I've started to come across this question a lot in the spaces I find myself in and instead of trying to re-write the instructions all the time, I figured I could just write it down and refer people here instead. Seems easier.
So, here's the question. I want to make THIS THING, but I want to modify it THIS WAY. How do I calculate yardage?
Fortunately, you can figure that out in just 32 easy steps (it may actually be fewer, but it won't seem like it in the moment, haha!)
I want to break it down as simply and as easily as I can. I personally use Spreadsheets for this, but in order to do that, you do need to know how to use formulas in your spreadsheets. I won't bog this down with formula talk. I can do that in a different post if there is interest.
You want to modify. Here's what you do:
1. Get the dimensions of what you want to make. You want this so you can calculate the area of the piece. If you're wanting to add or lengthen a sleeve or use a different weight yarn on a shawl, I wouldn't worry so much about trying to calculate the decreases or the shape. Getting the overall length and width is fine with the understanding that you will be OVER-calculating your yarn needs. I like that because it avoids yarn chicken.
2. Work up a swatch in the yarn you want to use with the hook or needle you want to use. This is VERY important. From this will be the basis for all your future calculations. If all you need is an alteration of the initial size, for example for a shawl you're changing yarn weight on, you can work a swatch in the first section of whatever stitch that is. If you're wanting to add sleeves, the work it in the stitch pattern of the sleeve. If you're doing a complicated stitch and you swatch in single crochet or stockinette stitch, your measurements will be incorrect.
3. Measure (in cm) and weigh the swatch (in grams). If blocking is an essential piece, then block exactly how you will need to for the final product before you measure. Make sure it is COMPLETELY DRY before you weigh. Patience is the name of this game. I personally use a kitchen scale for all my yarn measuring. It's portable and has always been accurate enough for my purposes. I've never had issues.
4. Then calculate your stitches per cm (I find sts/cm to be more accurate than sts/inch). I recommend counting your stitched per 4 or 5 cms then dividing, in case 1.5 sts/cm and not 1. Over time, that little bit of difference can add up to be a lot.
5. Then divide the weight of the swatch by your sts/cm. That's will give your yarn weight per cm. To break that down more, take the GRAMS number and divide it by the number you got in STEP 4. That's why your final number will be the GRAMS per CENTIMETER. I found that thinking about it that way was helpful for me to conceptualize which number to divide by which other number.
6. Divide the yardage/meterage of the yarn by the weight of the skein to get your yds or meters/gram weight. This information is found right on your skein of yarn. I live in the U.S. and so my brain works on yardage. I'm slowly working my way to grasp a better conception of meterage so that I'm working in metric completely instead of switching to imperial at the end. I find I get a more accurate number when I stick with metric, but it's really a brain thing for me. To be entirely honest with you, I calculate in both as a check.
7. Multiply your g/cm by the yds or meters/gram to get yardage per cm. In other words, multiply the number from STEP 5 with the number from STEP 6. This number is how many yards or meters are in each cm of your swatch.
8. Multiply your yds or m per cm by the total area of the garment (length x width) and you'll have an estimate of how many yards or meters you need. I know it probably feels a bit overwhelming and like a LOT of complicated math, but I promise you can do it. Just take it one step at a time, write EVERYTHING DOWN and CLEARLY LABEL each number so that you know where to reference back when it's time to move on. The longest step is working up the swatch and if you do that before you do everything else, then it's just a matter of sitting down and working through the math.
If you need to change the size of something because you're working in a different weight of yarn, then you have extra steps.
1. Determine the length of the beg chain/cast on edge
2. Multiply that by your stitches per cm
It can get a little more complicated if you're working on, for example, a shawl that starts with 4 stitches and grows to 374 stitches. I can break that down in a different post. I want this one to be short and sweet and helpful.
I have a picture here of my own personal spreadsheet for an actual project of mine. It's how I estimated the yardage requirements for that project. I measured both in metric and imperial as a checks and balances of my own math and honestly, it worked out. I was actually amazed and felt like a a bit of a wizard. You also can feel magical when you get this done the first time.
Feel free to comment with questions or get a hold of my in the contact form. I'm happy to try to help you further if you need it.