A sure way to fix runny yogurt is to strain it as if you were making yogurt cheese. Line a colander or strainer with a clean dish towel or cheesecloth. Place the cloth-lined strainer over a bowl. Dump or spoon the yogurt into the strainer. Excess whey, the watery part of yogurt, will drip through the cloth into the bowl. Let the yogurt drain for 30 to 60 minutes until it has the consistency you want. This method works best for yogurt that has been refrigerated or purchased.

If you made your own yogurt and it is still warm, adding unflavored gelatine may thicken it. Add 1 to 3 teaspoons of gelatine per jar and whisk it thoroughly into the warm yogurt: 1 teaspoon for an 8-ounce jar, 2 for a pint jar and 3 teaspoons for a quart jar. There are no hard and fast rules for how long this takes. Keep whisking until the yogurt starts to thicken to the consistency you want. Scrape the bottom of the jar often to distribute the gelatine particles throughout the yogurt. Failure to incorporate the gelatine throughout the product may result in a gummy layer on the bottom and a runny layer at the top of the jar.

Add some cream to the milk used in making homemade yogurt to reduce the chances of having a runny product. Stirring some nonfat dry milk into the fresh milk you are using may also help prevent runniness.

Use a fresh batch of starter if your homemade yogurt is consistently runny. The lactobacillus organisms that ferment and create yogurt lose their strength over time. Buy fresh starter once a month. Discard old starter if you have not made yogurt in a while. Keep powdered yogurt starters cool and dry when you are not using them. Incubate your homemade yogurt at a high enough temperature and for a sufficient length of time to get a creamy consistency. Do not jostle or shake your yogurt while it is incubating or cooling. Put cooled jars in the refrigerator or in the freezer briefly to firm up a thin batch.