Google Apps Script入門
最終行を取得して繰り返す

function mySample5() {
  var sheet = SpreadsheetApp.getActiveSheet()
  var tannka,suuryou,lastRow
  lastRow = sheet.getLastRow()
  for (var i=2; i<=lastRow; i++) {
    tannka = sheet.getRange(i, 2).getValue()
    suuryou = sheet.getRange(i, 3).getValue()
    sheet.getRange(i, 4).setValue(tannka * suuryou) 
  }
}

function mySample6() {
  var sheet = SpreadsheetApp.getActiveSheet()
  var tannka,suuryou,lastRow
  lastRow = sheet.getLastRow()
  for (var i=2; i<=lastRow; i++) {
    if (sheet.getRange(i, 2).getValue() == "") break
    tannka = sheet.getRange(i, 2).getValue()
    suuryou = sheet.getRange(i, 3).getValue()
    sheet.getRange(i, 4).setValue(tannka * suuryou) 
  }
}

function lastRowColumn() {
  var sheet = SpreadsheetApp.getActiveSheet()
  var lastRow,lastCol
  //先頭行から下方向
  lastRow = sheet.getRange(1, 2).getNextDataCell(SpreadsheetApp.Direction.DOWN).getRow();
  Logger.log(lastRow);
 
  //最終行から上方向
  lastRow = sheet.getRange(sheet.getMaxRows(), 2).getNextDataCell(SpreadsheetApp.Direction.UP).getRow();
  Logger.log(lastRow);

  //先頭列から右方向
  lastCol = sheet.getRange(2, 1).getNextDataCell(SpreadsheetApp.Direction.NEXT).getColumn();
  Logger.log(lastCol);
 
  //最終列から左方向
  lastCol = sheet.getRange(2, sheet.getMaxColumns()).getNextDataCell(SpreadsheetApp.Direction.PREVIOUS).getColumn();
  Logger.log(lastCol);
}