DemographicsGrantham is approximately 49.28 square kilometres. In 2016 the population of Grantham was 634. The population is 48% male and 52% female. On average the monthly mortgage repayments in Grantham are $1000 to $1399. The most common weekly income bracket is $300 to $399 per week. Weekly rents in the area are on average $225 to $275.