IOSACal: open source radiocarbon calibration

IOSACal is an open source tool for radiocarbon calibration. It has a command-line program:

$ iosacal -d 7505 -s 93 --id "P-769"

which also creates nice plots like this:

$ iosacal -d 7505 -s 93 --id "P-769" -p

The same features are available as a Python 3 library:

>>> from iosacal import R
>>> from iosacal.text import single_text
>>> r = R(7505, 93, 'P-769')
>>> cal_r = r.calibrate('intcal13')
>>> single_text(cal_r)

# P-769

Calibration of P-769: 7505 ± 93 BP

## Calibrated age

Atmospheric data from Reimer et al (2013);

### 68.2% probability

* 8392 CalBP ‒ 8284 CalBP (45.6%)
* 8264 CalBP ‒ 8205 CalBP (22.3%)

### 95.4% probability

* 8516 CalBP ‒ 8493 CalBP (1.1%)
* 8480 CalBP ‒ 8157 CalBP (92.7%)
* 8115 CalBP ‒ 8114 CalBP (0.1%)
* 8089 CalBP ‒ 8058 CalBP (1.5%)


IOSACal v0.3

Multiple determinations from the same event can be checked for consistency and combined:

>>> from iosacal import R, combine
>>> from iosacal.text import single_text
>>> r1 = R(4430, 100, 'Birm-637')
>>> r2 = R(4430, 120, 'Birm-638')
>>> r3 = R(4400, 100, 'Birm-639')
>>> r4 = R(4350, 130, 'Birm-636')
>>> r_combined = combine([r1, r2, r3, r4])
>>> print(r_combined)

RadiocarbonSample( Combined from Birm-637, Birm-638, Birm-639, Birm-636 with test statistic 0.287 : 4406 ± 55 )

>>> r_combined_cal = r_combined.calibrate('intcal13')
>>> print(single_text(r_combined_cal))

# Combined from Birm-637, Birm-638, Birm-639, Birm-636 with test statistic 0.287

Calibration of Combined from Birm-637, Birm-638, Birm-639, Birm-636 with test statistic 0.287: 4406.465763017056 ± 55.163983474505095 BP

## Calibrated age

Atmospheric data from Reimer et al (2013);

### 68.2% probability

* 5210 CalBP ‒ 5203 CalBP (2.2%)
* 5047 CalBP ‒ 4870 CalBP (65.8%)

### 95.4% probability

* 5280 CalBP ‒ 5164 CalBP (19.3%)
* 5135 CalBP ‒ 5105 CalBP (3.6%)
* 5077 CalBP ‒ 4855 CalBP (72.5%)


IOSACal v0.3

IOSACal can be used interactively with Jupyter Notebook (formerly known as IPython Notebook).


If you have Python 3 just:

pip3 install iosacal

or get the source at

If none of the above makes sense, follow the Installation instructions.

Why another 14C calibration software ?

Most available programs for radiocarbon calibration, like OxCal, CALIB and others, are freeware. You don’t have to pay for them, but on the other side you’re not free to modify them as you need, nor to access and study the source code.

This is the main motivation behind IOSACal: creating a free-as-in-freedom radiocarbon calibration software, with a clean programming library, that enables experiments and integration in existing archaeological information systems.

Furthermore, writing this software from scratch is an alternative way of learning how 14C calibration works, not only in strict mathematical terms, but also from a practical point of view.


IOSACal takes a radiocarbon determination and outputs a calibrated age as a set of probability intervals. A radiocarbon date is represented by a date in years BP (before present, that is before 1950 AD) and a standard deviation, like 2430±170. The combination of these two values is a numerical representation of a laboratory measure performed on the original organic material.

The main task of the calibration process is to convert this measure into a set of calendar dates by means of a calibration curve. Users can choose whether they want results as a plot, a short textual summary or both (the plot includes the summary).

IOSACal reads calibration curves in the common .14c format used also by other programs. Should you have calibration data in another format, it would be easy to either convert them to that format or modify the source code of IOSACal to adapt it to your needs.

IOSACal is based on current calibration methods, like those described in [RAM2008].

[RAM2008]C. Bronk Ramsey, Radiocarbon dating: revolutions in understanding, Archaeometry 50,2 (2008) pp. 249–275

Can I cite IOSACal?

Glad you asked. Yes, each release of IOSACal has a DOI on Zenodo. The latest release is 0.3 [IOSACALv03]. While you could easily cite this website or the git repository, a DOI helps us get some recognition for creating and maintaining this software free for everyone.

[IOSACALv03]Costa S. et al. (2016). IOSACal: v0.3. Zenodo.