diff --git a/src/Number/Vat.php b/src/Number/Vat.php
index a31b703..cd0c01b 100644
--- a/src/Number/Vat.php
+++ b/src/Number/Vat.php
@@ -18,7 +18,7 @@ public function diff(float $number): float
 
 	public function without(float $number): float
 	{
-		return Utils\Percentage::without($this->ratio, $number);
+		return Utils\Percentage::without($number, $this->ratio);
 	}
 
 }
diff --git a/src/Utils/Percentage.php b/src/Utils/Percentage.php
index 8e56603..bab7cbf 100644
--- a/src/Utils/Percentage.php
+++ b/src/Utils/Percentage.php
@@ -25,6 +25,18 @@ public static function ratio(float $smallRatio): float
 	}
 
 
+	public static function calculate(float $part, float $total): float
+	{
+		return self::without($part, $total) * 100;
+	}
+
+
+	public static function calculateRemainder(float $part, float $total): float
+	{
+		return 100 - self::calculate($part, $total);
+	}
+
+
 	/**
 	 * N + 17.5% -> N * 1.175
 	 */
@@ -44,11 +56,15 @@ public static function diffWith(float $ratio, float $number): float
 
 
 	/**
+	 * Safe division
 	 * If you have price with percentage, and you need price without percentage.
 	 * N / 1.175
 	 */
-	public static function without(float $ratio, float $number): float
+	public static function without(float $number, float $ratio): float
 	{
+		if ($ratio === 0.0) {
+			return 0.0;
+		}
 		return $number / $ratio;
 	}
 
@@ -58,7 +74,7 @@ public static function without(float $ratio, float $number): float
 	 */
 	public static function diffWithout(float $ratio, float $number): float
 	{
-		return $number - self::without($ratio, $number);
+		return $number - self::without($number, $ratio);
 	}
 
 
@@ -76,6 +92,6 @@ public static function deduct(float $percentage, float $number): float
 	 */
 	public static function diffDeduct(float $percentage, float $number): float
 	{
-		return self::smallRatio($number * $percentage);
+		return self::with(self::smallRatio($percentage), $number);
 	}
 }
diff --git a/tests/src/Utils/PercentageTest.php b/tests/src/Utils/PercentageTest.php
new file mode 100644
index 0000000..c47382a
--- /dev/null
+++ b/tests/src/Utils/PercentageTest.php
@@ -0,0 +1,172 @@
+<?php declare(strict_types=1);
+
+namespace h4kuna\Format\Tests\Utils;
+
+use h4kuna\Format\Tests\TestCase;
+use h4kuna\Format\Utils\Percentage;
+use Tester\Assert;
+
+require __DIR__ . '/../../bootstrap.php';
+
+final class PercentageTest extends TestCase
+{
+
+	/**
+	 * @dataProvider providePercentage
+	 */
+	public function testPercentage(float $result, float $part, float $total): void
+	{
+		Assert::same($result, Percentage::calculate($part, $total));
+	}
+
+
+	/**
+	 * @return array<mixed>
+	 */
+	protected function providePercentage(): array
+	{
+		return [
+			[0.0, 1.0, 0.0],
+			[0.0, 0.0, 1.0],
+			[-100.0, 1.0, -1.0],
+			[120, 108, 90],
+			[120, 120, 100],
+			[120, -120, -100],
+		];
+	}
+
+	/**
+	 * @dataProvider provideCalculateRemainder
+	 */
+	public function testCalculateRemainder(float $result, float $part, float $total): void
+	{
+		Assert::same($result, Percentage::calculateRemainder($part, $total));
+	}
+
+
+	/**
+	 * @return array<mixed>
+	 */
+	protected function provideCalculateRemainder(): array
+	{
+		return [
+			[100.0, 1.0, 0.0],
+			[100.0, 0.0, 1.0],
+			[200.0, 1.0, -1.0],
+			[20, 80, 100],
+			[-20, 120, 100],
+			[-20, -120, -100],
+		];
+	}
+
+
+	/**
+	 * @dataProvider provideSmallRatio
+	 */
+	public function testSmallRatio(float $expected, float $percentage): void
+	{
+		Assert::same($expected, Percentage::smallRatio($percentage));
+	}
+
+
+	/**
+	 * @return array<mixed>
+	 */
+	protected function provideSmallRatio(): array
+	{
+		return [
+			[0, 0],
+			[0.2, 20],
+			[-0.2, -20],
+		];
+	}
+
+
+	/**
+	 * @dataProvider provideRatio
+	 */
+	public function testRatio(float $expected, float $percentage): void
+	{
+		Assert::same($expected, Percentage::ratio($percentage));
+	}
+
+
+	/**
+	 * @return array<mixed>
+	 */
+	protected function provideRatio(): array
+	{
+		return [
+			[1.2, 0.2],
+			[1.0, 0.0],
+			[0.8, -0.2],
+		];
+	}
+
+
+	/**
+	 * @dataProvider provideDiffDeduct
+	 */
+	public function testDiffDeduct(float $expected, float $percentage, float $number): void
+	{
+		Assert::same($expected, Percentage::diffDeduct($percentage, $number));
+	}
+
+
+	/**
+	 * @return array<mixed>
+	 */
+	protected function provideDiffDeduct(): array
+	{
+		return [
+			[20, 20, 100],
+			[24, 20, 120],
+			[0, 20, 0],
+		];
+	}
+
+
+	/**
+	 * @dataProvider provideDeduct
+	 */
+	public function testDeduct(float $expected, float $percentage, float $number): void
+	{
+		Assert::same($expected, Percentage::deduct($percentage, $number));
+	}
+
+
+	/**
+	 * @return array<mixed>
+	 */
+	protected function provideDeduct(): array
+	{
+		return [
+			[80, 20, 100],
+			[96, 20, 120],
+			[0, 20, 0],
+		];
+	}
+
+
+	/**
+	 * @dataProvider provideWithout
+	 */
+	public function testWithout(float $expected, float $ratio, float $number): void
+	{
+		Assert::same($expected, Percentage::without($number, $ratio));
+	}
+
+
+	/**
+	 * @return array<mixed>
+	 */
+	protected function provideWithout(): array
+	{
+		return [
+			[100, 1.2, 120],
+			[0, 1.2, 0],
+		];
+	}
+}
+
+(new PercentageTest())->run();