How to calcuate a close ration, by comparing number of calls made (prospects) to actual sales (closed).

If you made 10 calls and sold 5 of them you have a 50% closing ratio. Divide sales into calls to get the ratio.

Exactly. And there are other things obtainable the same way;

Calls to interview ratios; Sales calls to interviews:
(100 calls to 20 interviews: 20%)

Interviews to bid ratio; Interviews to bids
(20 interviews to 10 bids: 50%)

Bid to close ratio; Bids to actual sales
(10 bids to 4 sales: 40%)

Everything can be broken down to percentages to judge your efforts and results, for a certain period of time; weekly, monthly, quarterly, etc

Stringer