Archaeological date converter
Convert between BP, b2k, BCE and CE. Type a number and it converts as you go.
What BP actually means
BP is "Before Present", and the present is fixed at 1950. It is not "before now" and it does not move - 1950 was chosen because atmospheric nuclear testing from the 1950s onwards altered the carbon-14 in the atmosphere, so it is the last year a radiocarbon measurement can be referred to without correction.
So 3000 BP is 3000 years before 1950.
b2k is the same idea with the datum at 2000 instead, used in ice-core chronologies. A b2k figure is always 50 higher than the BP figure for the same year.
The missing year zero, which is why most converters are wrong
The calendar has no year zero. 1 BCE is followed directly by 1 CE. So subtracting straight across the boundary is off by one, and a great many online converters do exactly that.
This one uses astronomical year numbering internally, where 1 BCE is year 0 and 2 BCE is year −1, and then converts back. The practical result: 3000 BP is 1051 BCE, not the 1050 BCE a naive subtraction gives.
| Years BP | BCE / CE |
|---|---|
| 1,000 | 950 CE |
| 1,949 | 1 CE |
| 1,950 | 1 BCE |
| 1,951 | 2 BCE |
| 3,000 | 1051 BCE |
| 5,000 | 3051 BCE |
| 10,000 | 8051 BCE |
Note the step from 1,949 to 1,950: 1 CE to 1 BCE with nothing between them. That is the missing year zero, visible.
Calibrated and uncalibrated
One thing this page cannot do for you. A raw radiocarbon result is
reported in uncalibrated radiocarbon years, written BP, and those
are not calendar years - atmospheric carbon-14 has varied, so the two
drift apart by centuries at older dates. Converting an uncalibrated
figure with the arithmetic above gives a number that looks like a
calendar date and is not one.
Calibration needs a curve such as IntCal20 and software like OxCal or
CALIB. If your figure is written cal BP, it has already been
calibrated and this page applies. If it is written plain BP with an
error range, calibrate it first.