diff --git a/gui/main_gui.ui b/gui/main_gui.ui
index 72eb4aa..defbf8a 100644
--- a/gui/main_gui.ui
+++ b/gui/main_gui.ui
@@ -7,19 +7,19 @@
     <x>0</x>
     <y>0</y>
     <width>420</width>
-    <height>250</height>
+    <height>228</height>
    </rect>
   </property>
   <property name="minimumSize">
    <size>
     <width>420</width>
-    <height>250</height>
+    <height>228</height>
    </size>
   </property>
   <property name="maximumSize">
    <size>
     <width>420</width>
-    <height>250</height>
+    <height>228</height>
    </size>
   </property>
   <property name="windowTitle">
@@ -35,7 +35,7 @@
       <x>10</x>
       <y>0</y>
       <width>401</width>
-      <height>191</height>
+      <height>171</height>
      </rect>
     </property>
     <layout class="QFormLayout" name="formLayout">
@@ -58,6 +58,28 @@
        </property>
       </widget>
      </item>
+     <item row="0" column="1">
+      <widget class="QDoubleSpinBox" name="value_input">
+       <property name="frame">
+        <bool>true</bool>
+       </property>
+       <property name="buttonSymbols">
+        <enum>QAbstractSpinBox::ButtonSymbols::NoButtons</enum>
+       </property>
+       <property name="specialValueText">
+        <string>£</string>
+       </property>
+       <property name="correctionMode">
+        <enum>QAbstractSpinBox::CorrectionMode::CorrectToNearestValue</enum>
+       </property>
+       <property name="prefix">
+        <string>£</string>
+       </property>
+       <property name="maximum">
+        <double>999999999.990000009536743</double>
+       </property>
+      </widget>
+     </item>
      <item row="1" column="0" colspan="2">
       <widget class="QLabel" name="mix_lab">
        <property name="text">
@@ -91,13 +113,6 @@
        </property>
       </widget>
      </item>
-     <item row="3" column="1">
-      <widget class="QComboBox" name="share_trades_combo">
-       <property name="editable">
-        <bool>true</bool>
-       </property>
-      </widget>
-     </item>
      <item row="4" column="0">
       <widget class="QLabel" name="fund_trades_lab">
        <property name="text">
@@ -114,31 +129,19 @@
      </item>
      <item row="5" column="0" colspan="2">
       <widget class="QPushButton" name="calc_but">
+       <property name="enabled">
+        <bool>false</bool>
+       </property>
        <property name="text">
         <string>Calculate</string>
        </property>
       </widget>
      </item>
-     <item row="0" column="1">
-      <widget class="QDoubleSpinBox" name="value_input">
-       <property name="frame">
+     <item row="3" column="1">
+      <widget class="QComboBox" name="share_trades_combo">
+       <property name="editable">
         <bool>true</bool>
        </property>
-       <property name="buttonSymbols">
-        <enum>QAbstractSpinBox::ButtonSymbols::NoButtons</enum>
-       </property>
-       <property name="specialValueText">
-        <string>£</string>
-       </property>
-       <property name="correctionMode">
-        <enum>QAbstractSpinBox::CorrectionMode::CorrectToNearestValue</enum>
-       </property>
-       <property name="prefix">
-        <string>£</string>
-       </property>
-       <property name="maximum">
-        <double>999999999.990000009536743</double>
-       </property>
       </widget>
      </item>
     </layout>
@@ -172,6 +175,13 @@
    </property>
   </action>
  </widget>
+ <tabstops>
+  <tabstop>value_input</tabstop>
+  <tabstop>mix_slider</tabstop>
+  <tabstop>share_trades_combo</tabstop>
+  <tabstop>fund_trades_combo</tabstop>
+  <tabstop>calc_but</tabstop>
+ </tabstops>
  <resources/>
  <connections/>
 </ui>
diff --git a/src/main_window.py b/src/main_window.py
index 5d458b3..14060e3 100644
--- a/src/main_window.py
+++ b/src/main_window.py
@@ -1,5 +1,5 @@
 from PyQt6.QtGui import QIntValidator
-from PyQt6.QtWidgets import QMainWindow
+from PyQt6.QtWidgets import QMainWindow, QWidget
 from PyQt6 import uic
 
 import output_window
@@ -7,7 +7,7 @@ import output_window
 
 class SIPPCompare(QMainWindow):
     # Receive instance of PlatformEdit() as parameter
-    def __init__(self, plat_edit_win):
+    def __init__(self, plat_edit_win: QWidget):
         super().__init__()
         # Import Qt Designer UI XML file
         uic.loadUi("gui/main_gui.ui", self)
@@ -39,6 +39,9 @@ class SIPPCompare(QMainWindow):
         self.actionEdit_Platforms.triggered.connect(self.show_platform_edit)
         # Update percentage mix label when slider moved
         self.mix_slider.valueChanged.connect(self.update_slider_lab)
+        #self.value_input.valueChanged.connect(self.check_valid)
+        self.share_trades_combo.currentTextChanged.connect(self.check_valid)
+        self.fund_trades_combo.currentTextChanged.connect(self.check_valid)
 
         # Set validators
         self.share_trades_combo.setValidator(QIntValidator(0, 999))
@@ -50,6 +53,13 @@ class SIPPCompare(QMainWindow):
         mix_lab_str = f"Investment mix (funds {slider_val}% / shares {100 - slider_val}%)"
         self.mix_lab.setText(mix_lab_str)
 
+    def check_valid(self):
+        if self.share_trades_combo.currentText() != "" \
+        and self.fund_trades_combo.currentText() != "":
+            self.calc_but.setEnabled(True)
+        else:
+            self.calc_but.setEnabled(False)
+
     # Get variables from platform editor input fields
     def init_variables(self):
         self.plat_name                  = self.platform_win.get_plat_name()
@@ -69,7 +79,7 @@ class SIPPCompare(QMainWindow):
         self.fund_plat_fees = 0
         value_num = float(self.value_input.value())
         # Funds/shares mix
-        slider_val = self.mix_slider.value()
+        slider_val: int = self.mix_slider.value()
         funds_value = (slider_val / 100) * value_num
         fund_trades_num = int(self.fund_trades_combo.currentText())
         self.fund_deal_fees = fund_trades_num * self.fund_deal_fee