Women who are underweight, or did not gain sufficient weight during pregnancy should NOT be using such a calculator.
Due to inefficient tissue stores gained during pregnancy – all lactation needs must be met through diet alone. These women should therefore be consuming more food.
These calculators are at best a rough guide and should never be used as accurate or restrictive rules. Please consult your physician or nutritionist for a more accurate approach that takes into account your own particular situation.
Mean weight gain during the 9 months of pregnancy is 26 pounds (12 kilograms). Some of this is for the development of the baby, and some is for the addition of maternal energy stores to be used throughout lactation.
Provided that a woman (well-nourished) has gained this amount of weight – then fat stores accumulated during pregnancy may be used to cover part of the energy cost of breastfeeding. The remaining energy will be met with diet.
The calculator will calculate daily calorie needs for maintenance (based on the above factors) – then add in the energy cost of lactation. A factor of 15% is then subtracted. Therefore, gradual weight loss should occur (see more below).
Additional energy needs for an exclusively breastfeeding woman are approximately 670 Calories per day . If gradual weight loss is required – a figure of an additional 500 Calories per day is recommended.
Research of healthy breastfeeding women has shown that – while lactating – women have a greater energy output (~2718 Calories) than when lactation has ceased (~2528 Calories). This increase in energy output is from milk production – BMR (Basal Metabolic Rate) remains largely the same whether breastfeeding or not .
Where energy input from diet is not enough, tissue stores will be mobilized.
It is generally not necessary to consume extra fluids, following your body’s natural cues is sufficient to meet lactation needs (source). Although assumed, caffeine does not generally lead to a diuretic affect (reseach is inconsistent). However, caffeine can be found in breast milk, so consumption of coffee and energy drinks should be managed carefully.
The most recent and thorough set of research concludes that 
For exclusive breastfeeding through 5 months postpartum, the energy cost of lactation (based on mean milk production) is 454 Calories per day (over non-pregnant, non-lactating women). This amount takes into account the energy released from tissue stores.
A review of over 17 studies  found that mean rates of weight change in the first 6 months postpartum are greater in well-nourished women (-0.8 kg or -1.76 lbs per month) than in undernourished women (-0.1 kg or -0.2 lbs per month).
Research into changes of Basal Metabolic Rate during breastfeeding has proved contradictory and ambiguous. Most studies have shown similar BMR in the lactating and non-lactating state.