diff --git a/src/TokenStream.js b/src/TokenStream.js
index 721eb87..59a3e1a 100644
--- a/src/TokenStream.js
+++ b/src/TokenStream.js
@@ -1,9 +1,10 @@
 const SYMBOL_MATCH = 'SYMBOL_MATCH'
 
 export default class TokenStream {
-  constructor(nodes, parent) {
+  constructor(nodes, units, parent) {
     this.index = 0
     this.nodes = nodes
+    this.units = units
     this.functionName = parent != null ? parent.value : null
     this.lastValue = null
     this.rewindIndex = -1
@@ -20,7 +21,7 @@ export default class TokenStream {
 
     for (let i = 0; i < tokenDescriptors.length; i += 1) {
       const tokenDescriptor = tokenDescriptors[i]
-      const value = tokenDescriptor(node)
+      const value = tokenDescriptor(node, this.units)
       if (value !== null) {
         this.index += 1
         this.lastValue = value
@@ -43,7 +44,7 @@ export default class TokenStream {
   matchesFunction() {
     const node = this.nodes[this.index]
     if (node.type !== 'function') return null
-    const value = new TokenStream(node.nodes, node)
+    const value = new TokenStream(node.nodes, this.units, node)
     this.index += 1
     this.lastValue = null
     return value
diff --git a/src/__tests__/flex.js b/src/__tests__/flex.js
index 2e1d594..21f8461 100644
--- a/src/__tests__/flex.js
+++ b/src/__tests__/flex.js
@@ -1,5 +1,17 @@
 import transformCss from '..'
 
+it('transforms with viewport units', () => {
+  expect(
+    transformCss([['flex', '1 2 10vw']], undefined, {
+      vw: 3.2,
+    })
+  ).toEqual({
+    flexGrow: 1,
+    flexShrink: 2,
+    flexBasis: 32,
+  })
+})
+
 it('transforms flex shorthand with 3 values', () => {
   expect(transformCss([['flex', '1 2 3px']])).toEqual({
     flexGrow: 1,
diff --git a/src/index.js b/src/index.js
index 0afce0a..7860d81 100644
--- a/src/index.js
+++ b/src/index.js
@@ -4,6 +4,7 @@ import camelizeStyleName from 'camelize'
 import transforms from './transforms/index'
 import devPropertiesWithoutUnitsRegExp from './devPropertiesWithoutUnitsRegExp'
 import TokenStream from './TokenStream'
+import { userUnitRe } from './tokenTypes'
 
 // Note if this is wrong, you'll need to change tokenTypes.js too
 const numberOrLengthRe = /^([+-]?(?:\d*\.)?\d+(?:e[+-]?\d+)?)(?:px)?$/i
@@ -13,7 +14,7 @@ const nullRe = /^null$/i
 const undefinedRe = /^undefined$/i
 
 // Undocumented export
-export const transformRawValue = (propName, value) => {
+export const transformRawValue = (propName, value, units) => {
   if (process.env.NODE_ENV !== 'production') {
     const needsUnit = !devPropertiesWithoutUnitsRegExp.test(propName)
     const isNumberWithoutUnit = numberOnlyRe.test(value)
@@ -30,6 +31,12 @@ export const transformRawValue = (propName, value) => {
   const numberMatch = value.match(numberOrLengthRe)
   if (numberMatch !== null) return Number(numberMatch[1])
 
+  const userUnitsMatch = value.match(userUnitRe)
+  if (userUnitsMatch != null) {
+    const { 1: length, 2: unit } = userUnitsMatch
+    return unit in units ? Number(length) * units[unit] : null
+  }
+
   const boolMatch = value.match(boolRe)
   if (boolMatch !== null) return boolMatch[0].toLowerCase() === 'true'
 
@@ -42,30 +49,35 @@ export const transformRawValue = (propName, value) => {
   return value
 }
 
-const baseTransformShorthandValue = (propName, value) => {
+const baseTransformShorthandValue = (propName, value, units) => {
   const ast = parse(value)
-  const tokenStream = new TokenStream(ast.nodes)
+  const tokenStream = new TokenStream(ast.nodes, units)
   return transforms[propName](tokenStream)
 }
 
 const transformShorthandValue =
   process.env.NODE_ENV === 'production'
     ? baseTransformShorthandValue
-    : (propName, value) => {
+    : (propName, value, units) => {
         try {
-          return baseTransformShorthandValue(propName, value)
+          return baseTransformShorthandValue(propName, value, units)
         } catch (e) {
           throw new Error(`Failed to parse declaration "${propName}: ${value}"`)
         }
       }
 
