Maintenance calories are the number of calories you can eat per day on average without your weight changing. They equal your total daily energy expenditure (TDEE) and are the starting point for every lean bulk and every cut: in a bulk you add a surplus on top, in a cut you subtract a deficit.
Estimate or measure
A calorie calculator estimates maintenance from your BMR and an activity factor. That is a reasonable starting value, but for you it can be off by a few hundred calories. Your own data are more accurate: log what you eat for at least two weeks, weigh yourself daily and compare your average intake with the change in your trend weight.
The calculation is: maintenance = average daily intake - (trend change in kg × 7,700 ÷ number of days). The 7,700 kcal per kilogram is an approximation (Hall 2008), but it is good enough for adjusting in small steps.
Example: Over 14 days you eat 2,650 kcal on average, and your trend drops from 82.0 to 81.6 kg. That is -0.4 kg × 7,700 = -3,080 kcal, or about -220 kcal per day. Your maintenance is therefore around 2,650 + 220 = 2,870 kcal. These numbers are a worked example.
What to watch out for
Only use trend weight, never two single daily readings. Leave out weeks with water steps, such as starting creatine or the first days after switching phases. And recalculate every two to three weeks: your needs drop with your weight during a cut and rise during a bulk.