diff --git a/src/builtin/math.rs b/src/builtin/math.rs
index 6d7d122..aa75042 100644
--- a/src/builtin/math.rs
+++ b/src/builtin/math.rs
@@ -30,4 +30,10 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             _ => todo!("expected number in builtin function `floor()`")
         }
     });
+    decl!(f "abs", |args, _| {
+        match arg!(args, 0, "number").eval() {
+            Value::Dimension(n, u) => Some(Value::Dimension(n.abs(), u)),
+            _ => todo!("expected number in builtin function `abs()`")
+        }
+    });
 }
diff --git a/src/value/number.rs b/src/value/number.rs
index 0f73979..7e068b7 100644
--- a/src/value/number.rs
+++ b/src/value/number.rs
@@ -4,6 +4,7 @@ use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Rem, RemAssign, S
 
 use num_bigint::BigInt;
 use num_rational::BigRational;
+use num_traits::sign::Signed;
 
 const PRECISION: usize = 10;
 
@@ -42,6 +43,12 @@ impl Number {
             val: self.val.floor(),
         }
     }
+
+    pub fn abs(self) -> Self {
+        Number {
+            val: self.val.abs(),
+        }
+    }
 }
 
 impl fmt::LowerHex for Number {
diff --git a/tests/math.rs b/tests/math.rs
index 4218c2d..a98d766 100644
--- a/tests/math.rs
+++ b/tests/math.rs
@@ -48,3 +48,18 @@ test!(
     "a {\n  color: ceil(10.6px);\n}\n",
     "a {\n  color: 11px;\n}\n"
 );
+test!(
+    abs_positive,
+    "a {\n  color: abs(10);\n}\n",
+    "a {\n  color: 10;\n}\n"
+);
+test!(
+    abs_negative,
+    "a {\n  color: abs(-10);\n}\n",
+    "a {\n  color: 10;\n}\n"
+);
+test!(
+    abs_unit,
+    "a {\n  color: abs(-10px);\n}\n",
+    "a {\n  color: 10px;\n}\n"
+);