-export const getStylesForProperty = (propName, inputValue, allowShorthand) => {
+export const getStylesForProperty = (
+  propName,
+  inputValue,
+  allowShorthand,
+  units
+) => {
   const isRawValue = allowShorthand === false || !(propName in transforms)
   const value = inputValue.trim()
 
   const propValues = isRawValue
-    ? { [propName]: transformRawValue(propName, value) }
-    : transformShorthandValue(propName, value)
+    ? { [propName]: transformRawValue(propName, value, units) }
+    : transformShorthandValue(propName, value, units)
 
   return propValues
 }
@@ -78,13 +90,13 @@ export const getPropertyName = propName => {
   return camelizeStyleName(propName)
 }
 
-export default (rules, shorthandBlacklist = []) =>
+export default (rules, shorthandBlacklist = [], units = {}) =>
   rules.reduce((accum, rule) => {
     const propertyName = getPropertyName(rule[0])
     const value = rule[1]
     const allowShorthand = shorthandBlacklist.indexOf(propertyName) === -1
     return Object.assign(
       accum,
-      getStylesForProperty(propertyName, value, allowShorthand)
+      getStylesForProperty(propertyName, value, allowShorthand, units)
     )
   }, {})
diff --git a/src/tokenTypes.js b/src/tokenTypes.js
index 497001d..0b2bab7 100644
--- a/src/tokenTypes.js
+++ b/src/tokenTypes.js
@@ -34,7 +34,7 @@ const identRe = /(^-?[_a-z][_a-z0-9-]*$)/i
 const numberRe = /^([+-]?(?:\d*\.)?\d+(?:e[+-]?\d+)?)$/i
 // Note lengthRe is sneaky: you can omit units for 0
 const lengthRe = /^(0$|(?:[+-]?(?:\d*\.)?\d+(?:e[+-]?\d+)?)(?=px$))/i
-const unsupportedUnitRe = /^([+-]?(?:\d*\.)?\d+(?:e[+-]?\d+)?(ch|em|ex|rem|vh|vw|vmin|vmax|cm|mm|in|pc|pt))$/i
+export const userUnitRe = /^([+-]?(?:\d*\.)?\d+(?:e[+-]?\d+)?)(ch|em|ex|rem|vh|vw|vmin|vmax|cm|mm|in|pc|pt)$/i
 const angleRe = /^([+-]?(?:\d*\.)?\d+(?:e[+-]?\d+)?(?:deg|rad))$/i
 const percentRe = /^([+-]?(?:\d*\.)?\d+(?:e[+-]?\d+)?%)$/i
 
@@ -66,7 +66,16 @@ export const NONE = regExpToken(noneRe)
 export const AUTO = regExpToken(autoRe)
 export const NUMBER = regExpToken(numberRe, Number)
 export const LENGTH = regExpToken(lengthRe, Number)
-export const UNSUPPORTED_LENGTH_UNIT = regExpToken(unsupportedUnitRe)
+export const USER_LENGTH_UNIT = (node, units) => {
+  if (node.type !== 'word') return null
+
+  const match = node.value.match(userUnitRe)
+  if (match === null) return null
+
+  const { 1: length, 2: unit } = match
+
+  return unit in units ? Number(length) * units[unit] : null
+}
 export const ANGLE = regExpToken(angleRe, angle => angle.toLowerCase())
 export const PERCENT = regExpToken(percentRe)
 export const IDENT = regExpToken(identRe)
diff --git a/src/transforms/border.js b/src/transforms/border.js
index d4c77b4..c846624 100644
--- a/src/transforms/border.js
+++ b/src/transforms/border.js
@@ -3,7 +3,7 @@ import {
   NONE,
   COLOR,
   LENGTH,
-  UNSUPPORTED_LENGTH_UNIT,
+  USER_LENGTH_UNIT,
   SPACE,
 } from '../tokenTypes'
 
