diff --git a/parser.jison b/parser.jison
index 36cb668..b46b718 100644
--- a/parser.jison
+++ b/parser.jison
@@ -51,6 +51,8 @@
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)cqb\b             return 'CQBS';
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)cqmin\b           return 'CQMINS';
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)cqmax\b           return 'CQMAXS';
+(([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)lh\b              return 'LHS';
+(([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)rlh\b             return 'RLHS';
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)cm\b              return 'LENGTH';
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)mm\b              return 'LENGTH';
 (([0-9]+("."[0-9]+)?|"."[0-9]+)(e(\+|-)[0-9]+)?)Q\b               return 'LENGTH';
@@ -150,7 +152,9 @@ expression
     | CQIS { $$ = { type: 'CqiValue', value: parseFloat($1), unit: 'cqi' }; }
     | CQBS { $$ = { type: 'CqbValue', value: parseFloat($1), unit: 'cqb' }; }
     | CQMINS { $$ = { type: 'CqminValue', value: parseFloat($1), unit: 'cqmin' }; }
-    | CQMAXS { $$ = { type: 'CqmaxValue', value: parseFloat($1), unit: 'cqmax' }; }            
+    | CQMAXS { $$ = { type: 'CqmaxValue', value: parseFloat($1), unit: 'cqmax' }; }
+    | LHS { $$ = { type: 'LhValue', value: parseFloat($1), unit: 'lh' }; }
+    | RLHS { $$ = { type: 'RlhValue', value: parseFloat($1), unit: 'rlh' }; }
     | PERCENTAGE { $$ = { type: 'PercentageValue', value: parseFloat($1), unit: '%' }; }
     | ADD dimension { var prev = $2; $$ = prev; }
     | SUB dimension { var prev = $2; prev.value *= -1; $$ = prev; }
diff --git a/src/lib/reducer.js b/src/lib/reducer.js
index b7a6e28..92cc4a3 100644
--- a/src/lib/reducer.js
+++ b/src/lib/reducer.js
@@ -47,6 +47,8 @@ function isValueType(node) {
     case 'CqminValue':
     case 'CqmaxValue':
     case 'PercentageValue':
+    case 'LhValue':
+    case 'RlhValue':
     case 'Number':
       return true;
   }
diff --git a/src/parser.d.ts b/src/parser.d.ts
index 5ab6579..e3a19fb 100644
--- a/src/parser.d.ts
+++ b/src/parser.d.ts
@@ -51,7 +51,9 @@ export interface DimensionExpression {
     | 'CqbValue'
     | 'CqiValue'
     | 'CqminValue'
-    | 'CqmaxValue';    
+    | 'CqmaxValue'
+    | 'LhValue'
+    | 'RlhValue';
   value: number;
   unit: string;
 }
diff --git a/test/index.js b/test/index.js
index 635155e..a2f3d16 100644
--- a/test/index.js
+++ b/test/index.js
@@ -305,6 +305,26 @@ test(
   testValue('calc(100svmax - 44.5svh)', 'calc(100svmax - 44.5svh)')
 );
 
+test(
+  'should add numbers with lh units',
+  testValue('calc(1lh + 4lh)', '5lh')
+);
+
+test(
+  'should add numbers with rlh units',
+  testValue('calc(1rlh + 4rlh)', '5rlh')
+);
+
+test(
+  'should not combine different lh units',
+  testValue('calc(1lh + 4rlh)', 'calc(1lh + 4rlh)')
+);
+
+test(
+  'should not combine different lh units',
+  testValue('calc(1lh + 20px)', 'calc(1lh + 20px)')
+);
+
 test(
   'should parse fractions without leading zero',
   testValue('calc(2rem - .14285em)', 'calc(2rem - 0.14285em)')