BC to AD Calculator
Convert a BC year to its astronomical year number and understand how the BC/AD timeline works. Because there is no year zero, the year 1 BC is immediately followed by AD 1. Astronomers use a year-numbering system where 1 BC = year 0, 2 BC = −1, and so on. Enter a BC year to see its astronomical equivalent.
Formula
Astronomical Year = 1 − BC Year
- The BC/AD calendar has no year zero: 1 BC is followed directly by AD 1.
- Astronomical year numbering fills that gap by defining 1 BC as year 0, 2 BC as −1, and so on.
- So a BC year converts to its astronomical number with: astronomical year = 1 − (BC year).
- This matters when counting spans across the BC/AD boundary — e.g. from 1 BC to AD 1 is one year, not two.
The year 44 BC (assassination of Julius Caesar)
Inputs
- Year (BC): 44
Astronomical year = 1 − 44 = −43. In other words, 44 BC corresponds to astronomical year −43, because there is no year 0 in the historical BC/AD system.
Frequently asked questions
Why is there no year zero?
The BC/AD system was devised before zero was widely used in Europe, so the timeline jumps straight from 1 BC to AD 1 with no year 0 in between.
What is an astronomical year number?
It's a numbering scheme used by astronomers that includes a year 0. In it, 1 BC = 0, 2 BC = −1, 3 BC = −2, and so on, which makes arithmetic across the boundary simpler.
How do I count years across the BC/AD line?
Remember there's no year zero. From 1 BC to AD 1 is a single year. Using astronomical numbering avoids the off-by-one error.
Is 1 BC the same as 0 AD?
There is no 0 AD in the traditional system. In astronomical numbering, however, 1 BC is written as year 0.