Usually someone has created and put on the net everything you might possibly want, but there doesn't seem to be a formula for heat index in Celsius yet. So here it is:

HI=

-0.000003582 * RH² * T²

0.0022117 * RH * T²

-0.0123 * T²

0.0007254 * RH² * T

-0.14611 * RH * T

1.611 * T

-0.01643 * RH²

2.34 * RH

-8.9

where RH is relative humidity in percents, and T is temperature in
Celsius. Notice that the formula stops working below ~26 °C and ~40 %.

The above is slightly prettified from the original formula, but is accurate to 0.03 °C, which should be accurate enough, considering the formula is based on subjective impressions of heat.