Whole column conditional formatting.

I have two columns of figures. I would like to apply conditional formatting to the figures in column 'B' when they fall below the figures in column 'A' I need this to apply to the whole worksheet. 'B1' less than 'A1' and so on down the sheet?
Is there a quick way to do this without having to enter individual formulae for each row which would take forever over nearly 1000 rows.
This sheet is to show when stock falls below a pre-defined minimum in column 'A'