BP: How Do Archaeologists Count Backward Into the Past?

What Do Archaeologists Mean by BP, and Why Do They Do That?

First successful Caesium Atomic Clock, 1955 (National Physical Laboratory)
First successful Caesium Atomic Clock, 1955 (National Physical Laboratory).

Richard Ash / Flickr

The initials BP (or bp and rarely B.P.), when placed after a number (as in 2500 BP), means "years Before the Present." Archaeologists and geologists generally use this abbreviation to refer to dates that were obtained through the radiocarbon dating technology. While BP is also used generally as an imprecise estimate of an age of an object or event, the use of it in science was made necessary by the quirks of the radiocarbon methodology.

Radiocarbon's Effects

Radiocarbon dating was invented in the late 1940's, and within a few decades, it was discovered that while the dates retrieved from the method have a sound, repeatable progression, they are not a one-to-one match with calendar years. Most importantly, researchers discovered that radiocarbon dates are affected by the amount of carbon in the atmosphere, which has fluctuated greatly in the past for both natural and human-caused reasons (such as the invention of iron smelting, the Industrial Revolution, and the invention of the combustion engine).

Tree rings, which keep a record of the amount of carbon in the atmosphere when they are created, are used to calibrate or fine-tune radiocarbon dates to their calendar dates. Scholars use the science of dendrochronology, which matches those annular rings to known carbon fluctuations. That methodology has been refined and improved several times over the last few years. BP was first established as a way to clarify the relationship between calendar years and radiocarbon dates.

Advantages and Disadvantages

One advantage to using BP is it avoids the occasionally irate philosophical debate about whether, in this multicultural world of ours, it is more appropriate to use AD and BC, with their explicit references to Christianity, or to use the same calendar but without the explicit references: CE (Common Era) and BCE (Before the Common Era). The problem is, of course, that CE and BCE still use the estimated date of the birth of Christ as the reference points for its numbering system: the two years 1 BCE and 1 CE are numerically equivalent to 1 BC and 1 AD.

However, a major disadvantage of using BP is that the present year, of course, changes every twelve months. If it was a simple matter of counting backward, what was accurately measured and published as 500 BP today in fifty years would be 550 BP. We need a fixed point in time as a starting point so that all the BP dates are equivalent no matter when they are published. Since the BP designation was originally associated with radiocarbon dating, archaeologists chose the year 1950 as a reference point for 'the present.' That date was chosen because radiocarbon dating was invented in the late 1940s. At the same time, atmospheric nuclear testing, which throws huge amounts of carbon into our atmosphere, was begun in the 1940's. Radiocarbon dates after 1950 are virtually useless unless and until we can figure out a way to calibrate for the excessive amount of carbon still being deposited in our atmosphere.

Nonetheless, 1950 is a long time ago now—should we adjust the starting point to 2000? No, the same problem would have to be addressed again in the coming years. Scholars now typically cite both raw, uncalibrated radiocarbon dates as years RCYBP (radiocarbon years before the present as 1950), alongside calibrated versions of those dates as cal BP, cal AD and cal BC (calibrated or calendar years BP, AD, and BC). That probably seems excessive, but it will always be useful to have a stable starting point in the past to hook our dates on, despite the outmoded religious underpinnings of our modern, multiculturally-shared calendar. So, when you see 2000 cal BP, think "2000 years before the calendar year 1950" or what calculates to the calendar year 50 BCE. No matter when that date is published, it will always mean that. 

Thermoluminescence Dating

Thermolumiscence dating, on the other hand, has a unique situation. Unlike radiocarbon dates, TL dates are calculated in straight calendar years—and the dates measured range from a few years to hundreds of thousands of years. It might not matter if a 100,000-year-old luminescence date was measured in 1990 or 2010.

But scholars still need a starting point, because, for a TL date of 500 years ago, even 50 years difference would be an important distinction. So, how do you record that? Current practice is to quote the age along with the date it was measured, but other options are being considered. Among those are using 1950 as a reference point; or better still, use 2000, cited in the literature as b2k, to segregate it out from radiocarbon dating. A TL date of 2500 b2k would be 2,500 years before 2000, or 500 BCE. 

Long after the Gregorian calendar was established throughout most of the world, atomic clocks have allowed us to adjust our modern calendars with leap seconds to correct for the slowing spin of our planet and other corrections. But, perhaps the most interesting outcome of all this investigation is the wide variety of modern mathematicians and programmers who have taken a crack at perfecting the matches between ancient calendars using modern technology.

Other Common Calendar Designations

  • A.D. (Anno Domini, "Year of Our Lord," dating from the birth of Jesus Christ, Christian calendar)
  • A.H. (Anno Hegira, "Year of the Journey" in Latin, dating from Mohammad's journey to Mecca, Islamic calendar)
  • A.M. (rarely used, but meaning Anno Mundi, "Year of the World," dating from the calculated date of the world's creation, Hebrew calendar)
  • B.C. "Before Christ," (before his birth, Christian calendar)
  • B.C.E. (Before the Common Era, Western revised Christian calendar)
  • C.E. (Common Era, Western revised Christian calendar)
  • RCYBP (RadioCarbon Years Before the Present, scientific nomenclature)
  • cal BP (Calibrated or Calendar years Before the Present, scientific nomenclature)


  • Duller GAT. 2011. What date is it? Should there be an agreed datum for luminescence ages? Ancient TL 29(1).
  • Peters JD. 2009. Calendar, clock, tower. MIT6 Stone and Papyrus: Storage and Transmission . Cambridge: Massachusetts Institute of Technology.
  • Reimer PJ, Bard E, Bayliss A, Beck JW, Blackwell PG, Bronk Ramsey C, Buck CE, Cheng H, Edwards RL, Friedrich M et al. 2013. IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0–50,000 Years cal BP. Radiocarbon 55(4):1869–1887.
  • Taylor T. 2008. Prehistory vs. Archaeology: Terms of Engagement. Journal of World Prehistory 21:1–18.
mla apa chicago
Your Citation
Hirst, K. Kris. "BP: How Do Archaeologists Count Backward Into the Past?" ThoughtCo, Jul. 29, 2021, thoughtco.com/bp-how-do-archaeologists-count-backward-170250. Hirst, K. Kris. (2021, July 29). BP: How Do Archaeologists Count Backward Into the Past? Retrieved from https://www.thoughtco.com/bp-how-do-archaeologists-count-backward-170250 Hirst, K. Kris. "BP: How Do Archaeologists Count Backward Into the Past?" ThoughtCo. https://www.thoughtco.com/bp-how-do-archaeologists-count-backward-170250 (accessed February 1, 2023).