diff --git a/servo/components/style/values/computed/length.rs b/servo/components/style/values/computed/length.rs
index 8c90d38eefa0..5878fde0f512 100644
--- a/servo/components/style/values/computed/length.rs
+++ b/servo/components/style/values/computed/length.rs
@@ -223,7 +223,7 @@ impl specified::CalcLengthOrPercentage {
         let mut length = Au(0);
 
         if let Some(absolute) = self.absolute {
-            length += zoom_fn(absolute);
+            length += zoom_fn(absolute.to_computed_value(context));
         }
 
         for val in &[self.vw.map(ViewportPercentageLength::Vw),
@@ -269,7 +269,7 @@ impl ToComputedValue for specified::CalcLengthOrPercentage {
     fn from_computed_value(computed: &CalcLengthOrPercentage) -> Self {
         specified::CalcLengthOrPercentage {
             clamping_mode: computed.clamping_mode,
-            absolute: Some(computed.length),
+            absolute: Some(AbsoluteLength::from_computed_value(&computed.length)),
             percentage: computed.percentage,
             ..Default::default()
         }
diff --git a/servo/components/style/values/specified/calc.rs b/servo/components/style/values/specified/calc.rs
index 93db7730f375..408a5f4ab5b4 100644
--- a/servo/components/style/values/specified/calc.rs
+++ b/servo/components/style/values/specified/calc.rs
@@ -6,7 +6,6 @@
 //!
 //! [calc]: https://drafts.csswg.org/css-values/#calc-notation
 
-use app_units::Au;
 use cssparser::{Parser, Token, BasicParseError};
 use parser::ParserContext;
 use std::ascii::AsciiExt;
@@ -16,7 +15,8 @@ use style_traits::values::specified::AllowedLengthType;
 use values::{CSSInteger, CSSFloat};
 use values::computed;
 use values::specified::{Angle, Time};
-use values::specified::length::{FontRelativeLength, NoCalcLength, ViewportPercentageLength};
+use values::specified::length::{AbsoluteLength, FontRelativeLength, NoCalcLength};
+use values::specified::length::ViewportPercentageLength;
 
 /// A node inside a `Calc` expression's AST.
 #[derive(Clone, Debug)]
@@ -68,7 +68,7 @@ pub enum CalcUnit {
 #[allow(missing_docs)]
 pub struct CalcLengthOrPercentage {
     pub clamping_mode: AllowedLengthType,
-    pub absolute: Option<Au>,
+    pub absolute: Option<AbsoluteLength>,
     pub vw: Option<CSSFloat>,
     pub vh: Option<CSSFloat>,
     pub vmin: Option<CSSFloat>,
@@ -118,6 +118,17 @@ impl ToCss for CalcLengthOrPercentage {
             };
         }
 
+        macro_rules! serialize_abs {
+            ( $( $val:ident ),+ ) => {
+                $(
+                    if let Some(AbsoluteLength::$val(v)) = self.absolute {
+                        first_value_check!(v);
+                        AbsoluteLength::$val(v.abs()).to_css(dest)?;
+                    }
+                )+
+            };
+        }
+
         dest.write_str("calc(")?;
 
         // NOTE(emilio): Percentages first because of web-compat problems, see:
@@ -129,18 +140,17 @@ impl ToCss for CalcLengthOrPercentage {
 
         // NOTE(emilio): The order here it's very intentional, and alphabetic
         // per the spec linked above.
-        serialize!(ch, em, ex);
+        serialize!(ch);
+        serialize_abs!(Cm);
+        serialize!(em, ex);
+        serialize_abs!(In);
 
