Service level target comparison calculator
How much does a higher service level target actually cost? Enter your call volume, AHT, and three SL targets to see the headcount and weekly cost difference side by side. Uses Erlang C — the standard inbound voice model.
Volume & Handle Time
Service level targets to compare
Labour cost (optional)
Side-by-side comparison
70% / 20 s SL target
Seated agents
7
Scheduled agents
10
incl. 25% shrinkage
Predicted SL
74%
Occupancy
71%
80% / 20 s SL target
+1 vs lowest targetSeated agents
8
Scheduled agents
11
incl. 25% shrinkage
Predicted SL
88%
Occupancy
63%
90% / 20 s SL target
+2 vs lowest targetSeated agents
9
Scheduled agents
12
incl. 25% shrinkage
Predicted SL
95%
Occupancy
56%
Bottom line: Moving from a 70% to a 90% SL target at this volume requires 2 more scheduled agents.
Model: Erlang C (M/M/N queue) — Poisson arrivals, exponential handle times, no abandonment. Scheduled agents = seated agents ÷ (1 − shrinkage). Weekly cost = scheduled agents × 40 hrs × fully-loaded rate. Headcount shown is for this single interval — a real schedule must account for intraday demand variation across all intervals.
Choosing a service level target
Why SL targets are non-linear
The relationship between headcount and service level follows an Erlang C curve — not a straight line. Moving from 70% to 80% SL typically costs fewer agents than moving from 80% to 90%. This is because the first few extra agents have a large effect on queue wait time; additional agents have a diminishing return as the queue is already being cleared quickly. The hockey-stick shape means the final 5–10 percentage points of SL improvement are disproportionately expensive.
The 80/20 standard
The “80% of calls answered within 20 seconds” standard originated from AT&T research in the 1980s and has become the default for many industries. It is not a regulatory requirement and is not optimal for every contact type. High-value sales queues, healthcare triage, and emergency lines often target 90/15 or better. Lower-priority or cost-sensitive queues may use 70/30 or 80/30 to reduce headcount. Use this calculator to make an explicit business case for whatever target you choose.
What shrinkage does here
Shrinkage converts the Erlang C “seated” requirement — agents who must be on the phones at any moment — into the larger number who must be scheduled. At 25% shrinkage, you must schedule 4 agents to have 3 seated. Shrinkage amplifies the headcount cost of a higher SL target: each extra seated agent requires 1/(1-shrinkage) scheduled agents.
When to challenge the SL target
If the cost difference between targets is small (1–2 agents), the choice matters less — pick the higher target. If the gap is large (4+ agents per interval for a high-volume queue), the SL target becomes a genuine budget decision that should involve finance and operations leadership. This calculator gives you the numbers to have that conversation with evidence.
This is an interval-level calculation
The headcount shown is for a single interval at the volume you entered. A real schedule must account for how call volume changes across the day and week. Your peak interval may need 3× the agents of your off-peak interval, and you cannot simply staff every interval at the peak level. Use the full Turnella app to compute the staffing requirement interval-by-interval and build a shift schedule around the resulting demand profile.
See the full intraday staffing picture
This calculator compares targets at a single demand level. Turnella runs the Erlang C calculation across every interval in your forecast and produces a shift schedule and weekly cost breakdown at your chosen SL target — automatically.