1) Choose a temperature scale.
2) Enter values in 2 of the 3 boxes.
3) Press "Calculate" to find the missing value.

* Values are calculated using the August-Roche-Magnus approximation.

* Equations for each unknown written in terms of the two knowns.

* Spreadsheet-ready equations for each unknown in terms of the two knowns:
    RH: =100*(EXP((17.625*TD)/(243.04+TD))/EXP((17.625*T)/(243.04+T)))
    TD: =243.04*(LN(RH/100)+((17.625*T)/(243.04+T)))/(17.625-LN(RH/100)-((17.625*T)/(243.04+T)))
     T: =243.04*(((17.625*TD)/(243.04+TD))-LN(RH/100))/(17.625+LN(RH/100)-((17.625*TD)/(243.04+TD)))
     (• replace "T", "TD", and "RH" with your actual cell references)
     (• T and TD inputs/outputs to the equations are in °Celsius)
     (• EXP and LN are the exponential and natural logarithm functions)

References:
Alduchov, O. A., and R. E. Eskridge, 1996: Improved Magnus' form approximation of saturation vapor pressure. J. Appl. Meteor., 35, 601–609.
August, E. F., 1828: Ueber die Berechnung der Expansivkraft des Wasserdunstes. Ann. Phys. Chem., 13, 122–137.
Magnus, G., 1844: Versuche über die Spannkräfte des Wasserdampfs. Ann. Phys. Chem., 61, 225–247.