diff --git a/src/package.h b/src/package.h
--- a/src/package.h
+++ b/src/package.h
@@ -309,6 +309,27 @@
     */
     qint64 availableInstalledSize() const;
 
+/**
+    * Returns the amount of hard drive space that this package takes up
+    * when installed.
+    * If the package is installed, then it is the size of the currently
+    * installed version. Otherwise, it is the size of the candidate
+    * version.
+    * This is human-unreadable, so KDE applications may wish to run this
+    * through the KFormat().formatByteSize() function to get a
+    * localized, human-readable number.
+    *
+    * Returns -1 on error.
+    *
+    * @return The installed size of the package
+    *
+    * @see currentInstalledSize()
+    * @see availableInstalledSize()
+    *
+    * @since 3.1
+    */
+    qint64 installedSize() const;
+
    /**
     * Returns the download size of the package archive in bytes.
     * This is human-unreadable, so KDE applications may wish to run this
diff --git a/src/package.cpp b/src/package.cpp
--- a/src/package.cpp
+++ b/src/package.cpp
@@ -710,6 +710,17 @@
     return qint64(State.CandidateVerIter(*d->backend->cache()->depCache())->InstalledSize);
 }
 
+qint64 Package::installedSize() const
+{
+    const pkgCache::VerIterator &ver = d->packageIter.CurrentVer();
+
+    if (!ver.end()) {
+        return qint64(ver->InstalledSize);
+    }
+
+    return availableInstalledSize();
+}
+
 qint64 Package::downloadSize() const
 {
     pkgDepCache::StateCache &State = (*d->backend->cache()->depCache())[d->packageIter];