diff --git a/core/ui/src/main/java/com/sadellie/unitto/core/ui/common/textfield/ExpressionTransformer.kt b/core/ui/src/main/java/com/sadellie/unitto/core/ui/common/textfield/ExpressionTransformer.kt
index 16481555..668772ce 100644
--- a/core/ui/src/main/java/com/sadellie/unitto/core/ui/common/textfield/ExpressionTransformer.kt
+++ b/core/ui/src/main/java/com/sadellie/unitto/core/ui/common/textfield/ExpressionTransformer.kt
@@ -34,21 +34,24 @@ class ExpressionTransformer(private val formatterSymbols: FormatterSymbols) : Vi
     }
 
     inner class ExpressionMapping(
-        private val unformatted: String,
-        private val formatted: String
+        private val original: String,
+        private val transformed: String
     ) : OffsetMapping {
         // Called when entering text (on each text change)
         // Basically moves cursor to the right position
         //
         // original input is "1000" and cursor is placed at the end "1000|"
-        // the formatted is "1,000" where cursor should be? - "1,000|"
+        // the transformed is "1,000" where cursor should be? - "1,000|"
         override fun originalToTransformed(offset: Int): Int {
-            val unformattedSubstr = unformatted.take(offset)
+            if (offset <= 0) return 0
+            if (offset >= original.length) return transformed.length
+
+            val unformattedSubstr = original.take(offset)
             var buffer = ""
             var groupings = 0
 
             run {
-                formatted.forEach {
+                transformed.forEach {
                     when (it) {
                         formatterSymbols.grouping.first() -> groupings++
                         formatterSymbols.fractional.first() -> buffer += "."
@@ -58,18 +61,21 @@ class ExpressionTransformer(private val formatterSymbols: FormatterSymbols) : Vi
                 }
             }
 
-            return formatted.fixCursor(buffer.length + groupings, formatterSymbols.grouping)
+            return transformed.fixCursor(buffer.length + groupings, formatterSymbols.grouping)
         }
 
-        // Called when clicking formatted text
+        // Called when clicking transformed text
         // Snaps cursor to the right position
         //
-        // the formatted is "1,000" and cursor is placed at the end "1,000|"
+        // the transformed is "1,000" and cursor is placed at the end "1,000|"
         // original input is "1000" where cursor should be? - "1000|"
         override fun transformedToOriginal(offset: Int): Int {
+            if (offset <= 0) return 0
+            if (offset >= transformed.length) return original.length
+
             val grouping = formatterSymbols.grouping.first()
-            val fixedCursor = formatted.fixCursor(offset, formatterSymbols.grouping)
-            val addedSymbols = formatted.take(fixedCursor).count { it == grouping }
+            val fixedCursor = transformed.fixCursor(offset, formatterSymbols.grouping)
+            val addedSymbols = transformed.take(fixedCursor).count { it == grouping }
             return fixedCursor - addedSymbols
         }
     }
diff --git a/core/ui/src/test/java/com/sadellie/unitto/core/ui/ExpressionTransformerTest.kt b/core/ui/src/test/java/com/sadellie/unitto/core/ui/ExpressionTransformerTest.kt
new file mode 100644
index 00000000..430a76c3
--- /dev/null
+++ b/core/ui/src/test/java/com/sadellie/unitto/core/ui/ExpressionTransformerTest.kt
@@ -0,0 +1,68 @@
+/*
+ * Unitto is a unit converter for Android
+ * Copyright (c) 2023 Elshan Agaev
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.sadellie.unitto.core.ui
+
+import com.sadellie.unitto.core.ui.common.textfield.ExpressionTransformer
+import com.sadellie.unitto.core.ui.common.textfield.FormatterSymbols
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class ExpressionTransformerTest {
+
+    private val expr = ExpressionTransformer(FormatterSymbols.Comma)
+
+    private fun origToTrans(orig: String, trans: String, offset: Int): Int =
+        expr.ExpressionMapping(orig, trans).originalToTransformed(offset)
+
+    private fun transToOrig(trans: String, orig: String, offset: Int): Int =
+        expr.ExpressionMapping(orig, trans).transformedToOriginal(offset)
+
+    @Test
+    fun `test 1234`() {
+        // at the start
+        assertEquals(0, origToTrans("1,234", "1234", 0))
+        assertEquals(0, transToOrig("1,234", "1234", 0))
+
+        // somewhere in inside, no offset needed
+        assertEquals(1, origToTrans("1234", "1,234", 1))
+        assertEquals(1, transToOrig("1,234", "1234", 1))
+
+        // somewhere in inside, offset needed
+        assertEquals(1, transToOrig("1,234", "1234", 2))
+
+        // at the end
+        assertEquals(5, origToTrans("1234", "1,234", 4))
+        assertEquals(4, transToOrig("1,234", "1234", 5))
+    }
+
+    @Test
+    fun `test 123`() {
+        // at the start
+        assertEquals(0, origToTrans("123", "123", 0))
+        assertEquals(0, transToOrig("123", "123", 0))
+
+        // somewhere in inside
+        assertEquals(1, origToTrans("123", "123", 1))
+        assertEquals(1, transToOrig("123", "123", 1))
+
+        // at the end
+        assertEquals(3, origToTrans("123", "123", 3))
+        assertEquals(3, transToOrig("123", "123", 3))
+    }
+}