IIS Compression Configuration Guide

Last reviewed on 2026-10-03

Enable static and dynamic compression on Microsoft IIS for optimal performance

Quick Start: Enable IIS Compression

IIS supports both static and dynamic compression, but Dynamic Content Compression is not installed by default. Here's how to enable both. For a detailed walkthrough of installing the features (Server Manager, PowerShell, DISM, Windows 11) and fixing the "IIS Temporary Compressed Files" error, see Install Dynamic Content Compression in IIS.

1. Install Compression Features

# PowerShell (Run as Administrator)
# Windows Server 2019/2022
Install-WindowsFeature -Name Web-Dyn-Compression, Web-Stat-Compression

# Windows 10/11
Enable-WindowsOptionalFeature -Online -FeatureName IIS-HttpCompressionStatic
Enable-WindowsOptionalFeature -Online -FeatureName IIS-HttpCompressionDynamic

# Verify installation
Get-WindowsFeature | Where-Object {$_.Name -like "*Compression*"}

2. Enable via IIS Manager

  1. Open IIS Manager (inetmgr.exe)
  2. Select your server or site
  3. Double-click "Compression" in the IIS section
  4. Check "Enable dynamic content compression"
  5. Check "Enable static content compression"
  6. Click Apply

3. Basic web.config Configuration

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <urlCompression doStaticCompression="true" doDynamicCompression="true" />
        <staticContent>
            <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00" />
        </staticContent>
    </system.webServer>
</configuration>

Advanced Compression Configuration

Fine-tune compression settings for optimal performance:

ApplicationHost.config Settings

<system.webServer>
    <httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files"
                     minFileSizeForComp="2700"
                     noCompressionForHttp10="false"
                     noCompressionForProxies="false">
        
        <!-- Static compression settings -->
        <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" 
                staticCompressionLevel="9" 
                dynamicCompressionLevel="4" />
        
        <!-- Dynamic content types -->
        <dynamicTypes>
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />
            <add mimeType="application/javascript" enabled="true" />
            <add mimeType="application/json" enabled="true" />
            <add mimeType="application/xml" enabled="true" />
            <add mimeType="application/xhtml+xml" enabled="true" />
            <add mimeType="*/*" enabled="false" />
        </dynamicTypes>
        
        <!-- Static content types -->
        <staticTypes>
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />
            <add mimeType="application/javascript" enabled="true" />
            <add mimeType="application/json" enabled="true" />
            <add mimeType="application/xml" enabled="true" />
            <add mimeType="application/xhtml+xml" enabled="true" />
            <add mimeType="application/atom+xml" enabled="true" />
            <add mimeType="application/rss+xml" enabled="true" />
            <add mimeType="font/otf" enabled="true" />
            <add mimeType="font/ttf" enabled="true" />
            <add mimeType="image/svg+xml" enabled="true" />
            <add mimeType="*/*" enabled="false" />
        </staticTypes>
    </httpCompression>
    
    <urlCompression doStaticCompression="true" 
                    doDynamicCompression="true" 
                    dynamicCompressionBeforeCache="true" />
</system.webServer>

PowerShell Configuration

# Import IIS module
Import-Module WebAdministration

# Set compression levels
Set-WebConfigurationProperty -Filter "system.webServer/httpCompression/scheme[@name='gzip']" `
    -Name "staticCompressionLevel" -Value 9
Set-WebConfigurationProperty -Filter "system.webServer/httpCompression/scheme[@name='gzip']" `
    -Name "dynamicCompressionLevel" -Value 4

# Enable compression for specific MIME types
Add-WebConfigurationProperty -Filter "system.webServer/httpCompression/dynamicTypes" `
    -Name "." -Value @{mimeType='application/json'; enabled='True'}

# Set minimum file size for compression (bytes)
Set-WebConfigurationProperty -Filter "system.webServer/httpCompression" `
    -Name "minFileSizeForComp" -Value 1024

