FERS guide
FERS annuity formula
A plain-English guide to the basic FERS annuity formula and how service, age, and high-three salary fit together.
Short answer
The basic FERS annuity formula combines creditable service and high-three average salary, with different formula treatment for certain age and service combinations.
Last reviewed 2026-05-01
Why the formula is only the starting point
A formula article can explain the main inputs, but your estimate also depends on eligibility path, retirement date, service history, and the limits of what the tool supports.
When a calculator result appears, it is produced by tested calculator logic and shown with methodology, sources, warnings, and a last-updated note.
When a simple formula is not enough
Individual records can include deposits, redeposits, part-time service, military service, sick leave, and other details that deserve official review.
Use the calculator for an educational estimate, then use official records and agency benefits-office guidance before making a retirement decision.
How to compare dates without hand math
If you are deciding between two retirement dates, avoid doing a quick page-copy calculation and treating it as reliable. The same formula inputs can sit inside different eligibility and timing questions.
Run one date at a time, keep the assumptions visible, and compare the result pages side by side. That gives you a cleaner conversation with HR or a financial professional.
Example: formula inputs before the math
Before calculating anything, the important inputs are the retirement path, birth date, service start date, planned retirement date, and high-three salary estimate.
FERS Planner uses those inputs inside the calculator so the estimate is handled consistently instead of guessed from page copy.
Formula inputs to review
| Input | Why it matters | Common planning mistake |
|---|---|---|
| High-three salary | It is part of the basic annuity computation. | Using gross taxable income instead of a basic-pay estimate. |
| Creditable service | Service length affects the computation and eligibility path. | Ignoring breaks, deposits, redeposits, part-time service, or sick-leave treatment. |
| Age and retirement date | Age can affect eligibility path and formula treatment. | Comparing dates without checking the retirement path. |
Sources
This independent educational page explains formula inputs. It does not provide an official annuity computation or personalized financial advice.