BC to AD Calculator
Enter a BC year and an AD year to find how many years apart they are, accounting for the fact that there is no year 0 on the historical timeline. You can also solve in reverse: give the year difference and one date to find the other. Results show years and centuries with a full step-by-step breakdown.
Formula
Worked example
Julius Caesar was assassinated in 44 BC. To find how many years before 2026 AD that was: 44 + 2026 - 1 = 2069 years. In centuries: 2069 / 100 = 20.69 centuries. If you only know that two events are 500 years apart and one occurred in 2026 AD, the BC year is 500 - 2026 + 1 = -1525, but since that is negative the BC year would be invalid (meaning the earlier date is actually an AD date). For a valid example: 500 years apart, later date 200 AD gives BC = 500 - 200 + 1 = 301 BC.
Why you subtract 1 when crossing from BC to AD
The Julian and Gregorian calendars as used historically have no year 0. After 1 BC comes AD 1 immediately, with nothing in between. This means if you simply add a BC year to an AD year you overcount by one year. For example, from 1 BC to AD 1 is exactly one year, not two. The correction is straightforward: years apart = BC year + AD year - 1. The same gap from 10 BC to AD 10 is 10 + 10 - 1 = 19 years, not 20. Astronomers sometimes use a "year zero" convention (where 1 BC = year 0, 2 BC = year -1, and so on), but historical writing and most everyday calculations use the no-year-zero system this calculator applies.
BC/AD and BCE/CE: what the abbreviations mean
BC stands for "Before Christ" and AD stands for "Anno Domini" (Latin for "in the year of the Lord"). BCE (Before Common Era) and CE (Common Era) are equivalent secular terms that map to exactly the same years: 44 BC = 44 BCE, and AD 2026 = 2026 CE. No mathematical conversion is needed between the two systems. The calculator accepts either framing because the numbers are identical regardless of which abbreviation you prefer.
The three solve modes: difference, find BC, find AD
Most users want the year gap between two known dates, but sometimes you know the span and only one endpoint. The "find BC year" mode rearranges the formula to BC = span - AD + 1, so if you know an event was 300 years before AD 100, the BC year is 300 - 100 + 1 = 201 BC. The "find AD year" mode does the reverse: AD = span - BC + 1. The "how long ago" mode measures from the current year (2026 AD), which is useful for quickly placing ancient dates in context. All modes show the working so you can verify or extend the calculation by hand.
Century and millennium conversions
The calculator also expresses the span in centuries (divide by 100) and you can infer millennia (divide by 1000). This is useful for archaeological or geological contexts where raw year counts become unwieldy. Note that in everyday usage "the 1st century BC" runs from 100 BC to 1 BC (100 years), and "the 1st century AD" runs from AD 1 to AD 100. A span crossing the BC/AD boundary will therefore straddle two century labels even if it is less than 200 years.
Limitations and what this calculator does not cover
This calculator works with whole years and uses the proleptic Julian/Gregorian system, meaning it extends the modern calendar backward without applying historical calendar reforms. It does not account for the 10 to 13 day adjustment when converting between the Julian and Gregorian calendars (the Julian calendar was in use until various countries adopted the Gregorian between 1582 and 1923). It also does not incorporate radiocarbon or other scientific dating uncertainties, which can span centuries for events in deep antiquity. For exact day-level calculations spanning calendar reforms, use a dedicated astronomical date tool.
Notable historical dates and their year span from today (2026 AD)
| Event | Date | Years from 2026 | Centuries from 2026 |
|---|---|---|---|
| First Olympic Games | 776 BC | 2801 | 28.0 |
| Founding of Rome (traditional) | 753 BC | 2778 | 27.8 |
| Battle of Marathon | 490 BC | 2515 | 25.1 |
| Assassination of Julius Caesar | 44 BC | 2069 | 20.7 |
| Birth of Augustus (traditional) | 63 BC | 2088 | 20.9 |
| Construction of the Parthenon begins | 447 BC | 2472 | 24.7 |
| Death of Alexander the Great | 323 BC | 2348 | 23.5 |
| Birth of Confucius (traditional) | 551 BC | 2576 | 25.8 |
These examples show how the BC + AD - 1 formula applies to well-known historical events. All spans are calculated from 2026 AD.
Frequently asked questions
Why is the answer one less than simply adding the two years?
Because there is no year 0. The calendar moves from 1 BC directly to AD 1, so the span between them is one year, not two. Adding BC + AD counts that transition twice, so you subtract 1 to correct it. This is the standard rule used in historical and archaeological literature.
How long ago was 44 BC?
Using 2026 as the current year: 44 + 2026 - 1 = 2069 years ago. Julius Caesar was assassinated 2069 years before 2026 AD. Each year the answer increases by one; in 2027 it will be 2070 years.
How do I find the BC year if I know the span and the AD year?
Rearrange the formula: BC = span - AD + 1. For example, if an event was 500 years before AD 200, the BC year is 500 - 200 + 1 = 301 BC. You can verify this: 301 + 200 - 1 = 500.
Are BCE and CE the same as BC and AD?
Yes, they are exactly the same years. 44 BCE = 44 BC and 2026 CE = 2026 AD. No conversion is needed. BCE and CE are secular alternatives to the Christian-origin BC and AD labels, but they share an identical numbering system and the same no-year-zero rule.
Does "100 BC to AD 100" equal 200 years or 199 years?
199 years. Applying the formula: 100 (BC) + 100 (AD) - 1 = 199. The intuitive guess of 200 years overcounts because it implicitly assumes a year 0 between 1 BC and AD 1.
What is the difference between BC and the astronomical year numbering?
Astronomers use a continuous year count that includes year 0: in this system 1 BC = year 0, 2 BC = year -1, 3 BC = year -2, and so on. If you are working with astronomical software or Julian Day Numbers you may see these negative year values. To convert an astronomical year to a BC year, negate it and add 1 (e.g. year -43 = 44 BC). This calculator uses the historical no-year-zero convention.
How many years is one century in BC or AD?
A century is always 100 years, in either era. The 1st century BC runs from 100 BC to 1 BC. The 1st century AD runs from AD 1 to AD 100. Centuries crossing the BC/AD boundary are simply split: for example, a 150-year span from 50 BC to AD 100 covers the last 50 years of the 1st century BC and the full 1st century AD.