Single granny square blanket

[Neens]

Wanting to make a 42 inch square granny square blanket, just one square, one colour.  How much yarn would be required?

Yarn is 8ply 100g @ 283m a ball 

[Granny Square]

Hi, and welcome to the 'ville! 

There are various ways to go about this.  And the unit you need to think in is yards, versus skeins, in what I'm about to say.  Also, there's no pat formula for this, there are too many variables, yarn weights are a range not an absolute and peoples' stitch gauges vary.

You could look at a similar pattern about the same size and stitch pattern  (err on choosing a bigger rather than smaller blanket pattern, better to have leftovers and make a hat than not being able to find the same dye lots later) and buy the same total meterage.  The reason, not all skein brands are the same length of yarn, example if you find a pattern of the same weight of yarn and says x skeins of y yarn, but the skein put-ups for y yarn is 100 meters per ball, you'll need 3x the skeins.  

Or, make a swatch, unravel, and do a bit of math for your own personal yarn requirement for your gauge and that yarn.  42" on each side is 1764 square inches. That, divided by 6, is 294 squares, if you were making a pieced blanket.  Make a 6" granny square, mark the bit of yarn coming out of the last stitch with a paper clip or something that will stay put, unravel the square, and measure the yardage you used.  Multiply that number by 294 - that should be the approximate minimum yardage you will need for a big square.  Remember that 'round up the skeins needed' thing.