Genealogists estimate 30 years for each generation and this is useful to guess birth dates when we have incomplete information. Let see on a real example how accurate the 30 years estimate may be.
We will use the well documented Spanish kings starting with Leonor de Borbon, (daughter of Philip de Borbon Prince of Asturias and grand-daughter of the happily-reigning Juan Carlos I). We will follow her ancestors for 45 generations completing 1325 years. After all these centuries we only find a three-year error between genealogic estimate and the actual birth dates.
| Generation Number | Name | Estimated Year | Birth Year | Error |
| 1 | Leonor de Borbon | 2000 | 2005 | -5 |
| 2 | Felipe de Borbon | 1970 | 1968 | 2 |
| 3 | Juan Carlos I of Spain | 1940 | 1938 | 2 |
| 4 | Juan of Spain | 1910 | 1913 | -3 |
| 5 | Alfonso XIII | 1880 | 1886 | -6 |
| 6 | Alfonso XII | 1850 | 1857 | -7 |
| 7 | Isabella II | 1820 | 1830 | -10 |
| 8 | Ferdinand VII | 1790 | 1784 | 6 |
| 9 | Charles IV | 1760 | 1748 | 12 |
| 10 | Charles III | 1730 | 1716 | 14 |
| 11 | Philip V | 1700 | 1683 | 17 |
| 12 | Luis Dauphin of France | 1670 | 1661 | 9 |
| 13 | Maria Theresa of Spain | 1640 | 1638 | 2 |
| 14 | Philip IV | 1610 | 1605 | 5 |
| 15 | Philip III | 1580 | 1578 | 2 |
| 16 | Philip II | 1550 | 1527 | 23 |
| 17 | Charles I | 1520 | 1500 | 20 |
| 18 | Joanna I | 1490 | 1479 | 11 |
| 19 | Isabella I | 1460 | 1451 | 9 |
| 20 | John II | 1430 | 1405 | 25 |
| 21 | Henry III | 1400 | 1379 | 21 |
| 22 | John I | 1370 | 1358 | 12 |
| 23 | Henry II | 1340 | 1333 | 7 |
| 24 | Alfonso XI | 1310 | 1311 | -1 |
| 25 | Ferdinand IV | 1280 | 1285 | -5 |
| 26 | Sancho IV | 1250 | 1258 | -8 |
| 27 | Alfonso X | 1220 | 1221 | -1 |
| 28 | Ferdinand III | 1190 | 1199 | -9 |
| 29 | Alfonso IX | 1160 | 1171 | -11 |
| 30 | Ferdinand II | 1130 | 1137 | -7 |
| 31 | Alfonso VII | 1100 | 1105 | -5 |
| 32 | Urraca of Leon | 1070 | 1081 | -11 |
| 33 | Alfonso VI | 1040 | 1040 | 0 |
| 34 | Sancha of Leon | 1010 | 1013 | -3 |
| 35 | Alfonso V | 980 | 989 | -9 |
| 36 | Bermudo II | 950 | 956 | -6 |
| 37 | Ordoño III | 920 | 925 | -5 |
| 38 | Ramiro II | 890 | 898 | -8 |
| 39 | Ordoño II | 860 | 873 | -13 |
| 40 | Alfonso III | 830 | 848 | -18 |
| 41 | Ordoño I | 800 | 821 | -21 |
| 42 | Ramiro I | 770 | 790 | -20 |
| 43 | Bermudo I | 740 | 739 | 1 |
| 44 | Fruela | 710 | 713 | -3 |
| 45 | Don Pedro | 680 | 683 | -3 |
