Counting proportion of promoters with polymorphism in Gnomad dataset

Question

[Jamie Watson]

I have a bed file of promoters and I want to find out the number of base pairs covered in that file. I know we can use the following if we two bed files:

bedtools intersect -a file1.bed -b file2.bed -wo

But I don't quite understand how to achieve this for just one file.
And then find proportion of promoters with polymorphism in GNOMAD vcf file? How can I do it efficiently?