Using Calculators for User Engagement
http://www.usabilityprofessionals.org/upa_publications/upa_voice/volumes/2008/august/using-calculators.html

Zhou, Yun and Cliff Anderson
Usability Professionals Association
2008

Abstract:

Calculators can play important roles on websites. They are especially popular for financial sites, where they can help users calculate mortgage payments, retirement needs, interest earned, and so on. They also appear on other sites, where users can calculate things as varied as their BMI (body mass index), carbon footprint, life expectancy, or gas mileage.