# Apply changes
Restart-WebAppPool -Name "DefaultAppPool"

Enable Brotli Compression on IIS

Add Brotli support using the IIS Compression module:

1. Install IIS Compression Module

# Download the installer from Microsoft's IIS Compression page:
# https://www.iis.net/downloads/microsoft/iis-compression

# Install silently (64-bit package)
msiexec /i iiscompression_amd64.msi /quiet /norestart

# Restart IIS
iisreset

2. Configure Brotli in ApplicationHost.config

<system.webServer>
    <httpCompression>
        <!-- Brotli scheme: list it before gzip so it wins -->
        <scheme name="br" dll="%ProgramFiles%\IIS\IIS Compression\iisbrotli.dll" 
                staticCompressionLevel="11" 
                dynamicCompressionLevel="5" />
        
        <!-- Keep existing gzip -->
        <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" 
                staticCompressionLevel="9" 
                dynamicCompressionLevel="4" />
    </httpCompression>
</system.webServer>

3. Do not rewrite Content-Encoding

The scheme provider sets Content-Encoding: br and Vary: Accept-Encoding itself. Older tutorials add an outbound URL Rewrite rule that sets Content-Encoding to br whenever the request accepts Brotli; that relabels gzip or uncompressed bodies and breaks decoding in browsers. Remove such rules if you have them. The complete walkthrough, including Azure App Service options, is in the Brotli on IIS guide; if you see decoding failures, see content decoding errors.

Per-Site Compression Configuration

Configure compression for specific sites or applications:

Site-Specific web.config

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <!-- Enable compression for this site -->
        <urlCompression doStaticCompression="true" doDynamicCompression="true" />
        
        <!-- Custom compression rules -->
        <httpCompression>
            <dynamicTypes>
                <clear />
                <add mimeType="text/*" enabled="true" />
                <add mimeType="application/javascript" enabled="true" />
                <add mimeType="application/json" enabled="true" />
                <add mimeType="application/xml" enabled="true" />
            </dynamicTypes>
        </httpCompression>
        
        <!-- Static file handling -->
        <staticContent>
            <!-- Enable compression for custom file types -->
            <mimeMap fileExtension=".json" mimeType="application/json" />
            <mimeMap fileExtension=".woff2" mimeType="font/woff2" />
            
            <!-- Client caching -->
            <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00" />
        </staticContent>
        
        <!-- Disable compression for specific paths -->
        <location path="api/stream">
            <system.webServer>
                <urlCompression doStaticCompression="false" doDynamicCompression="false" />
            </system.webServer>
        </location>
    </system.webServer>
</configuration>

ASP.NET Core Compression

// Program.cs or Startup.cs
using Microsoft.AspNetCore.ResponseCompression;

var builder = WebApplication.CreateBuilder(args);

// Add response compression services
builder.Services.AddResponseCompression(options =>
{
    options.EnableForHttps = true;
    options.Providers.Add();
    options.Providers.Add();
    options.MimeTypes = ResponseCompressionDefaults.MimeTypes.Concat(
        new[] { "application/json", "image/svg+xml" });
});

// Configure compression levels
builder.Services.Configure(options =>
{
    options.Level = CompressionLevel.Optimal;
});

builder.Services.Configure(options =>
{
    options.Level = CompressionLevel.Optimal;
});

var app = builder.Build();

// Use compression middleware
app.UseResponseCompression();

Performance Optimization

CPU Threshold

Disable compression when CPU is high:

<httpCompression 
    staticCompressionDisableCpuUsage="90"
    dynamicCompressionDisableCpuUsage="80">

Compression Levels

Recommended settings by content type:

  • Static HTML/CSS/JS: Level 9
  • Dynamic content: Level 4-6
  • API responses: Level 1-4

Cache Integration

Compress before caching:

