POISSON.DIST
The maths under Erlang.
Why planners need it
Call arrivals follow a Poisson process. This function gives the probability of exactly N (or up to N) arrivals in an interval — the foundation of staffing theory.
Syntax
=POISSON.DIST(x, mean, cumulative)
Worked example
If a 30-min interval averages 10 calls, the chance of 15+ :
=1 - POISSON.DIST(14, 10, TRUE) → ≈9%. The TRUE gives the cumulative (up to and including x).
Watch out: mean and x must be over the same interval length. Real arrivals can be "burstier" than Poisson — treat it as the baseline model, not gospel.