diff --git a/src/Invoice/Calculator/ActivityInvoiceCalculator.php b/src/Invoice/Calculator/ActivityInvoiceCalculator.php
new file mode 100644
index 00000000..22a23229
--- /dev/null
+++ b/src/Invoice/Calculator/ActivityInvoiceCalculator.php
@@ -0,0 +1,51 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Invoice\Calculator;
+
+use App\Entity\Timesheet;
+use App\Invoice\CalculatorInterface;
+
+/**
+ * A calculator that sums up the timesheet records by activity.
+ */
+class ActivityInvoiceCalculator extends AbstractMergedCalculator implements CalculatorInterface
+{
+    /**
+     * @return Timesheet[]
+     */
+    public function getEntries()
+    {
+        $entries = $this->model->getEntries();
+        if (empty($entries)) {
+            return [];
+        }
+
+        /** @var Timesheet[] $timesheets */
+        $timesheets = [];
+
+        foreach ($entries as $entry) {
+            if (!isset($timesheets[$entry->getActivity()->getId()])) {
+                $timesheets[$entry->getActivity()->getId()] = new Timesheet();
+            }
+            $timesheet = $timesheets[$entry->getActivity()->getId()];
+            $this->mergeTimesheets($timesheet, $entry);
+        }
+
+        return array_values($timesheets);
+    }
+
+    /**
+     * @return string
+     */
+    public function getId(): string
+    {
+        return 'activity';
+    }
+}
diff --git a/tests/Invoice/Calculator/ActivityInvoiceCalculatorTest.php b/tests/Invoice/Calculator/ActivityInvoiceCalculatorTest.php
new file mode 100644
index 00000000..1e2928c0
--- /dev/null
+++ b/tests/Invoice/Calculator/ActivityInvoiceCalculatorTest.php
@@ -0,0 +1,129 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Tests\Invoice\Calculator;
+
+use App\Entity\Activity;
+use App\Entity\Customer;
+use App\Entity\InvoiceTemplate;
+use App\Entity\Timesheet;
+use App\Entity\User;
+use App\Invoice\Calculator\ActivityInvoiceCalculator;
+use App\Model\InvoiceModel;
+use App\Repository\Query\InvoiceQuery;
+
+/**
+ * @covers \App\Invoice\Calculator\ActivityInvoiceCalculator
+ * @covers \App\Invoice\Calculator\AbstractCalculator
+ */
+class ActivityInvoiceCalculatorTest extends AbstractCalculatorTest
+{
+    public function testEmptyModel()
+    {
+        $this->assertEmptyModel(new ActivityInvoiceCalculator());
+    }
+
+    public function testWithMultipleEntries()
+    {
+        $customer = new Customer();
+        $template = new InvoiceTemplate();
+        $template->setVat(19);
+
+        $user = $this->getMockBuilder(User::class)->setMethods(['getId'])->disableOriginalConstructor()->getMock();
+        $user->method('getId')->willReturn(1);
+
+        $activity1 = $this->getMockBuilder(Activity::class)->setMethods(['getId'])->disableOriginalConstructor()->getMock();
+        $activity1->method('getId')->willReturn(1);
+
+        $activity2 = $this->getMockBuilder(Activity::class)->setMethods(['getId'])->disableOriginalConstructor()->getMock();
+        $activity2->method('getId')->willReturn(2);
+
+        $timesheet = new Timesheet();
+        $timesheet
+            ->setDuration(3600)
+            ->setRate(293.27)
+            ->setUser($user)
+            ->setActivity($activity1)
+        ;
+
+        $timesheet2 = new Timesheet();
+        $timesheet2
+            ->setDuration(400)
+            ->setRate(84.75)
+            ->setUser($user)
+            ->setActivity($activity2)
+        ;
+
+        $timesheet3 = new Timesheet();
+        $timesheet3
+            ->setDuration(1800)
+            ->setRate(111.11)
+            ->setUser($user)
+            ->setActivity($activity1)
+        ;
+
+        $timesheet4 = new Timesheet();
+        $timesheet4
+            ->setDuration(400)
+            ->setRate(1947.99)
+            ->setUser($user)
+            ->setActivity($activity2)
+        ;
+
+        $timesheet5 = new Timesheet();
+        $timesheet5
+            ->setDuration(400)
+            ->setRate(84)
+            ->setUser(new User())
+            ->setActivity(new Activity())
+        ;
+
+        $entries = [$timesheet, $timesheet2, $timesheet3, $timesheet4, $timesheet5];
+
+        $query = new InvoiceQuery();
+        $query->setActivity($activity1);
+
+        $model = new InvoiceModel();
+        $model->setCustomer($customer);
+        $model->setTemplate($template);
+        $model->setEntries($entries);
+        $model->setQuery($query);
+
+        $sut = new ActivityInvoiceCalculator();
+        $sut->setModel($model);
+
+        $this->assertEquals('activity', $sut->getId());
+        $this->assertEquals(3000.13, $sut->getTotal());
+        $this->assertEquals(19, $sut->getVat());
+        $this->assertEquals('EUR', $sut->getCurrency());
+        $this->assertEquals(2521.12, $sut->getSubtotal());
+        $this->assertEquals(6600, $sut->getTimeWorked());
+        $this->assertEquals(3, count($sut->getEntries()));
+
+        $entries = $sut->getEntries();
+        $this->assertEquals(404.38, $entries[0]->getRate());
+        $this->assertEquals(2032.74, $entries[1]->getRate());
+        $this->assertEquals(84, $entries[2]->getRate());
+    }
+
+    public function testDescriptionByTimesheet()
+    {
+        $this->assertDescription(new ActivityInvoiceCalculator(), false, false);
+    }
+
+    public function testDescriptionByActivity()
+    {
+        $this->assertDescription(new ActivityInvoiceCalculator(), false, true);
+    }
+
+    public function testDescriptionByProject()
+    {
+        $this->assertDescription(new ActivityInvoiceCalculator(), true, false);
+    }
+}
diff --git a/translations/invoice-calculator.de.xliff b/translations/invoice-calculator.de.xliff
index cd635696..a5b65df7 100644
--- a/translations/invoice-calculator.de.xliff
+++ b/translations/invoice-calculator.de.xliff
@@ -18,6 +18,10 @@
                 <source>user</source>
                 <target>Benutzer gruppiert - ein Eintrag pro Benutzer</target>
             </trans-unit>
+            <trans-unit id="activity">
+                <source>activity</source>
+                <target>Aktivitäten gruppiert - ein Eintrag pro Aktivität</target>
+            </trans-unit>
         </body>
     </file>
 </xliff>
diff --git a/translations/invoice-calculator.en.xliff b/translations/invoice-calculator.en.xliff
index c9405b27..52cfbeae 100644
--- a/translations/invoice-calculator.en.xliff
+++ b/translations/invoice-calculator.en.xliff
@@ -18,6 +18,10 @@
                 <source>user</source>
                 <target>User grouped - one entry per user</target>
             </trans-unit>
+            <trans-unit id="activity">
+                <source>activity</source>
+                <target>Activity grouped - one entry per activity</target>
+            </trans-unit>
         </body>
     </file>
 </xliff>