Monthly Premium Calculator Use this calculator to estimate the future value of a regular monthly investment. Monthly Investment (£): * Annual Growth/Interest (%): * Enter 6% as 6. Calculator Notes: This calculator assumes that charges are as per L&G 100 Index Fund. Your actual charges may be higher or lower. The calculator assumes that the 4, 6, 8 per cent growth rates rules apply, although actual growth could be more or less than this, or there could be no growth at all.