One maintenance figure, computed once
For an 80kg, 170cm, 30-year-old man, moderately active, the BMR/TDEE calculation gives exactly 2,662 kcal/day maintenance calories — computed the identical way whether it's looked up directly or as the first internal step of a deficit or surplus calculation for the same person.
Deficit and surplus are the same adjustment, opposite direction
A 0.5kg/week loss goal subtracts 550 kcal/day from that 2,662 figure, landing at 2,112. A 0.25kg/week gain goal adds 275 kcal/day to the same 2,662 figure, landing at 2,937. Both calculations use the identical "weekly kg change × 7,700 ÷ 7" formula for the adjustment size — one just subtracts, the other adds.
Why this matters for understanding your own numbers
Whether the calorie deficit or surplus calculator is used, the maintenance figure underneath is exactly the same calculation described in the companion article on maintenance calories — getting an accurate weight, height, age, and activity level into that calculation is what determines the accuracy of every deficit or surplus target built on top of it.
What changes between someone aiming to lose weight and someone aiming to gain
Only two things: which direction the adjustment goes, and how large the weekly rate goal is. Everything else about the calculation — the BMR formula, the activity multiplier, and the 7,700 kcal/kg approximation — is identical.