The Maths Of Weight Loss.

Once you've realised that all-important basic equation..

Gain weight = eat more calories
Lose weight = eat less calories

(its amazing how something that simple still eludes people  )

..the next step to properly manage your food intake is to figure out exactly what your daily calorific needs are. Once you know this, you know how much food your body *really* needs and can add/ subtract foods as required. Basically, in order to succeed, you need to take the guess work out of it and know that everything that goes into your mouth and into your belly needs to be there. 
Get Your Calculator Ready!

I know this formula doesn't have anything to do with this but  its what i think of when it comes to math.

1) So.. first off you need to figure out your resting metabolic rate (RMR) or the calories your body requires just to exist if you were lying down for 24 hours:

Age Men 
10-18 (body weight (kg) * 17.5) + 651 
19-30 (bw (kg) * 15.3) + 679 
31-60 (bw (kg) * 11.6) + 879 

Age Women
10-18 (bw (kg) * 12.2) + 746 
19-30 (bw (kg) * 14.7) + 496 
31-60 (bw (kg) * 8.7) + 829


I'll use myself as an example: 63kg, 20 year old man. 63*15.3 + 679 = 1642.9 is my RMR.


2) Then calculate your lifestyle daily energy needs:

Activity level 
mostly seated or standing RMR * 1.4
regular walking or equivalent RMR * 1.7
generally physically active RMR * 2.0


So, for me, 1643.9 * 1.7 = 2794.63


3) Then estimate your daily exercise expenditure by working out how much exercise you do a week and divide by seven for an average daily value.
I job about 2 hours a week = 1160 calories approx.
1160 / 7 = 165.


So add that to my daily energy needs: 2794.63 + 165 = 2959.63
My maintenance calorie needs every day are 2959 cals/ day.


For weight loss: I need to reduce my calories by about 15% a day: 2959*0.85 = 2515 cals/day.