diff --git a/src/models/units/units.model.js b/src/models/units/units.model.js
index 1d762351..acd3f0d2 100644
--- a/src/models/units/units.model.js
+++ b/src/models/units/units.model.js
@@ -64,12 +64,12 @@ const virtualFields = {
       }
 
       const serialNumberPattern = this.getDataValue('serialNumberPattern');
-      const [unitBlockStart, unitBlockEnd] = transformSerialNumberBlock(
+      const [, , unitCount] = transformSerialNumberBlock(
         serialNumberBlock,
         serialNumberPattern,
       );
 
-      return Number(unitBlockEnd) - Number(unitBlockStart);
+      return unitCount;
     },
   },
 };
diff --git a/src/utils/helpers.js b/src/utils/helpers.js
index a1121e15..48a24bc1 100644
--- a/src/utils/helpers.js
+++ b/src/utils/helpers.js
@@ -89,7 +89,7 @@ export const transformSerialNumberBlock = (
     return [null, null, null];
   }
 
-  const blockStart = Number(unitBlocks[1]);
+  const blockStart = Number(unitBlocks[1]) - 1; // Unit blocks are inclusive bound
   const blockEnd = Number(unitBlocks[2]);
   return [blockStart, blockEnd, blockEnd - blockStart];
 };