Idaho PERSI Retirement Calculator

The 42-month FAS window

Idaho is one of very few state pension systems that uses 42 months, not 36 or 60, as its final average salary window. Most states pick a round number. Idaho picked 3.5 years, which produces a different calculation than either of the common alternatives.

Compared to a 3-year window (36 months), the 42-month average includes 6 more months of salary history. In years where salary has been growing steadily, those extra 6 months pull the average down slightly by including lower earnings from earlier in the career. Compared to a 5-year window (60 months), Idaho's 42 months is more favorable because it excludes more of the lower-salary years from the average.

For most members, the practical difference between 42 months and 36 months is modest. If your salary grew 3% per year, the 42-month average might be $500 to $1,500 lower than the 36-month average at a $60,000-$80,000 salary range. That translates to $10 to $30 less per month in pension income. Not trivial over 25 years, but not a retirement-altering difference either. The window matters more if your salary grew rapidly in the final years before retirement.

What Idaho's 42-month window does avoid is the manipulation risk that affects some 3-year states, where a single year of abnormally high overtime or special pay can significantly inflate the FAS. The extra 6 months smooths that out somewhat.

Rule of 90: worked examples

The Rule of 90 lets you retire with a full unreduced benefit when your age plus years of service reaches 90, subject to a minimum retirement age of 55.

A member who is 55 with 35 years hits exactly 90. Full benefit at 55. That's the earliest possible Rule of 90 retirement. At a $65,000 FAS and 35 years, the benefit is 0.02 x 35 x $65,000 = $45,500 per year ($3,792/month).

A member who is 60 with 30 years also hits 90. Same path, different shape. Someone who started public service later in life follows this route. The benefit on 30 years is $39,000 per year ($3,250/month) at the same FAS.

What about 45 years of service? Theoretically someone could have 45 years of PERSI service, and 45 + 45 = 90. But they'd need to have started at 20 and worked continuously to 65. If they tried to retire at 55 with 35 years, they qualify. If somehow they accumulated 45 years by age 55 (starting at 10), the minimum age of 55 still applies as the floor. In practice, 45 years of PERSI service is nearly impossible because most workers start in their 20s at the earliest.

If the Rule of 90 isn't within reach, the other unreduced option is age 65 with any vested service (5+ years). The early retirement option applies at age 55 with 5+ years but carries a 3% reduction per year under age 65. Retiring at 60 instead of 65 means a 15% permanent reduction. That's not trivial.

The 1% COLA cap

Idaho PERSI's annual COLA is capped at 1% per year, non-compounding, based on CPI movement. It's one of the lowest caps in the United States for a state pension system. Montana MPERA offers 1.5% compounding. Wyoming PERS offers 3% on a portion. Oregon PERS offers up to 2% compounding. Idaho's 1% sits well below those neighbors.

The impact compounds over a long retirement. At 2% annual inflation, a retiree needs a 2% COLA just to maintain purchasing power. Idaho's 1% cap means real purchasing power erodes 1% per year in a 2% inflation environment. After 20 years, that retiree has lost roughly 18% of their original purchasing power. After 30 years, closer to 26%.

On a $3,000 monthly benefit, 18% erosion means the benefit is worth about $2,460 in today's dollars after 20 years of retirement. That gap matters more in Idaho, where the cost of living has risen faster than the national average in recent years due to population growth and housing price appreciation in the Boise metro area.

Planning around the 1% cap means PERSI retirees need more savings outside the pension to cover inflation risk. The PERSI Choice Plan (401k supplement) and Social Security together can offset some of this, but building a cushion is worth the effort.

Social Security participation

Unlike many state pension members, Idaho PERSI members pay into and receive Social Security. Your PERSI pension and Social Security are both part of your retirement picture, not alternatives to each other. This puts Idaho PERSI members in a more flexible position than teachers in Texas or California, who have no Social Security benefit from their covered employment.

The coordination between PERSI income and Social Security claiming age is worth thinking through carefully. If you retire under the Rule of 90 at 58, you could face 4 or more years between PERSI retirement and your earliest Social Security eligibility at 62. Having the PERSI Choice Plan savings to bridge that gap is useful.

The Choice 401(k) supplement

PERSI's Choice Plan is a 401(k)-style defined contribution account available to all active members. Employees choose how much to contribute (beyond the mandatory Base Plan contribution) and direct the investments among available options. There's no employer match on the Choice Plan, but the account is portable if you leave state employment.

Given PERSI's 1% COLA cap, the Choice Plan serves an important inflation-protection role. Members within 10 years of retirement should consider how much they're contributing and whether the balance is sufficient to supplement a flat-real pension benefit over a 25-year retirement. The tax deferral is valuable, and the investment options have historically been competitive on fees.

How the 2% formula compares to neighboring states

Idaho's 2.0% formula sits in the middle of its neighbors. Montana MPERA uses 1.785%, one of the lower formulas in the Mountain West. Wyoming PERS uses 2.125% for members who joined before 2012, slightly above Idaho. Oregon PERS is more complex, with formulas ranging from 1.5% to 2.0% depending on tier and calculation method. Nevada PERS uses 2.5% for most members, which is among the more generous in the region.

Idaho's 2.0% combined with its 42-month FAS and 1% COLA cap produces middle-of-pack retirement income relative to the region. The Social Security participation adds a significant floor that states like Nevada (where most PERS members don't get Social Security) don't have.