@@ -29,7 +29,7 @@ export default tokenStream => {
 
     if (
       borderWidth === undefined &&
-      tokenStream.matches(LENGTH, UNSUPPORTED_LENGTH_UNIT)
+      tokenStream.matches(LENGTH, USER_LENGTH_UNIT)
     ) {
       borderWidth = tokenStream.lastValue
     } else if (borderColor === undefined && tokenStream.matches(COLOR)) {
diff --git a/src/transforms/flex.js b/src/transforms/flex.js
index cf9bdbf..3fb9114 100644
--- a/src/transforms/flex.js
+++ b/src/transforms/flex.js
@@ -3,7 +3,7 @@ import {
   AUTO,
   NUMBER,
   LENGTH,
-  UNSUPPORTED_LENGTH_UNIT,
+  USER_LENGTH_UNIT,
   PERCENT,
   SPACE,
 } from '../tokenTypes'
@@ -43,7 +43,7 @@ export default tokenStream => {
       }
     } else if (
       flexBasis === undefined &&
-      tokenStream.matches(LENGTH, UNSUPPORTED_LENGTH_UNIT, PERCENT)
+      tokenStream.matches(LENGTH, USER_LENGTH_UNIT, PERCENT)
     ) {
       flexBasis = tokenStream.lastValue
     } else if (flexBasis === undefined && tokenStream.matches(AUTO)) {
diff --git a/src/transforms/font.js b/src/transforms/font.js
index ae5e5dd..01503da 100644
--- a/src/transforms/font.js
+++ b/src/transforms/font.js
@@ -3,7 +3,7 @@ import {
   regExpToken,
   SPACE,
   LENGTH,
-  UNSUPPORTED_LENGTH_UNIT,
+  USER_LENGTH_UNIT,
   SLASH,
 } from '../tokenTypes'
 
@@ -42,10 +42,10 @@ export default tokenStream => {
     numStyleWeightVariantMatched += 1
   }
 
-  const fontSize = tokenStream.expect(LENGTH, UNSUPPORTED_LENGTH_UNIT)
+  const fontSize = tokenStream.expect(LENGTH, USER_LENGTH_UNIT)
 
   if (tokenStream.matches(SLASH)) {
-    lineHeight = tokenStream.expect(LENGTH, UNSUPPORTED_LENGTH_UNIT)
+    lineHeight = tokenStream.expect(LENGTH, USER_LENGTH_UNIT)
   }
 
   tokenStream.expect(SPACE)
diff --git a/src/transforms/index.js b/src/transforms/index.js
index 66320ae..ca55ba9 100644
--- a/src/transforms/index.js
+++ b/src/transforms/index.js
@@ -3,7 +3,7 @@ import {
   WORD,
   COLOR,
   LENGTH,
-  UNSUPPORTED_LENGTH_UNIT,
+  USER_LENGTH_UNIT,
   PERCENT,
   AUTO,
 } from '../tokenTypes'
@@ -35,7 +35,7 @@ const borderRadius = directionFactory({
 })
 const borderWidth = directionFactory({ prefix: 'border', suffix: 'Width' })
 const margin = directionFactory({
-  types: [LENGTH, UNSUPPORTED_LENGTH_UNIT, PERCENT, AUTO],
+  types: [LENGTH, USER_LENGTH_UNIT, PERCENT, AUTO],
   prefix: 'margin',
 })
 const padding = directionFactory({ prefix: 'padding' })
diff --git a/src/transforms/util.js b/src/transforms/util.js
index b195567..512458a 100644
--- a/src/transforms/util.js
+++ b/src/transforms/util.js
@@ -1,6 +1,6 @@
 import {
   LENGTH,
-  UNSUPPORTED_LENGTH_UNIT,
+  USER_LENGTH_UNIT,
   PERCENT,
   COLOR,
   SPACE,
@@ -8,7 +8,7 @@ import {
 } from '../tokenTypes'
 
 export const directionFactory = ({
-  types = [LENGTH, UNSUPPORTED_LENGTH_UNIT, PERCENT],
+  types = [LENGTH, USER_LENGTH_UNIT, PERCENT],
   directions = ['Top', 'Right', 'Bottom', 'Left'],
   prefix = '',
   suffix = '',
@@ -65,16 +65,16 @@ export const parseShadow = tokenStream => {
 
     if (
       offsetX === undefined &&
-      tokenStream.matches(LENGTH, UNSUPPORTED_LENGTH_UNIT)
+      tokenStream.matches(LENGTH, USER_LENGTH_UNIT)
     ) {
       offsetX = tokenStream.lastValue
       tokenStream.expect(SPACE)
-      offsetY = tokenStream.expect(LENGTH, UNSUPPORTED_LENGTH_UNIT)
+      offsetY = tokenStream.expect(LENGTH, USER_LENGTH_UNIT)
 
       tokenStream.saveRewindPoint()
       if (
         tokenStream.matches(SPACE) &&
-        tokenStream.matches(LENGTH, UNSUPPORTED_LENGTH_UNIT)
+        tokenStream.matches(LENGTH, USER_LENGTH_UNIT)
       ) {
         radius = tokenStream.lastValue
       } else {