There's a handy BMI (Body Mass Index) calculator in both metric and imperial at http://nhlbisupport.com/bmi/bmicalc.htm - for the mathematically minded, the formula is
BMI = Weight in Pounds / (height in inches * height in inches) x 703 or Weight in kilos / (height in metres * height in metres)
I'm now at a BMI of 28. My target weight gives me a BMI of 23.6