The survey viability (SV) score is simply the ratio of survey invites needed to the survey invites available.
Survey Viability = Sampling Frame / (Target Sample Size / Response Rate)
If your score is above 1, you have enough invites to get the responses you need. If your score is below 1, then your study will be underpowered.
If you're not sure how to get the right values to enter in the calculator, read the full article here