I'm on a weaving designing roll at the moment, and thought I'd take you on the path with me.
Here's three yarn hand dyed by fellow Canberra Spinners and Weavers member Liz. Liz has a great eye for colour and makes amazing yarns and drum carded batts for spinning.
These yarns are all 50g and 116m. For my rigid heddle I've found that I use about 20% more yarn in my warp than what I use in my weft. So when I'm working out my "mileage" from a ball (or more) of yarn I say 60% of yarn is for warp, 40% for weft.
Warp and weft can be confusing for new weavers and non-weavers. Warp is the long threads that run from the front to the back of the loom, perpendicular to the weaver. They can be really long (up to 30m!). Weft is the yarn laid down by the shuttle, as it moves back and forth through the warp. It's usually short (a few cm up to one metre). I find this little meme helpful to remember:
So, back to project planning. These balls are 116m each so working out (166mx0.6 = 69.9m) I have about 70m per ball for the warp, or 210m in total.
So that's how much 'fuel' I have. Now I have to work out how far I'm going. For one scarf, lets say it's 180cm, including fringes. I can use almost all the warp for fringes so I won't include any loom waste (yarn that you need to make the warp but doesn't actually end up in your project). With 210m I can get a 180cm scarf that's (210m/1.8m=116 ends) that's 116 ends wide.
As this is 8 ply/DK weight yarn I'll weave it in a 7.5 dent (ends per inch) heddle. 116 ends at 7.5 ends per inch [(116 ends/7.5 ends per inch)x2.54=39.3cm] is just over 39cm wide. That's waaay too wide for a scarf. I'd want about half that.
So what if I doubled the warp for 2 scarves? I could get 58 ends (210m/3.6m=58 ends) out of the warp, which is [(58 ends/7.5 ends per inch)x2.54=19.6cm] 19cm wide. Perfect! And yay, two scarves.
Now I can work out how many ends per colour (70m/3.6m=19 ends), which is 19. Now time to get designing.
You can spend hundreds of dollars on some very fancy weaving software, but honestly my favourite is the one you get with a subscription to Handweaving.net. For $US29.99 Handweaving.net is an utter bargain, and that is my not-sponspored-not-affiliated opinion.
The draft editor and colour editor is worth the price of admission alone. With the colour editor you can upload a photo and then use the colour picker to select colours right out of the photo. This is always my first stop when designing a warp (unless I'm using Maurice Brassard cotton and I can grab the colour RGB numbers straight out of Lunatic Fringe's list to type into FiberWorks).
Once I have my 3 colours picked I know I have up to 19 ends per colour to play with and design a stripe pattern.
This is what I came up with.
The weft colour pattern is just the warp colours copied across (again, something that's super easy to do in Handweaving.net).
That's half the brain work done. Next it's time to work out of this weft will actually work with the yarn I have.