<urlCompression 
    dynamicCompressionBeforeCache="true" />

Testing IIS Compression

PowerShell Testing

# Test compression with PowerShell
$response = Invoke-WebRequest -Uri "https://localhost/test.css" `
    -Headers @{"Accept-Encoding"="gzip, deflate, br"}

# Check encoding
$response.Headers["Content-Encoding"]

# Compare sizes
$uncompressed = Invoke-WebRequest -Uri "https://localhost/test.css" `
    -Headers @{"Accept-Encoding"="identity"}

Write-Host "Compressed: $($response.RawContent.Length) bytes"
Write-Host "Uncompressed: $($uncompressed.RawContent.Length) bytes"
Write-Host "Ratio: $(($response.RawContent.Length / $uncompressed.RawContent.Length * 100))%"

Failed Request Tracing

<system.webServer>
    <tracing>
        <traceFailedRequests>
            <add path="*">
                <traceAreas>
                    <add provider="WWW Server" areas="Compression" verbosity="Verbose" />
                </traceAreas>
                <failureDefinitions statusCodes="200-999" />
            </add>
        </traceFailedRequests>
    </tracing>
</system.webServer>

Common IIS Compression Issues

Compression Not Working

  • Verify features are installed: Get-WindowsFeature *compression*
  • Check applicationHost.config for correct MIME types
  • Ensure file size is above minFileSizeForComp threshold
  • Verify no conflicting modules or handlers

Performance Issues

  • Monitor CPU usage during compression
  • Adjust compression levels based on server capacity
  • Use static compression for frequently accessed files
  • Consider hardware compression accelerators

Debugging Commands

# Check IIS compression status
%windir%\system32\inetsrv\appcmd list config -section:httpCompression

# View compression statistics
Get-Counter "\Web Service(_Total)\Total Compressed Files"
Get-Counter "\Web Service(_Total)\Total Compression Attempts"

# Check Event Viewer for compression errors
Get-EventLog -LogName System -Source W3SVC | Where-Object {$_.Message -like "*compression*"}

IIS Compression Best Practices

<!-- Optimal production configuration -->
<system.webServer>
    <httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files"
                     minFileSizeForComp="1024"
                     staticCompressionDisableCpuUsage="95"
                     dynamicCompressionDisableCpuUsage="85">
        <scheme name="br" dll="%ProgramFiles%\IIS\IIS Compression\iisbrotli.dll" 
                staticCompressionLevel="11" dynamicCompressionLevel="5" />
        <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" 
                staticCompressionLevel="9" dynamicCompressionLevel="4" />
    </httpCompression>
    <urlCompression doStaticCompression="true" doDynamicCompression="true" 
                    dynamicCompressionBeforeCache="true" />
</system.webServer>

Frequently asked questions

How do I enable compression in IIS?

Install the Static and Dynamic Content Compression features, then enable both in IIS Manager under Compression or with <urlCompression doStaticCompression="true" doDynamicCompression="true" />. Make sure the MIME types you need are listed in staticTypes and dynamicTypes.

How do I add application/json to IIS compression?

Add it to dynamicTypes in applicationHost.config, for example with appcmd set config -section:system.webServer/httpCompression /+"dynamicTypes.[mimeType='application/json',enabled='True']" /commit:apphost.

How do I disable IIS compression for one path?

Use a <location path="..."> element with <urlCompression doStaticCompression="false" doDynamicCompression="false" />, as shown in the site-specific web.config example above.

How do I stop IIS compressing a file type such as .xlsx?

Make sure its MIME type is not enabled in staticTypes or dynamicTypes and that the catch-all */* entry is enabled="false". Office files such as .xlsx are already ZIP-compressed and gain nothing.

Can IIS compress HTTP headers?

Content compression only applies to response bodies. Header compression happens at the protocol level: HTTP/2 (HPACK), which IIS 10 supports over HTTPS, compresses headers automatically.

Does IIS support Brotli?

Yes, with Microsoft's IIS Compression module, which adds the iisbrotli.dll scheme provider. See the Brotli on IIS guide.