Once you know your maintenance calories, the rest is arithmetic: eat somewhat less to lose weight, somewhat more to gain it, or match it to hold steady. This calculator does that math for you and suggests a gradual, sustainable adjustment rather than an aggressive one.
This tool first estimates your TDEE, then applies an adjustment: roughly 250 to 500 calories below maintenance for weight loss, or the same amount above it for weight gain. A 500-calorie daily deficit works out to about 0.5kg of fat loss a week, since a kilogram of fat is roughly 7,700 calories.