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 2026-04-28
Why the formula should not live in page copy
A formula article can explain the inputs, but actual calculator behavior belongs in tested rules code. That protects users from hidden presentation-layer math.
When a calculator result appears, it is produced by the existing rules engine 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.
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 rather than asking the UI to invent retirement math.
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.