Hi! I’m a beginner knitter and have some 100g wool that I wanted to try use to knit a little winter hat for my daughter(almost 1 year old). but the pattern I’ve found recommends 50g. What will happen if I do it with the 100g?Will it be bigger so I should do less stitches? Should I ditch the plan and buy new wool or try find a new pattern? Maybe I’m making it all too complicated! Thanks!
Hi! this truely is too complicated.
The 50 g or 100g is just the weight of the skein / ball you got. That means you have 2 times as much as recommended.
Patterns usually say how many skeins you need. That is why patterns that may use 20 g really, say they need 50 g, if they use 70 g they may say “100g” (since the writer assumes you buy complete skeins / balls).
So, what we mean by weight of the yarn and the relevance for the pattern, is more the ratio between weight and lenght.
Imagine a thick yarn: a piece of a certain length is much heavier than with a very thin yarn. (in extreme: ship ropes and sewing thread).
Also: in 50 g of yarn (or 100 g if you buy a bigger skein) there are less meters or yards.
You have to know how many yards there are so that you can find out how much yarn you need to buy.
What you have to really compare (now that you have a pattern) is the stitch ratio (it usually says on the wrapper of the yarn. (e.g. 20 stitches 24 rows (on a swatch of 4 inches by 4 inches or 10 cm by 10cm // or maybe 10 stitches and 12 rows on the same swatch…).
If you do not know, you make a swatch. Knit up a piece that is approx bigger than 4 by 4 inches and count how many stitches and rows you have within 4 by 4 inches (there are a lot of threads here about that). Ask if you have any questions.
changing the needle size will make your knitting bigger or smaller so you can partially adjust the number of stitches and rows (but not infintely of course.)
So, maybe you can link the pattern? And maybe you can even tell us the brand and type of that yarn? if so, we can be incredibly more helpful 
50 and 100g refer to the weight of the skein, and it just means the 100 has twice as much yardage as the 50. Yarn weight means the thickness and you’re confusing the two.
50 gr of worsted weight has about 110 yds, 100 gr has 220. But if it’s fingering weight, the 50 g has 220 yds and 100 would be twice that. Bulky weight might have only 60 yds in the 50g and 120 in the 100. So we really need to know a lot more about what yarn the pattern calls for and what yarn you want to use. As long as they’re the same thickness, you’d probably only use half the 100 g.
You don’t have “some 100g wool.” You have 100g of wool. As others have pointed out, that is merely the weight of the wool, and not necessarily the measurement you need to consider for your hat.
You need to look at the size of your wool—is it worsted weight, fingering weight, bulky?? If the hat pattern calls for 50g of worsted and you have 100g of worsted, there’s no need to adjust the pattern…you’ll have enough yarn for two hats, or for your hat and something else you wish to make.
But if the pattern calls for a different weight than what you have, you may need to make adjustments or find yarn to go with the pattern.
Depending on how tightly you knit, you might need to go up or down a needle size to get gauge, even if you have the right yarn.
You can search/google to find information on gauge that may be helpful to you.