diff --git a/adc.h b/adc.h
index 8850508..827bf59 100644
--- a/adc.h
+++ b/adc.h
@@ -61,6 +61,10 @@ class ADCBase {
             *buffer++ = ADCBase::readFast();
         }
     };
+    // calibration function, return corrected AREF based on Vbg measured against it.
+    uint16_t calibrateAREF(){
+        return 0;
+    }
 
     /*
      * ADC Information
diff --git a/adc_avr.cpp b/adc_avr.cpp
index 9afdc48..c5d896f 100644
--- a/adc_avr.cpp
+++ b/adc_avr.cpp
@@ -26,6 +26,10 @@
 #include "adc_avr.h"
 #if defined(ADCSRA) && defined(ADCL)
 
+// Actual voltage of Vbg (internal 1.1V ref) in mV. This is different from chip to chip.
+// = 5000*1100/VREFmeasured (with accurate multimeter). 3300*1100/VREFmeasured for 3.3V systems.
+#define INTERNAL_REF_MV 1090
+
 const uint8_t ADCInput::prescalers[] = {7,6,5,4,3,2}; // 1:8MHz clock is out of ADC spec for 16MHz AVR
 
 bool ADCInput::init(uint8_t newInput, uint8_t mode){
@@ -48,24 +52,28 @@ uint8_t ADCInput::getModeCount(){
 /*
  * Set ADC prescaler
  */
-void ADCInput::setPrescaler(uint8_t mode){
+bool ADCInput::setPrescaler(uint8_t mode){
     if (mode < ADCInput::getModeCount()){
-        curMode = mode;
         uint8_t prescaler = prescalers[mode];
         prescaler & 4 ? sbi(ADCSRA,ADPS2) : cbi(ADCSRA,ADPS2);
         prescaler & 2 ? sbi(ADCSRA,ADPS1) : cbi(ADCSRA,ADPS1);
         prescaler & 1 ? sbi(ADCSRA,ADPS0) : cbi(ADCSRA,ADPS0);
         sbi(ADCSRA, ADEN);
+        delay(10); // allow the ADC to settle
+    } else {
+        return false;
     }
-    delay(10); // allow the ADC to settle
+    return true;
 }
 
 /*
  * Configure ADC for given mode.
  */
 bool ADCInput::setMode(uint8_t mode){
-    ADCInput::setPrescaler(mode);
-    return 1;
+    if (ADCInput::setPrescaler(mode)){
+        curMode = mode;
+    }
+    return (curMode = mode);
 }
 
 /*
@@ -90,6 +98,31 @@ void ADCInput::readMulti(uint16_t *buffer, unsigned size){
     cbi(ADCSRA, ADATE);  // Disable free running
 }
 
+/*
+ * Read internal reference voltage.
+ */
+uint16_t ADCInput::calibrateAREF(){
+    // reset ADC to default mode for highest precision.
+    setPrescaler(0);
+    read(); // allow the port to be configured (discard result)
+
+    ADMUX = (ADMUX & 0xf0) | 0x0e; // switch to reading the internal 1.1V reference
+
+    // wait for the reference input to stabilize
+    readFast();
+    delay(200);
+    uint16_t reference = readFast();
+
+    // reference should represent 1100mV
+    uint16_t rangemV = INTERNAL_REF_MV * ((1L<<bits)-1) / long(reference);
+
+    // set ADC to previous mode
+    setPrescaler(curMode);
+    read();
+
+    return rangemV;
+}
+
 /*
  * Return ADC clock in Hz. This is only useful to estimate sampling rate.
  */
diff --git a/adc_avr.h b/adc_avr.h
index 3507660..e0c98c0 100644
--- a/adc_avr.h
+++ b/adc_avr.h
@@ -29,7 +29,7 @@
 class ADCInput : public ADCBase {
 protected:
     uint8_t curMode;
-    void setPrescaler(uint8_t mode);
+    bool setPrescaler(uint8_t mode);
 public:
     static const uint8_t prescalers[];
     uint8_t input;                           // analog input port
@@ -51,6 +51,7 @@ class ADCInput : public ADCBase {
         loop_until_bit_is_clear(ADCSRA, ADSC); // Conversion finished
         return ADCL | (ADCH << 8);
     }
+    uint16_t calibrateAREF();
 };
 
 #endif /* ADC_AVR_H_ */
diff --git a/capture.h b/capture.h
index 2a998c4..88a47db 100644
--- a/capture.h
+++ b/capture.h
@@ -20,6 +20,7 @@ class Capture {
     uint16_t *data;
 
     int init(ADCInput adc, unsigned samples, uint16_t rangemV);
+    void calibrate();
     void capture();
     void tomV();
 };
diff --git a/scope.h b/scope.h
index 2e82f41..e500325 100644
--- a/scope.h
+++ b/scope.h
@@ -16,7 +16,7 @@
 // Maximum number of horizontal grid lines
 #define MAX_X_GRID_LINES 10
 // Maximum number of vertical grid lines
-#define MAX_Y_GRID_LINES 5
+#define MAX_Y_GRID_LINES 6
 
 class Scope {
 protected:
diff --git a/tiny_scope.ino b/tiny_scope.ino
index e5045b4..629548f 100644
--- a/tiny_scope.ino
+++ b/tiny_scope.ino
@@ -61,7 +61,7 @@ void displaySplash(){
     display.print(F("Tiny Scope"));
 
     display.setTextSize(1);
-    display.printf(F("\nINPUT A%d VREF "), capture.adc.input);
+    display.printf(F("\nINPUT %d VREF "), capture.adc.input);
     display.printSmallUnits(1000L*capture.rangemV, "V\n"); // printSmallUnits expects micro[V]
     if (capture.adc.getClock()){
         display.print(F("ADC CLOCK "));
@@ -73,8 +73,10 @@ void displaySplash(){
 }
 
 void setup(){
-    int success = capture.init(ADCInput(), SCREEN_WIDTH, AREF_MV);
-    capture.adc.init(ADC_PIN, EEPROM.read(ADC_MODE_ADDR));
+    ADCInput adc = ADCInput();
+    adc.init(ADC_PIN, EEPROM.read(ADC_MODE_ADDR));
+    uint16_t aref = adc.calibrateAREF();
+    int success = capture.init(adc, SCREEN_WIDTH, (aref) ? aref : AREF_MV);
     delay(100);  // give time for display to init; if display blank increase delay
     display.begin(SSD1306_SWITCHCAPVCC, DISPLAY_I2C_ADDRESS);
     display.setRotation(2);