I’m training for a rugby tournament so I’ve started taking whey protein powder from a site called MyProtein. I’m aiming to take a total of about 60g a day. The problem is, they didn’t give me a scoop with it, so I’ve been using a scoop that I had knocking around which says 90ML on it. I thought each scoop of these scoops was 15g so I’ve been taking 4 scoops a day: but someone’s just told me they think each 90ml scoop will be 30g of powder, meaning I’d be taking twice as much as I’m aiming to! I have a hard time believing that because I’ve used these powders briefly before and it doesn’t feel like I’m taking more than I was back then, but I’d like to be sure. Anyone know how to work it out? 
If I’m not mistaken, ml is a measure of volume while g is a measure of mass. Simple answer: Get a scale, weigh an empty scoop, weigh a scoop full of powder and calculate the difference. More complicated, less accurate answer: Look for a net weight on the powder’s packaging and divide it by the number of servings per container (assuming each seving is 90ml) – everything you need should be on the Nutrition Facts label or somewhere else on the container.
If this is the product you’re taking:
They say that their scoop:
is 70 cc (same as 70 ml) and gives you about 30 g of powder. Your 90 ml scoop is about 1.28 times as much, which means you’re getting 38 grams of powder per scoop.
And the nutritional information for the protein powder says that 30g (1 scoop) has the following:
Energy 116.0Kcal
Protein (dry basis) 24.9g
Protein (as-is) 23.7g
Lactose (Carbs) 1.2g
Fat 2.1g
Cholesterol 92.0mg
Which means you’re getting a little over 30 g of protein per 90 ml scoop.
As Reply said, a milliliter (ml) is a measure of volume, while grams (g) is a measure of mass (or weight, makes no difference for your needs). There is no way to tell you have much you need without the density of the powder. Best way is what Reply said, get a scale. Now, if you don’t have one, you can estimate it by making a balance, 30ml of water weighs 30g. Measure out 30ml of water, and put it on a homemade balance and see when it evens out.
That’s exactly what I wanted- I hadn’t realized the site tells you how many grams are in their 70ml scoop. Thanks a lot.