Or more broadly and easily... C = R x L / G

Where C is physical coverage, R is the total number of reads, L is (average) read length, and G is the genome size.

So for example, if you have a 100 Mbp genome and an Illumina library with 10 M reads of length 150 bp, the coverage of the genome is 15X (10,000,000 * 150 bp / 100,000,000 bp).

you can use bedtools like this calculate coverage

Or GATK