conv, [256, 3, 2]]
    - [[-1, 14], 1, Concat, [1]] # cat head P4
    - [-1, 3, C3, [512, False]] # 20 (P4/16-medium)

    - [-1, 1, Conv, [512, 3, 2]]
    - [[-1, 10], 1, Concat, [1]] # cat head P5
    - [-1, 3, C3, [1024, False]] # 23 (P5/32-large)

    - [[17, 20, 23], 1, Detect, [nc]] # Detect(P3, P4, P5)
```

so we can define the freeze list to contain all modules with 'model.0.' - 'model.9.' in their names:

```bash
python train.py --freeze 10
```

## Freeze All Layers

To freeze the full model except for the final output convolution layers in Detect(), we set freeze list to contain all modules with 'model.0.' - 'model.23.' in their names:

```bash
python train.py --freeze 24
```

## Results

We train YOLOv5m on VOC on both of the above scenarios, along with a default model (no freezing), starting from the official COCO pretrained `--weights yolov5m.pt`:

```bash
train.py --batch 48 --weights yolov5m.pt --data voc.yaml --epochs 50 --cache --img 512 --hyp hyp.finetune.yaml
```

### Accuracy Comparison

The results show that freezing speeds up training, but reduces final accuracy slightly.

![Freezing training mAP50 results](https://user-images.githubusercontent.com/26833433/98394454-11579f80-205b-11eb-8e57-d8318e1cc2f8.png)

![Freezing training mAP50-95 results](https://user-images.githubusercontent.com/26833433/98394459-13216300-205b-11eb-871b-49e20691a423.png)

<img width="922" alt="Table results" src="https://user-images.githubusercontent.com/26833433/98394485-22081580-205b-11eb-9e37-1f9869fe91d8.png">

### GPU Utilization Comparison

Interestingly, the more modules are frozen the less GPU memory is required to train, and the lower GPU utilization. This indicates that larger models, or models trained at larger --image-size may benefit from freezing in order to train faster.

![Training GPU memory allocated percent](https://user-images.githubusercontent.com/26833433/98394920-c2f6d080-205b-11eb-9611-fd68522b4e0e.png)

![Training GPU memory utilization percent](https://user-images.githubusercontent.com/26833433/98394918-bf634980-205b-11eb-948d-311036ef9325.png)

## Supported Environments

Ultralytics provides a range of ready-to-use environments, each pre-installed with essential dependencies such as [CUDA](https://developer.nvidia.com/cuda), [CUDNN](https://developer.nvidia.com/cudnn), [Python](https://www.python.org/), and [PyTorch](https://pytorch.org/), to kickstart your projects.

- **Free GPU Notebooks**: <a href="https://bit.ly/yolov5-paperspace-notebook"><img src="https://assets.paperspace.io/img/gradient-badge.svg" alt="Run on Gradient"></a> <a href="https://colab.research.google.com/github/ultralytics/yolov5/blob/master/tutorial.ipynb"><img src="https://colab.research.google.com/assets/colab-badge.svg" alt="Open In Colab"></a> <a href="https://www.kaggle.com/ultralytics/yolov5"><img src="https://kaggle.com/static/images/open-in-kaggle.svg" alt="Open In Kaggle"></a>
- **Google Cloud**: [GCP Quickstart Guide](../environments/google_cloud_quickstart_tutorial.md)
- **Amazon**: [AWS Quickstart Guide](../environments/aws_quickstart_tutorial.md)
- **Azure**: [AzureML Quickstart Guide](../environments/azureml_quickstart_tutorial.md)
- **Docker**: [Docker Quickstart Guide](../environments/docker_image_quickstart_tutorial.md) <a href="https://hub.docker.com/r/ultralytics/yolov5"><img src="https://img.shields.io/docker/pulls/ultralytics/yolov5?logo=docker" alt="Docker Pulls"></a>

## Project Status

<a href="https://github.com/ultralytics/yolov5/actions/workflows/ci-testing.yml"><img src="https://github.com/ultralytics/yolov5/actions/workflows/ci-testing.yml/badge.svg" alt="YOLOv5 CI"></a>

This badge indicates that all [YOLOv5 GitHub Actions](https://github.com/ultralytics/yolov5/actions) Continuous Integration (CI) tests are successfully passing. These CI tests rigorously check the functionality and performance of YOLOv5 across various key aspects: [training](https://github.com/ultralytics/yolov5/blob/master/train.py), [validation](https://github.com/ultralytics/yolov5/blob/master/val.py), [inference](https://github.com/ultralytics/yolov5/blob/master/detect.py), [export](https://github.com/ultralytics/yolov5/blob/master/export.py), and [benchmarks](https://github.com/ultralytics/yolov5/blob/master/benchmarks.py). They ensure consistent and reliable operation on macOS, Windows, and Ubuntu, with tests conducted every 24 hours and upon each new commit.
