1 teaspoon
No use tarragon instead
How big a sprig? You must be specific. But 1/2 tsp dry ground thyme = 1 tsp fresh thyme if that is any help. That's the leaves not including the stem.
Dried herbs are normally twice as strong as fresh, so 1/2 a tsp.
I will repeat an answer found within the other good answers: Rule of thumb is 1 tsp dried herb substituted for 1 tablespoon fresh herb.
Typically, when substituting thyme powder for fresh thyme, use about one-third the amount of powder as you would fresh. Since dried herbs are more concentrated, you would need approximately 1 teaspoon of thyme powder to replace 1 tablespoon of chopped fresh thyme. Adjust to taste, as the intensity of flavor can vary.
When substituting dried thyme for fresh thyme, use about one-third the amount called for in the recipe. A typical bundle of fresh thyme is roughly equivalent to 3 to 4 sprigs, which translates to about 1 to 1.5 teaspoons of dried thyme. Adjust to taste, as dried herbs can be more potent than fresh.
How much ground thyme do I use in place of chopped thyme
1 teaspoon ground = 1 tablespoon fresh. Ground or dried herbs are actually stronger than their fresh counterparts.
You can't really measure it that way, despite what the books say, which I very much refute on. It's said that 1 tsp of ground thyme is equivalent to 2 and half tablespoon of fresh thyme. But it changes when you blanch your fresh thyme first - That's when they are activated. Their smell goes stronger, their 'stem' bursts out in flavor and everything in the herb goes wild. Same goes to other strong herbs like dill, rosemary, etc.
When substituting ground thyme for rubbed thyme, use a 1:1 ratio; however, ground thyme is more concentrated in flavor, so you may want to start with slightly less. Typically, about 1 teaspoon of ground thyme can replace 1 teaspoon of rubbed thyme. Adjust according to your taste preference, as the potency can vary.
One fresh bunch of thyme is roughly equivalent to one teaspoon of dried thyme.