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Perfectly normal for native speakers Mohammad although a little bit "old fashioned". Some examples: "I reckon Brazil will win the world cup" - In my opinion, or I consider that Brazil will win the world cup. "What do you reckon?" - What do you think?
"How much do you reckon that shopping will cost"? - How much do you think or calculate that shopping will cost?
"Wait a minute I will reckon it up" - Wait a minute and I will calculate the cost.
These expressions were certainly used more when I was younger than they are today!