Using the knit gauge for the pattern you've chosen (sts per inch and rows per inch - should be stated in the pattern), convert the numbers to inches or centimetres. You should then be able to come up with a rough sketch (if none is provided) of the size and shape of the garment piece(s). (Be sure you're using the measurements / stitch count for the same size garment all the way through.)
Then the fun part - play around with different stitches and yarns and hooks to get a similar fabric (light and drapery, airy and open, heavy and solid, etc.) A stitch dictionary can be very helpful here, if you have one, or go through your collection of crochet patterns and magazines for ideas and inspiration.
Pick the stitch pattern (or patterns) you like best, carefully measure your gauge in crochet, then convert the number of stitches and rows required, etc. A calculator makes the job go much faster. Also, the larger the swatch, the more accurate your measurements will be.
If a schematic is provided for the knit pattern, photocopy it and write the results of your calculations and other notes right on the pattern. Do take lots of notes as you work on the item in case you decide to make it again some time in future. Don't forget to note the yarn type (and when you're done, yarn quantity) and hook you used!
Measure your work regularly as you go and compare it to the original pattern, just to be sure your sizing is on track, and you should eventually come up with a comparable garment. Tada!
A few things to keep in mind:
Be sure you have lots of extra yarn if converting from knit to crochet. I believe crochet generally uses about one third more yarn than a comparable item done in knit with the same yarn, ie if you need about 1200 metres to knit it, plan on at least 1600 metres if crocheted. Afghan (Tunisian) crochet uses even more!
Knitters get a break here. You'll generally need less yarn than stated when converting a crochet pattern to knitting.
Remember, this is with the same yarn. You can't do a straight math calculation if you're substituting sport weight for worsted, etc. If that's the case, you should measure and then ravel out your swatch and measure the number of yards / metres which cover that particular area (square inches / cm) and go from there (though it's a bit more complicated if the pattern has lots of shaping). Always err on the side of too much yarn rather than too little no matter which way you're converting.
Remember, you will get a comparable item, not an exact copy of the original pattern. If that's what you want, you'll have to learn the other craft!
This will work both ways... from knit to crochet, or crochet to knit.
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