I have a recipe that calls for garlic powder, but I only have fresh garlic on hand.
What ratio should I use to substitute?
I'd say about 1 clove for every half-teaspoon of garlic powder. There isn't going to be anything particularly scientific about this, since garlic cloves vary in size, strength and flavor but that will be in the ballpark.
The other option I can think of...
Finely chop up some garlic and cook them in med-to-high temp oil. You may put in the garlic when the oil is warm, so you don't over cook. As soon as you see the colour of garlic turning to brown colour, remove it asap and get the garlic out of oil.
After that, you can use the same amount of the cooked garlic as the powder.
Another way is to roast your garlic and then chop it up which is closer to garlic powder