diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f4427e8..e0d3b51 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -11,8 +11,15 @@ Please note that this project is released with a [Contributor Code of Conduct][c
 ## Submitting a pull request
 
 1. [Fork][fork] and clone the repository
-2. Configure and install the dependencies: `script/bootstrap`
-3. Make sure the tests pass on your machine: `make test`
+
+
+
+2. Configure and install the dependencies
+   - On Unix-like machines: `script/bootstrap`
+   - On Windows: `script/bootstrap.ps1` (requires PowerShell 7+) 
+3. Make sure the tests pass on your machine
+   - On Unix-like machines: `make test`
+   - On Windows machines: `make -f Makefile.win test` (because there's a different Makefile when building on Windows)
 4. Create a new branch: `git checkout -b my-branch-name`
 5. Make your change, add tests, and make sure the tests still pass
 6. Push to your fork and [submit a pull request][pr]
diff --git a/Makefile.win b/Makefile.win
new file mode 100644
index 0000000..b772251
--- /dev/null
+++ b/Makefile.win
@@ -0,0 +1,57 @@
+# Windows-specific Makefile for git-sizer designed for PowerShell
+
+PACKAGE := github.com/github/git-sizer
+GO111MODULES := 1
+
+# Use the project's go wrapper script via the -File parameter to avoid loading your profile
+GOSCRIPT := $(CURDIR)/script/go.ps1
+GO := pwsh.exe -NoProfile -ExecutionPolicy Bypass -File $(GOSCRIPT)
+
+# Get the build version from git using try/catch instead of "||"
+BUILD_VERSION := $(shell pwsh.exe -NoProfile -ExecutionPolicy Bypass -Command "try { git describe --tags --always --dirty 2>$$null } catch { Write-Output 'unknown' }")
+LDFLAGS := -X github.com/github/git-sizer/main.BuildVersion=$(BUILD_VERSION)
+GOFLAGS := -mod=readonly
+
+ifdef USE_ISATTY
+GOFLAGS := $(GOFLAGS) --tags isatty
+endif
+
+# Find all Go source files
+GO_SRC_FILES := $(shell powershell -NoProfile -ExecutionPolicy Bypass -Command "Get-ChildItem -Path . -Filter *.go -Recurse | Select-Object -ExpandProperty FullName")
+
+# Define common PowerShell command
+PWSH := @powershell -NoProfile -ExecutionPolicy Bypass -Command
+
+# Default target
+all: bin/git-sizer.exe
+
+# Main binary target - depend on all Go source files
+bin/git-sizer.exe: $(GO_SRC_FILES)
+	$(PWSH) "if (-not (Test-Path bin)) { New-Item -ItemType Directory -Path bin | Out-Null }"
+	$(GO) build $(GOFLAGS) -ldflags "$(LDFLAGS)" -a -o .\bin\git-sizer.exe .
+
+# Test target - explicitly run the build first to ensure binary is up to date
+test:
+	@$(MAKE) -f Makefile.win bin/git-sizer.exe
+	@$(MAKE) -f Makefile.win gotest
+
+# Run go tests
+gotest:
+	$(GO) test -timeout 60s $(GOFLAGS) -ldflags "$(LDFLAGS)" ./...
+
+# Clean up builds
+clean:
+	$(PWSH) "if (Test-Path bin) { Remove-Item -Recurse -Force bin }"
+
+# Help target
+help:
+	$(PWSH) "Write-Host 'Windows Makefile for git-sizer' -ForegroundColor Cyan"
+	$(PWSH) "Write-Host ''"
+	$(PWSH) "Write-Host 'Targets:' -ForegroundColor Green"
+	$(PWSH) "Write-Host '  all        - Build git-sizer (default)'"
+	$(PWSH) "Write-Host '  test       - Run tests'"
+	$(PWSH) "Write-Host '  clean      - Clean build artifacts'"
+	$(PWSH) "Write-Host ''"
+	$(PWSH) "Write-Host 'Example usage:' -ForegroundColor Green"
+	$(PWSH) "Write-Host '  make -f Makefile.win'"
+	$(PWSH) "Write-Host '  make -f Makefile.win test'"
diff --git a/git-sizer.go b/git-sizer.go
index 1ef9812..fe5876b 100644
--- a/git-sizer.go
+++ b/git-sizer.go
@@ -331,6 +331,19 @@ func mainImplementation(ctx context.Context, stdout, stderr io.Writer, args []st
 		return fmt.Errorf("error scanning repository: %w", err)
 	}
 
