How do you write out 1.57 mil in standard form?

1,570,000

Unless you are using mil to mean "millimetre" then 1.57 mil = 1.57 mm = 0.00157 m

Or you are using mil to mean "millilitre" then 1.57 mil = 1.57 ml = 0.00157 l.

Or you are using mil to mean "thousandths of an inch" then 1.57 mil = 0.00157 inch.