diff --git a/index.js b/index.js
index cf6b894..2a0615b 100644
--- a/index.js
+++ b/index.js
@@ -1,8 +1,18 @@
+// TODO: Remove this ignore comment.
+// eslint-disable-next-line no-mixed-operators
+const toHex = (red, green, blue, alpha) => ((blue | green << 8 | red << 16) | 1 << 24).toString(16).slice(1) + alpha;
+
 export default function rgbHex(red, green, blue, alpha) {
-	const isPercent = (red + (alpha || '')).toString().includes('%');
+	let isPercent = (red + (alpha || '')).toString().includes('%');
+
+	if (typeof red === 'string' && !green) { // Single string parameter.
+		const parsed = parseCssRgbString(red);
+		if (!parsed) {
+			throw new TypeError('Invalid or unsupported color format.');
+		}
 
-	if (typeof red === 'string') {
-		[red, green, blue, alpha] = red.match(/(0?\.?\d+)%?\b/g).map(component => Number(component));
+		isPercent = false;
+		[red, green, blue, alpha] = parsed;
 	} else if (alpha !== undefined) {
 		alpha = Number.parseFloat(alpha);
 	}
@@ -31,7 +41,33 @@ export default function rgbHex(red, green, blue, alpha) {
 		alpha = '';
 	}
 
-	// TODO: Remove this ignore comment.
-	// eslint-disable-next-line no-mixed-operators
-	return ((blue | green << 8 | red << 16) | 1 << 24).toString(16).slice(1) + alpha;
+	return toHex(red, green, blue, alpha);
 }
+
+const parseCssRgbString = input => {
+	const parts = input.replace(/rgba?\(([^)]+)\)/, '$1').split(/[,\s/]+/).filter(Boolean);
+	if (parts.length < 3) {
+		return;
+	}
+
+	const parseValue = (value, max) => {
+		value = value.trim();
+
+		if (value.endsWith('%')) {
+			return Math.min(Number.parseFloat(value) * max / 100, max);
+		}
+
+		return Math.min(Number.parseFloat(value), max);
+	};
+
+	const red = parseValue(parts[0], 255);
+	const green = parseValue(parts[1], 255);
+	const blue = parseValue(parts[2], 255);
+	let alpha;
+
+	if (parts.length === 4) {
+		alpha = parseValue(parts[3], 1);
+	}
+
+	return [red, green, blue, alpha];
+};
diff --git a/test.js b/test.js
index 615e522..324dfb1 100644
--- a/test.js
+++ b/test.js
@@ -4,16 +4,30 @@ import rgbHex from './index.js';
 test('main', t => {
 	t.is(rgbHex(0, 0, 0), '000000');
 	t.is(rgbHex(65, 131, 196), '4183c4');
-	t.is(rgbHex('255 154 253'), 'ff9afd');
-	t.is(rgbHex('rgb(40, 42, 54)'), '282a36');
 	t.is(rgbHex(0, 0, 0, 0), '00000000');
 	t.is(rgbHex(0, 0, 0, 0.5), '00000080');
 	t.is(rgbHex(0, 0, 0, '50%'), '00000080');
 	t.is(rgbHex(0, 0, 0, '100%'), '000000ff');
 	t.is(rgbHex(65, 131, 196, 0.2), '4183c433');
-	t.is(rgbHex('255 154 253, 0.8'), 'ff9afdcc');
-	t.is(rgbHex('160 82 45 .4'), 'a0522d66');
 	t.is(rgbHex(40, 42, 54, 0.75), '282a36bf');
 	t.is(rgbHex(40, 42, 54, '75%'), '282a36bf');
+});
+
+test('string - value', t => {
 	t.is(rgbHex('rgba(40, 42, 54, 75%)'), '282a36bf');
+	t.is(rgbHex('rgba(      40,      42, 54, 75% )'), '282a36bf');
+	t.is(rgbHex('40, 42, 54, 75%'), '282a36bf');
+	t.is(rgbHex('40, 42, 54'), '282a36');
+	t.is(rgbHex('255 154 253'), 'ff9afd');
+	t.is(rgbHex('255 154 253, 0.8'), 'ff9afdcc');
+	t.is(rgbHex('160 82 45 .4'), 'a0522d66');
+});
+
+test('string - percentage', t => {
+	t.is(rgbHex('rgb(30%, 20%, 50%)'), '4c337f');
+	t.is(rgbHex('100%, 50%, 0%'), 'ff7f00');
+	t.is(rgbHex('30%, 20%, 50%'), '4c337f');
+	t.is(rgbHex('30%, 20%, 50%, 50%'), '4c337f80');
+	t.is(rgbHex('30% 20% 50%'), '4c337f');
+	t.is(rgbHex('30% 20% 50% / 50%'), '4c337f80');
 });