+	// Calculate the actual size of the .git directory.
+	gitDir, err := repo.GitDir()
+	if err != nil {
+		return fmt.Errorf("error getting Git directory path: %w", err)
+	}
+
+	gitDirSize, err := sizes.CalculateGitDirSize(gitDir)
+	if err != nil {
+		return fmt.Errorf("error calculating Git directory size: %w", err)
+	}
+
+	historySize.GitDirSize = gitDirSize
+
 	if jsonOutput {
 		var j []byte
 		var err error
diff --git a/git/git.go b/git/git.go
index 096ce81..5fc8f34 100644
--- a/git/git.go
+++ b/git/git.go
@@ -150,8 +150,11 @@ func (repo *Repository) GitCommand(callerArgs ...string) *exec.Cmd {
 
 // GitDir returns the path to `repo`'s `GIT_DIR`. It might be absolute
 // or it might be relative to the current directory.
-func (repo *Repository) GitDir() string {
-	return repo.gitDir
+func (repo *Repository) GitDir() (string, error) {
+	if repo.gitDir == "" {
+		return "", errors.New("gitDir is not set")
+	}
+	return repo.gitDir, nil
 }
 
 // GitPath returns that path of a file within the git repository, by
diff --git a/script/bootstrap.ps1 b/script/bootstrap.ps1
new file mode 100644
index 0000000..7f0413a
--- /dev/null
+++ b/script/bootstrap.ps1
@@ -0,0 +1,18 @@
+#!/usr/bin/env pwsh
+
+# Exit immediately if any command fails
+$ErrorActionPreference = "Stop"
+
+# Change directory to the parent directory of the script
+Set-Location -Path (Split-Path -Parent $PSCommandPath | Split-Path -Parent)
+
+# Set ROOTDIR environment variable to the current directory
+$env:ROOTDIR = (Get-Location).Path
+
+# Check if the operating system is macOS
+if ($IsMacOS) {
+    brew bundle
+}
+
+# Source the ensure-go-installed.ps1 script
+. ./script/ensure-go-installed.ps1
\ No newline at end of file
diff --git a/script/ensure-go-installed.ps1 b/script/ensure-go-installed.ps1
new file mode 100644
index 0000000..3d653c1
--- /dev/null
+++ b/script/ensure-go-installed.ps1
@@ -0,0 +1,64 @@
+# This script is meant to be sourced with ROOTDIR set.
+
+if (-not $env:ROOTDIR) {
+    $env:ROOTDIR = (Resolve-Path (Join-Path $scriptDir "..")).Path
+}
+
+# Function to check if Go is installed and at least version 1.21
+function GoOk {
+    $goVersionOutput = & go version 2>$null
+    if ($goVersionOutput) {
+        $goVersion = $goVersionOutput -match 'go(\d+)\.(\d+)' | Out-Null
+        $majorVersion = [int]$Matches[1]
+        $minorVersion = [int]$Matches[2]
+        return ($majorVersion -eq 1 -and $minorVersion -ge 21)
+    }
+    return $false
+}
+
+# Function to set up a local Go installation if available
+function SetUpVendoredGo {
+    $GO_VERSION = "go1.23.7"
+    $VENDORED_GOROOT = Join-Path -Path $env:ROOTDIR -ChildPath "vendor/$GO_VERSION/go"
+    if (Test-Path -Path "$VENDORED_GOROOT/bin/go") {
+        $env:GOROOT = $VENDORED_GOROOT
+        $env:PATH = "$env:GOROOT/bin;$env:PATH"
+    }
+}
+
+# Function to check if Make is installed and install it if needed
+function EnsureMakeInstalled {
+    $makeInstalled = $null -ne (Get-Command "make" -ErrorAction SilentlyContinue)
+    if (-not $makeInstalled) {
+        #Write-Host "Installing Make using winget..."
+        winget install --no-upgrade --nowarn -e --id GnuWin32.Make 
+        if ($LASTEXITCODE -ne 0 -and $LASTEXITCODE -ne 0x8A150061) {
+            Write-Error "Failed to install Make. Please install it manually. Exit code: $LASTEXITCODE"
+        }
+        # Refresh PATH to include the newly installed Make
+        $env:PATH = [System.Environment]::GetEnvironmentVariable("PATH", "Machine") + ";" + [System.Environment]::GetEnvironmentVariable("PATH", "User")
+    }
+
+    # Add GnuWin32 bin directory directly to the PATH
+    $gnuWin32Path = "C:\Program Files (x86)\GnuWin32\bin"
+    if (Test-Path -Path $gnuWin32Path) {
+        $env:PATH = "$gnuWin32Path;$env:PATH"
+    } else {
+        Write-Host "Couldn't find GnuWin32 bin directory at the expected location."
+        # Also refresh PATH from environment variables as a fallback
+        $env:PATH = [System.Environment]::GetEnvironmentVariable("PATH", "Machine") + ";" + [System.Environment]::GetEnvironmentVariable("PATH", "User")