         #[cfg(feature = "gecko")]
         {
             serialize!(mozmm);
         }
 
-        if let Some(val) = self.absolute {
-            first_value_check!(val);
-            val.abs().to_css(dest)?;
-        }
-
+        serialize_abs!(Mm, Pc, Pt, Px, Q);
         serialize!(rem, vh, vmax, vmin, vw);
 
         dest.write_str(")")
@@ -349,8 +359,10 @@ impl CalcNode {
                 match *l {
                     NoCalcLength::Absolute(abs) => {
                         ret.absolute = Some(
-                            ret.absolute.unwrap_or(Au(0)) +
-                            Au::from(abs).scale_by(factor)
+                            match ret.absolute {
+                                Some(value) => value + abs * factor,
+                                None => abs * factor,
+                            }
                         );
                     }
                     NoCalcLength::FontRelative(rel) => {
diff --git a/servo/components/style/values/specified/length.rs b/servo/components/style/values/specified/length.rs
index 6a0bd08b880f..da8fda5ca32d 100644
--- a/servo/components/style/values/specified/length.rs
+++ b/servo/components/style/values/specified/length.rs
@@ -13,7 +13,7 @@ use font_metrics::FontMetricsQueryResult;
 use parser::{Parse, ParserContext};
 use std::{cmp, fmt, mem};
 use std::ascii::AsciiExt;
-use std::ops::Mul;
+use std::ops::{Add, Mul};
 use style_traits::{ToCss, ParseError, StyleParseError};
 use style_traits::values::specified::AllowedLengthType;
 use stylesheets::CssRuleType;
@@ -278,6 +278,23 @@ impl AbsoluteLength {
             | AbsoluteLength::Pc(v) => v == 0.,
         }
     }
+
+    /// Convert this into a pixel value.
+    #[inline]
+    pub fn to_px(&self) -> CSSFloat {
+        use std::f32;
+
+        let pixel = match *self {
+            AbsoluteLength::Px(value) => value,
+            AbsoluteLength::In(value) => value * (AU_PER_IN / AU_PER_PX),
+            AbsoluteLength::Cm(value) => value * (AU_PER_CM / AU_PER_PX),
+            AbsoluteLength::Mm(value) => value * (AU_PER_MM / AU_PER_PX),
+            AbsoluteLength::Q(value) => value * (AU_PER_Q / AU_PER_PX),
+            AbsoluteLength::Pt(value) => value * (AU_PER_PT / AU_PER_PX),
+            AbsoluteLength::Pc(value) => value * (AU_PER_PC / AU_PER_PX),
+        };
+        pixel.min(f32::MAX).max(f32::MIN)
+    }
 }
 
 impl ToComputedValue for AbsoluteLength {
@@ -337,6 +354,24 @@ impl Mul<CSSFloat> for AbsoluteLength {
     }
 }
 
+impl Add<AbsoluteLength> for AbsoluteLength {
+    type Output = Self;
+
+    #[inline]
+    fn add(self, rhs: Self) -> Self {
+        match (self, rhs) {
+            (AbsoluteLength::Px(x), AbsoluteLength::Px(y)) => AbsoluteLength::Px(x + y),
+            (AbsoluteLength::In(x), AbsoluteLength::In(y)) => AbsoluteLength::In(x + y),
+            (AbsoluteLength::Cm(x), AbsoluteLength::Cm(y)) => AbsoluteLength::Cm(x + y),
+            (AbsoluteLength::Mm(x), AbsoluteLength::Mm(y)) => AbsoluteLength::Mm(x + y),
+            (AbsoluteLength::Q(x), AbsoluteLength::Q(y)) => AbsoluteLength::Q(x + y),
+            (AbsoluteLength::Pt(x), AbsoluteLength::Pt(y)) => AbsoluteLength::Pt(x + y),
+            (AbsoluteLength::Pc(x), AbsoluteLength::Pc(y)) => AbsoluteLength::Pc(x + y),
+            _ => AbsoluteLength::Px(self.to_px() + rhs.to_px()),
+        }
+    }
+}
+
 /// Represents a physical length (mozmm) based on DPI
 #[derive(Clone, Copy, Debug, PartialEq)]
 #[cfg(feature = "gecko")]