diff --git a/internal/blueprint/blueprint_test.go b/internal/blueprint/blueprint_test.go
index 21f71a6fc..719ae3ffa 100644
--- a/internal/blueprint/blueprint_test.go
+++ b/internal/blueprint/blueprint_test.go
@@ -1,6 +1,7 @@
 package blueprint
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/BurntSushi/toml"
@@ -21,6 +22,10 @@ version = "2.4.*"
 [[customizations.filesystem]]
 mountpoint = "/var"
 size = 2147483648
+
+[[customizations.filesystem]]
+mountpoint = "/opt"
+size = "20 GB"
 `
 
 	var bp Blueprint
@@ -29,6 +34,23 @@ size = 2147483648
 	assert.Equal(t, bp.Name, "test")
 	assert.Equal(t, "/var", bp.Customizations.Filesystem[0].Mountpoint)
 	assert.Equal(t, uint64(2147483648), bp.Customizations.Filesystem[0].MinSize)
+	assert.Equal(t, "/opt", bp.Customizations.Filesystem[1].Mountpoint)
+	assert.Equal(t, uint64(20*1000*1000*1000), bp.Customizations.Filesystem[1].MinSize)
+
+	blueprint = `{
+		"name": "test",
+		"customizations": {
+		  "filesystem": [{
+			"mountpoint": "/opt",
+			"minsize": "20 GiB"
+		  }]
+		}
+	  }`
+	err = json.Unmarshal([]byte(blueprint), &bp)
+	require.Nil(t, err)
+	assert.Equal(t, bp.Name, "test")
+	assert.Equal(t, "/opt", bp.Customizations.Filesystem[0].Mountpoint)
+	assert.Equal(t, uint64(20*1024*1024*1024), bp.Customizations.Filesystem[0].MinSize)
 }
 
 func TestDeepCopy(t *testing.T) {
diff --git a/internal/blueprint/customizations.go b/internal/blueprint/customizations.go
index a0768c2dc..58ace04ad 100644
--- a/internal/blueprint/customizations.go
+++ b/internal/blueprint/customizations.go
@@ -4,6 +4,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"reflect"
+
+	"github.com/osbuild/osbuild-composer/internal/common"
 )
 
 type Customizations struct {
@@ -90,8 +92,14 @@ func (fsc *FilesystemCustomization) UnmarshalTOML(data interface{}) error {
 	switch d["size"].(type) {
 	case int64:
 		fsc.MinSize = uint64(d["size"].(int64))
+	case string:
+		size, err := common.DataSizeToUint64(d["size"].(string))
+		if err != nil {
+			return fmt.Errorf("TOML unmarshal: size is not valid filesystem size (%w)", err)
+		}
+		fsc.MinSize = size
 	default:
-		return fmt.Errorf("TOML unmarshal: size must be integer, got %v of type %T", d["size"], d["size"])
+		return fmt.Errorf("TOML unmarshal: size must be integer or string, got %v of type %T", d["size"], d["size"])
 	}
 
 	return nil
@@ -116,8 +124,14 @@ func (fsc *FilesystemCustomization) UnmarshalJSON(data []byte) error {
 	case float64:
 		// Note that it uses different key than the TOML version
 		fsc.MinSize = uint64(d["minsize"].(float64))
+	case string:
+		size, err := common.DataSizeToUint64(d["minsize"].(string))
+		if err != nil {
+			return fmt.Errorf("JSON unmarshal: size is not valid filesystem size (%w)", err)
+		}
+		fsc.MinSize = size
 	default:
-		return fmt.Errorf("JSON unmarshal: minsize must be float64 number, got %v of type %T", d["minsize"], d["minsize"])
+		return fmt.Errorf("JSON unmarshal: minsize must be float64 number or string, got %v of type %T", d["minsize"], d["minsize"])
 	}
 
 	return nil