Illinois TRS Calculator: How to Use It
Both Illinois TRS tiers use the same 2.2% multiplier. That is where the similarity ends. Tier 2 members face an 8-year FAS window, a salary cap, and a retirement age 7 years later than Tier 1. The calculator shows exactly how large that gap is for your specific numbers.
What this calculator does
The Illinois TRS Calculator applies the 2.2% formula to your tier-appropriate final average salary and projected service years. For Tier 1, it uses the 4-year FAS average with no salary cap. For Tier 2, it applies the 8-year average and caps the salary at the current year limit. It checks eligibility for unreduced and early reduced retirement under each tier's rules, and projects your benefit with the correct COLA formula: 3% compounding for Tier 1, or lesser of 3% or half CPI non-compounding for Tier 2.
What each input means
Membership tier
Joined TRS before January 1, 2011: Tier 1. Joined on or after January 1, 2011: Tier 2. Your tier appears on your TRS annual benefit statement. If you left and returned to teaching in Illinois, the gap length and whether you took a refund of contributions determines your tier on return. Contact TRS if you have any uncertainty.
Current age and years of creditable service
Creditable service is your total TRS-credited years, including part-time service that has been converted to full-time equivalents. Decimals are accepted. The calculator projects your service at your planned retirement age by adding the gap between your current age and your target date.
Final average salary
For Tier 1: the average of your 4 highest consecutive years of creditable earnings. For Tier 2: the average of your 8 highest consecutive years, with each year capped at the current salary limit ($128,903.44 in 2026). Enter your current annual salary as a starting point. If your salary has been rising, the actual FAS at retirement will be higher. If you are Tier 2 and earn above the cap, the calculator automatically applies it.
Planned retirement age
Tier 1 members can retire unreduced at 60 with 10+ years or at 55 with 35+ years. Tier 2 members need 67 with 10+ years. The 12-year difference between Tier 1 and Tier 2 unreduced eligibility is the most striking structural gap between the tiers.
Understanding the outputs
The 75% of FAS benefit cap appears once you have accumulated enough service. Tier 1 members hit it at approximately 34 years (34 x 2.2% = 74.8%). Tier 2 members hit the same threshold at the same service level, but their FAS is lower due to the longer averaging window and salary cap, so the capped dollar amount is also lower.
The COLA projections show a meaningful long-term difference between the tiers. A Tier 1 member with a $4,000/month starting benefit receives 3% compounding, producing $6,436/month after 15 years. A Tier 2 member starting at $3,200/month with a non-compounding 1.5% COLA (typical in low-inflation years) would be at $3,920/month after 15 years. The purchasing power gap grows every year.
Illinois TRS members generally do not participate in Social Security from their teaching employment. WEP and GPO rules previously applied if you had other Social Security credits, but both were repealed in January 2025.
The 8-year FAS window in practice
A Tier 2 teacher who spent years 1-8 earning $45,000-$60,000 but now earns $90,000 faces an 8-year average that pulls heavily from the lower-salary years. A 4-year average would capture mostly the higher years. The difference can reduce the calculated FAS by $10,000 to $20,000, translating into $2,200 to $4,400 less per year in pension income at a 22-year service level.
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Frequently asked questions
What is the difference between Illinois TRS Tier 1 and Tier 2?
Both use 2.2% per year. Tier 1 (joined before Jan 1, 2011): 4-year FAS window, no salary cap, unreduced at 60/10 or 55/35, 3% compounding COLA. Tier 2 (joined Jan 1, 2011+): 8-year FAS window, salary capped at ~$129K (2026), unreduced at 67/10, non-compounding lesser-of-3%-or-half-CPI COLA.
How does the Illinois TRS benefit formula work?
2.2% x years of service x final average salary, capped at 75% of FAS. The cap is hit at approximately 34 years of service. Any additional service beyond that does not increase the benefit.
When can I retire without a penalty under Illinois TRS?
Tier 1: age 60 with 10+ years, or age 55 with 35+ years. Early option: age 55 with 20+ years at 0.5%/month reduction before 60. Tier 2: age 67 with 10+ years. Early option: age 62 with 10+ years at 0.5%/month before 67.
How does the Illinois TRS COLA work differently for each tier?
Tier 1: 3% compounding annually, starting one year after retirement. Tier 2: lesser of 3% or half annual CPI, applied as a simple (non-compounding) addition. In moderate-inflation years, Tier 2 COLA is often 1-1.5%.
What is the Illinois TRS salary cap for Tier 2 members?
$128,903.44 for 2026, indexed annually. Only salary below this threshold counts toward your FAS calculation. High earners have their effective FAS artificially reduced by the cap.