---
title: Fastly-IO-Info
summary: null
url: >-
  https://www.fastly.com/documentation/reference/http/http-headers/Fastly-IO-Info
---

This header is written by Fastly when a response is transformed by [Image Optimizer](https://docs.fastly.com/products/image-optimizer). It provides detail on the input format (`ifmt`), dimensions (`idim`), and size in bytes (`ifsz`), and also the output format (`ofmt`), dimensions (`odim`), and size in bytes (`ofsz`).

## Format

The header's value is a space-delimited string of key/value pairs. These pairs can appear in any order.

```
ifsz=<input size in bytes>
idim=<input dimensions - width x height>
ifmt=<input format>
ofsz=<output size in bytes>
odim=<output dimensions - width x height>
ofmt=<output format>
```

## Example

```
ifsz=108501 idim=827x788 ifmt=jpeg ofsz=13066 odim=300x286 ofmt=jpeg
```