Gaining weight is the same maths as losing it, reversed: find your real daily burn, then eat a consistent surplus (~400 kcal over). This calculator works out both — using the Mifflin-St Jeor equation. Eat the number daily, weigh weekly, adjust.
Your BMR (basal metabolic rate) is what your body burns doing nothing — calculated with the Mifflin-St Jeor equation, which studies consistently rate as the most accurate estimate for most people. Multiplying by your activity level gives your TDEE (total daily energy expenditure) — your real day-to-day burn.
To lose weight, eat ~500 kcal under your TDEE — roughly 0.5kg a week, sustainable for most people. To gain, eat ~400 over with enough protein. These are estimates: track for two or three weeks, watch the scales, and adjust by 100–200 kcal if the trend's off. That's exactly what Fuel Log's for.