diff --git a/ModbusThermometer.cpp b/ModbusThermometer.cpp
index 1992f94..bf415a7 100644
--- a/ModbusThermometer.cpp
+++ b/ModbusThermometer.cpp
@@ -18,9 +18,9 @@ const uint8_t ADC_1_RDY_PIN = 7;
 const uint8_t ADC_2_CS_PIN = 2;
 const uint8_t ADC_2_RDY_PIN = 3;
 
-const uint8_t CAL_ENABLE = ..;
-const uint8_t CAL_OFFSET_ENABLE = ..;
-const uint8_t CAL_FACTOR_ENABLE = ..;
+const uint8_t CAL_ENABLE = 18;
+const uint8_t CAL_OFFSET_ENABLE = 19;
+const uint8_t CAL_FACTOR_ENABLE = 20;
 
 const uint8_t MODBUS_TX_ENABLE_PIN = 6;
 const uint8_t MODBUS_ID = 3;
@@ -142,6 +142,22 @@ void loop() {
         break;
       case e_CAL_SET:
         // calculate and set according to selected calibration mode
+        if (digitalRead(CAL_OFFSET_ENABLE) != 0) {
+          // for offset calibration, the terminals needs to be shorten
+          // offset calibration needs to be performed first
+          float offset = calibrationValueSum / calibrationCycleCnt;
+          for (uint8_t i = 0; i < NUM_OF_CHANNELS; i++) {
+            ads1210[i].setCalOffset(offset);
+          }     
+        }
+        if (digitalRead(CAL_FACTOR_ENABLE) != 0) {
+          // for factor calibration, a 1000R resistor needs to be connected
+          // to the terminals
+          float factor = 1000.0 / (calibrationValueSum / calibrationCycleCnt);
+          for (uint8_t i = 0; i < NUM_OF_CHANNELS; i++) {
+            ads1210[i].setCalFactor(offset);
+          }     
+        }
         calibrationState = e_CAL_COMPLETE;
         break;
       default: