Question about yarn (hand waving frantically in the air)

I want to use a pattern that uses two strand of DK yarn on a size 9 needle and a 40 stitch cast on. I want to use just one strand of yarn, on a 4. Where is the calculator for conversion?

The pattern should have a gauge - sts per inch or per 4". That will tell you how many inches the 40 sts work out to. Then figure out how many sts per inch the size 4 with 1 strand is and multiply times the inches you need.

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LOL!