+    }    
+}
+
+SetUpVendoredGo
+
+if (-not (GoOk)) {
+    & ./script/install-vendored-go >$null
+    if ($LASTEXITCODE -ne 0) {
+        exit 1
+    }
+    SetUpVendoredGo
+}
+
+# Ensure Make is installed
+EnsureMakeInstalled
diff --git a/script/go.ps1 b/script/go.ps1
new file mode 100644
index 0000000..e2314b8
--- /dev/null
+++ b/script/go.ps1
@@ -0,0 +1,21 @@
+# Ensure that script errors stop execution
+$ErrorActionPreference = "Stop"
+
+# Determine the root directory of the project.
+$scriptDir = Split-Path -Parent $MyInvocation.MyCommand.Path
+$ROOTDIR = (Resolve-Path (Join-Path $scriptDir "..")).Path
+
+# Source the ensure-go-installed functionality.
+# (This assumes you have a corresponding PowerShell version of ensure-go-installed.
+#  If not, you could call the bash version via bash.exe if available.)
+$ensureScript = Join-Path $ROOTDIR "script\ensure-go-installed.ps1"
+if (Test-Path $ensureScript) {
+    . $ensureScript
+} else {
+    Write-Error "Unable to locate '$ensureScript'. Please provide a PowerShell version of ensure-go-installed."
+}
+
+# Execute the actual 'go' command with passed arguments.
+# This re-invokes the Go tool in PATH.
+$goExe = "go"
+& $goExe @args
\ No newline at end of file
diff --git a/sizes/dirsize.go b/sizes/dirsize.go
new file mode 100644
index 0000000..eb9eb99
--- /dev/null
+++ b/sizes/dirsize.go
@@ -0,0 +1,31 @@
+package sizes
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/github/git-sizer/counts"
+)
+
+// CalculateGitDirSize returns the total size in bytes of the .git directory.
+func CalculateGitDirSize(gitDir string) (counts.Count64, error) {
+	var totalSize counts.Count64
+
+	err := filepath.Walk(gitDir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			// Only skip errors for files we cannot access.
+			if os.IsNotExist(err) || os.IsPermission(err) {
+				return nil
+			}
+			return err
+		}
+
+		// Only count files, not directories.
+		if !info.IsDir() {
+			totalSize.Increment(counts.Count64(info.Size()))
+		}
+		return nil
+	})
+
+	return totalSize, err
+}
diff --git a/sizes/output.go b/sizes/output.go
index 933cc05..0538cb7 100644
--- a/sizes/output.go
+++ b/sizes/output.go
@@ -279,10 +279,10 @@ func (t *Threshold) Type() string {
 // A `pflag.Value` that can be used as a boolean option that sets a
 // `Threshold` variable to a fixed value. For example,
 //
-//		pflag.Var(
-//			sizes.NewThresholdFlagValue(&threshold, 30),
-//			"critical", "only report critical statistics",
-//		)
+//	pflag.Var(
+//		sizes.NewThresholdFlagValue(&threshold, 30),
+//		"critical", "only report critical statistics",
+//	)
 //
 // adds a `--critical` flag that sets `threshold` to 30.
 type thresholdFlagValue struct {
@@ -492,7 +492,7 @@ func (s *HistorySize) contents(refGroups []RefGroup) tableContents {
 	return S(
 		"",
 		S(
-			"Overall repository size",
+			"Repository statistics",
 			S(
 				"Commits",
 				I("uniqueCommitCount", "Count",
@@ -521,11 +521,18 @@ func (s *HistorySize) contents(refGroups []RefGroup) tableContents {
 				I("uniqueBlobCount", "Count",
 					"The total number of distinct blob objects",
 					nil, s.UniqueBlobCount, metric, "", 1.5e6),
-				I("uniqueBlobSize", "Total size",
+				I("uniqueBlobSize", "Uncompressed total size",
 					"The total size of all distinct blob objects",
 					nil, s.UniqueBlobSize, binary, "B", 10e9),
 			),
 
+			S(
+				"On-disk size",
+				I("gitDirSize", "Compressed total size",
+					"The actual on-disk size of the .git directory",
+					nil, s.GitDirSize, binary, "B", 1e9),
+			),
+
 			S(
 				"Annotated tags",
 				I("uniqueTagCount", "Count",
diff --git a/sizes/sizes.go b/sizes/sizes.go
index b3de0bc..73834fd 100644
--- a/sizes/sizes.go
+++ b/sizes/sizes.go
@@ -210,6 +210,9 @@ type HistorySize struct {
 
 	// The tree with the maximum expanded submodule count.
 	MaxExpandedSubmoduleCountTree *Path `json:"max_expanded_submodule_count_tree,omitempty"`
+
+	// The actual size of the .git directory on disk.
+	GitDirSize counts.Count64 `json:"git_dir_size"`
 }
 
 // Convenience function: forget `*path` if it is non-nil and overwrite