Today I bought some bathroom scales and found out that I was 85.1kg. I measured myself as 173cm so

BMI=85.1/1.73×1.73 = 28.43

you can do this easily using the BBC's BMI Calculator: http://www.bbc.co.uk/health/healthy_living/your_weight/bmimetric_index.shtml

the imperial version is here:
http://www.bbc.co.uk/health/healthy_living/your_weight/bmiimperial_index.shtml

Recommended BMI Chart
Underweight	BMI less than 18.5
Ideal	BMI 18.5-25
Overweight	BMI 25-30
Obese - should lose weight	BMI 30-40
Very obese - lose weight now	BMI greater than 40

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This puts me in the overweight category, which comes as no surprise. To get to BMI=25 I will need to be 1.73×1.73×25=74.8kg. OK So I have 85.1 - 74.8 = 10.3kg to lose, I'm giving myself 6 months to do this. Wish me luck!