Calories burned walking at 4.5 mph (level, firm surface, very, very brisk) for 20 minutes
163.3 calories are burned walking at 4.5 mph (level, firm surface, very, very brisk) for 20 minutes  Calories Burned Calculator.
Using the calories burned calculator below, you just need to fill some data and get the value of calories burned in Kcal.

How to calculate calories (burned)
The number of calories you burn while exercising is dependent on:
 the exercise you do
 your weight
 the time spent doing the activity
By multiplying the body weight in kg by the MET ^{(*)} value and duration of activity, you can estimate the energy expenditure in Kcal specific to a persons body weight. In this example, walking at 4.5 mph (level, firm surface, very, very brisk) at a 7 MET value, burns 7 Kcal/kg x body weight/h.
A 70 kg individual walking at 4.5 mph (level, firm surface, very, very brisk) for 20 minutes expends the following:
(7 METs x 70 kg body weight) x (20 min/60 min) = 163.3 Kcal.
7 is the value in METs for walking at 4.5 mph (level, firm surface, very, very brisk).
(*) MET  Metabolic equivalent