No, ovulation and fertility calculators should not be used as a method of contraception.
These tools are useful in trying to conceive, as they provide an estimate of the period in which pregnancy is most likely to occur. However, the woman's fertile days may vary from the result estimated by the calculator (even more so if the woman has irregular cycles), so the woman could become pregnant outside the days that the calculator has indicated as "fertile".
Therefore, to avoid pregnancy, a contraceptive method should always be used. In addition, if it is a barrier method of contraception such as a condom, it will also prevent the spread of sexually transmitted diseases (STDs).