Free Tool
Contact Center Staffing Calculator
Calculate exactly how many agents you need to hit your service level targets using the Erlang C formula — the same model used by Genesys, NICE, and every major WFM platform. Enter your call volume, handle time, and target, and get instant staffing numbers including shrinkage.
Your call center parameters
Your staffing requirement
Erlang C formula — ICMI 2024 & COPC 2024 benchmarks
Staffing breakdown
Industry benchmark: 80/20 service level (80% answered in 20 sec) used by 68% of contact centers (ICMI 2024). Optimal occupancy: 80–85% (ICMI 2024). Shrinkage benchmark: 30–35% (COPC 2024).
How it works
Why staffing isn't as simple as calls ÷ handle time
Call center staffing is a queuing problem. Random call arrival and service time variability mean you need more agents than the simple arithmetic suggests — and the Erlang C formula quantifies exactly how many more.
Traffic intensity (Erlangs)
The foundation of Erlang C is traffic intensity: calls per hour × average handle time in hours. At 120 calls/hour with 6-min AHT, traffic = 120 × 0.1 = 12 Erlangs. This represents the minimum number of agents needed in a perfect world — but queuing variability means you need more.
Source: Erlang (1909) original formula, A.K. Erlang, Danish telephone engineer
The 80/20 service level standard
80% of calls answered within 20 seconds is the global contact center benchmark, established by AT&T in the 1980s and now embedded in most customer SLAs. ICMI 2024 research shows 68% of contact centers use exactly this target. Erlang C tells you how many agents you need to hit it.
Source: ICMI Contact Center Management Research 2024
Shrinkage: 30–40% overhead
Breaks (15%), training (8%), coaching (5%), admin (7%), and unplanned absences (5%) mean agents are only available 60–70% of scheduled time. COPC 2024 benchmarks shrinkage at 30–35%. If Erlang C says you need 12 agents on phones, you must schedule 17-18 to account for shrinkage.
Source: COPC Global Performance Standards 2024
Occupancy: the 85% ceiling
Agent occupancy = traffic Erlangs ÷ agents on phones. At 85% occupancy, agents handle calls efficiently while maintaining enough idle time to avoid burnout and queue spikes. ICMI 2024 research shows occupancy above 90% correlates with higher turnover and degraded service levels.
Source: ICMI Contact Center Management Research 2024
Economy of scale in staffing
Larger contact centers are more efficient per agent due to statistical pooling. 100 calls/hour at 6-min AHT needs ~12 agents for 80/20 SL. 200 calls/hour (double the volume) needs only ~22 agents, not 24 — a 8% staffing efficiency gain. Erlang C captures this non-linear scaling.
Source: Erlang C queuing theory, validated by ICMI 2024 benchmarks
Shift staffing vs. concurrent agents
This calculator outputs concurrent agents needed on phones plus total scheduled headcount after shrinkage. For multi-shift planning, multiply by shift coverage: a center needing 15 agents concurrently across three 8-hour shifts covering 24 hours needs 45 agent-shifts (before accounting for days off and leave).
Source: COPC Global Performance Standards 2024 — workforce planning methodology
Methodology
How we calculate your staffing requirement
This calculator implements the full Erlang C formula with transparent formulas and citable sources — suitable for workforce planning documentation and business cases.
1. Traffic intensity (Erlangs)
Traffic A = calls per hour × (AHT in minutes ÷ 60). This gives Erlangs — the dimensionless measure of traffic load. At 120 calls/hour with 6-min AHT: A = 120 × 0.1 = 12 Erlangs. You must have more agents than Erlangs or the system is mathematically unstable (infinite queue).
2. Erlang C: P(waiting)
C(N, A) = P(waiting) is computed iteratively: sum from k=0 to N-1 of (A^k / k!), then add the Erlang term (A^N / N!) × (N / (N−A)). P(wait) = Erlang term × 1/sum. This converges to the probability that an arriving caller must wait for an available agent.
3. Service level formula
SL = 1 − C(N,A) × exp(−(N−A) × t/T), where N = agents, A = traffic Erlangs, t = target answer time in hours (targetSeconds/3600), T = AHT in hours (ahtMinutes/60). The calculator searches from ceil(A)+1 upward until SL ≥ target, then applies shrinkage.
4. Shrinkage & headcount
Staffed agents = ceil(base agents ÷ (1 − shrinkage%)). At 30% shrinkage (COPC 2024 benchmark), if Erlang C requires 12 agents on phones, you must schedule ceil(12 / 0.70) = 18 agents. Occupancy = A / N (traffic ÷ agents on phones). ICMI 2024 recommends ≤85% occupancy.
Primary sources
- Erlang, A.K. (1909) — original Erlang C queuing formula for telephone traffic engineering
- ICMI Contact Center Management Research 2024 — service level benchmarks, occupancy thresholds, 80/20 standard prevalence
- COPC Global Performance Standards 2024 — shrinkage rate benchmarks (30–35%), workforce planning methodology
FAQ
Common questions
What is the Erlang C formula?
Developed by Danish mathematician A.K. Erlang in 1909, Erlang C calculates the probability that a call will have to wait for an available agent in a queuing system. It takes into account call arrival rate, average handle time, and number of agents to determine service level and average wait time. It's the industry standard for contact center capacity planning used by Genesys, NICE, and virtually all WFM software.
What is a good service level target?
The most common industry benchmark is 80/20 — 80% of calls answered within 20 seconds — established by AT&T in the 1980s and now used as the default in most SLAs. ICMI 2024 research shows 68% of contact centers use this target. Higher-stakes verticals (emergency, financial) often target 90/15 or 95/10.
What is shrinkage in staffing?
Shrinkage is the percentage of scheduled time that agents are unavailable to handle calls: breaks (15%), training (8%), coaching (5%), admin tasks (7%), unplanned absences (5%) — totaling 30-40% shrinkage in a typical center. COPC 2024 Global Standards benchmark: 30-35%. If you need 10 agents on phones, you must schedule 14-15 agents to account for shrinkage.
What is agent occupancy?
Occupancy is the percentage of time agents are actually handling calls vs. waiting. ICMI 2024 research suggests optimal occupancy is 80-85% — high enough to minimize idle time but low enough to prevent burnout and queue buildup. Above 90% occupancy, service level degrades rapidly and agents experience higher stress and turnover.
How does call volume affect staffing?
Agent requirements scale non-linearly with call volume due to the queuing effect. A center handling 100 calls/hour at 6-min AHT needs roughly 12 agents for 80/20 service level. Doubling to 200 calls/hour doesn't require 24 agents — it typically requires 22-23, because larger teams handle variability more efficiently. This economy of scale is captured by the Erlang C formula.
Can I share or download my results?
Yes — inputs are encoded in the URL for sharing/bookmarking. Use "Download PDF report" for a branded printable report with inputs, results, and methodology. No email required; all calculations happen in your browser.
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