Recipes from outside the United States often specify ingredients by weight rather than by volume. If you don't have a kitchen food scale, you can estimate how much you need based on measuring the volume in common American measurements such as cups, teaspoons or tablespoons. You can also learn to develop an eye for how much certain amounts of some common ingredients weigh, so you can guess without measuring when you don't need to add a precise amount.
Read the label if the ingredient is still in its original package. Many food labels list a serving size in both grams and a volume measurement such as a teaspoon. Divide the number of grams by the volume measurement to figure the grams per unit. Divide the number of grams in the recipe by the result to figure how many units you need. For example, if your salt label says a serving is 1/4 tsp or 1.5g, divide 1.5 by 1/4 to get 6g per teaspoon. If a recipe calls for 3g of salt, divide 3 by 6 to determine you'll need 1/2 tsp of salt.
Consult an online conversion table and look for equivalent weights and volumes of the ingredient you want to measure, if the package isn't helpful. See the References for some examples. Divide the amount of weight in the recipe by the weight listed in the table, then multiply by the volume to calculate the amount you need. For example, if you need to add 227gr. of flour and flour contains 113gr. per cup, divide 8 by 4 to calculate that you need to add 2 cups of flour.
Learn weights of common ingredients so you can judge them by appearance. Use either method above to establish the weight of something, then hold it in your hand or observe its size to remember its volume. For example, you might remember that a small potato or a medium-size apple is 142gr. or that your handful of flour is 85.1gr. or the size of hamburger patty that you usually make contains 113gr. of minced meat.
To convert grams to ounces, divide the number of grams by 28.4. To convert ounces to grams, multiply the number of ounces by 28.4.