Quick answer
Western usage means completed age. Where a record or tradition uses running age, the two differ by one and only a label prevents the confusion.
The same person, two numbers
Someone who has had 31 birthdays has a completed age of 31 and is living through their 32nd year. Both statements are true. "31" and "in their 32nd year" describe an identical situation and differ by one because they count different things — birthdays reached versus years begun.
Everyday English uses completed age almost exclusively, which is why "in his 32nd year" reads oddly and gets misread as meaning 32. Historical and formal writing uses running age more often, and legal records in some traditions do too.
Use this guide with the right tool: Open the Age Calculator for exact chronological age in completed years, calendar months and remaining days. If the question shifts, compare it with the Age at Last Birthday Calculator or the Age at Next Birthday Calculator.
Where it turns up
Older gravestones, parish registers, formal biographies and translated documents are the common sources. Latin records using "anno aetatis suae" typically mean the running year, so "anno aetatis suae 32" means a completed age of 31.
Some insurance and actuarial contexts use age at nearest birthday rather than last birthday, which is a third convention again: it rounds to whichever birthday is closer, so it can be one higher than completed age for anyone past the midpoint of their year.
Reconciling a one-year discrepancy
When two records disagree by exactly one year, the cause is one of three things: a running-age convention, a birthday that had not yet passed on one of the two reference dates, or a nearest-birthday rounding. All three produce the same visible symptom.
The diagnostic is to compare the reference dates and the month of birth. If the reference dates straddle the birthday, that explains it. If they do not, a convention difference is the likely cause, and the older or more formal source is the one more likely to be using running age.
Writing it unambiguously
In your own records, write completed age and say so if there is any chance of confusion. "31 (completed)" costs four characters and removes the question permanently.
When transcribing a historical record, keep the original wording alongside your interpretation rather than replacing it. "Aged 32 years [running age; completed age 31]" preserves the source and shows your reasoning, which is what a later reader needs.
Worked example
Try it with the dates in front of you. Where the answer sits close to a threshold, treat that as a prompt to check the rule rather than to conclude.
Common mistakes to avoid
- Reading 'in her 32nd year' as 32. It means a completed age of 31. The phrasing counts the year in progress.
- Averaging two records that differ by one. Identify which convention each used. An average matches neither source.
- Assuming insurance age means completed age. Some contexts use age at nearest birthday, which rounds up past the midpoint of the year.
Use the right calculator
Three tools cover most of what this guide describes. Each shows its working, so a result can be checked rather than trusted.
Frequently asked questions
Which does this site use?
Completed age throughout, which is standard Western usage and what forms and legal thresholds mean.
What does 'anno aetatis suae' mean?
Literally 'in the year of his/her age' — a running-age formula, so the completed age is one less.
What is age at nearest birthday?
A convention used in some actuarial contexts that rounds to whichever birthday is closer, rather than counting only completed ones.