I'm quoting a 3 1/4" - 5"-7" oak floor . I don't do those often and am curious on the best formula to figure out how much of each width is needed. So say you need to cover 500 square feet total with the three different widths?

I have that on my confuser somewhere. Go search Kens website or floormasters. I know it's on floormasters.

more on random width plank hardwood flooring and how to measure

Add the total width to come up with one number.

Your situation- 3.25 + 5 + 7= 15.25

500 SF

Divide each plank width by 15.25 or

3.25 divided by 15.25 = .213

Multiply .213 x 500 = 106.55 SF 3 1/4"

5 divided by 15.25 = .328

.328 x 500 = 163.93 SF 5"

7 divided by 15.25 = .46

.46 x 500 = 230 SF 7"

It looks like my math is right. Total 500 SF

Overage. 10% to be safe.