First things first, NEVER put dry salt mix directly into your saltwater aquarium. You always dissolve the salt mix in fresh RO/DI water first, then use that saltwater inside your aquarium. That said, the amount of salt mix you need just varies based on a few factors. 

  1. What is the salinity/specific gravity you need?
  2. What is the recommended amount of salt per gallon instructed by the manufacturer?
  3. Total water volume - How much water are you mixing?

Salt Mix Instructions
Aquaforest Salt Mix requires 405 grams of salt mix per 2.7 gallons of water to reach a salinity level of 35 ppt.

This equates to 150 grams of salt per gallon of water.

So how much salt mix do I need?

Reef aquariums should contain saltwater mixed to 1.024 - 1.026 specific gravity or 35 ppt salinity. Every brand of salt mix will be slightly different in terms of how much salt mix you need to use per gallon of water to reach a particular salinity level. Most brands have a chart or scale that lists common salinity levels with a corresponding amount of salt per gallon. You can then just use some basic math based on how much water you need to mix up.  

Example: To mix 30 gallons of saltwater @ 35 ppt using Aquaforest Salt Mix (pictured above).

    • 30 gallons / 2.7 = 11.1
    • 11.1 x 405 g = 4495.5 g or roughly 4.5 kg

Since most of us are mixing even gallons of water, you could just as easily find out how much salt per gallon of water and then multiply that by how many gallons you need.

    • 405g / 2.7 = 150 g per gallon
    • 150 g x 30 = 4.5 KG

Weighting your salt mix is the most accurate way to measure it and a kitchen scale works fine to weigh out the salt mix. You only really need to weigh it out once; just use the same measuring cup every time and after you weigh the salt the first time, you will know how many "measuring cups" it takes to get the exact amount you need.

Some salt mix brands use a volume measurement instead (such as 1/2 cup salt mix per gallon of water). Either way, follow the manufacturer's instructions carefully. As long as you stick to the instructions, you should get into the general ballpark and you will always verify with a refractometer before using the water in your tank anyway. 

After slowly dissolving the appropriate amount of dry salt mix into your full mixing container, verify the salinity using a refractometer or salinity meter. If the salinity is too low, add a little more salt mix. If salinity is too high, add a little fresh water until you hit your target.