Hydration, in bread baking terms, means how much water or liquid there is in a given amount of flour, or the ratio of water to flour by weight. More water means a higher hydration or wetter dough. Less water means a lower hydration or drier, stiffer dough. Hydration is figured out by the weight of the water as a percentage of the weight of the flour. Both ingredients are weighed and then the total amount of water by weight is divided by the total amount of flour by weight.

There will be times that you will need to just adjust the hydration because the dough is just too wet or dry. Instead of guessing how much to reduce or increase the liquids follow this simple formula below. First find out what is the hydration of the recipe and go from there.

Example
Pizza Recipe with no sourdough starter
260g water
303g hard white
3g yeast (heaping tsp.)
3g salt (heaping tsp.)

This recipe is at 86% 
260g water        = .858 x 100 = 86%
303g hard white 

How much extra water do you need for 90% hydration
x (water)             =.9 
303g hard white

x= .9 (303)
x= 273g water

So you will need to increase the water in this recipe to 273g to have the recipe at 90% hydration.

To check 273g water     = .90 x 100 = 90%
                303 hard white
​
Tips:
The lower hydration doughs are firmer and drier in the 50 – 64 % range of hydration.  The medium hydration doughs are stickier and wetter at around 65% to 76% hydration (loaf Breads). High hydration doughs are more like a thick batter and are extremely wet and sticky at 77% and up (Pizza dough). 

The hydration of the dough is a simple way of saying the ratio of water to flour. Most loaf breads are around 65-75 percent while most pizza doughs are 80-85 plus percent. 
Step 1:
You must know the hydration of your recipe along with the hydration of your starter. Most folks use a very wet starter with a hydration oat 100%. This means equal amount of water and flour by weight. You will need a scale to do this. Mine starter is at 75% so it is more dough like.

How to figure the hydration of your recipe
Basic Pizza Recipe
260g water
303g hard white or hard red wheat flour
1/4 tsp (heaping) yeast 3g
1/4 tsp. salt

260g water = .858 x 100 = 86%
303g flour  

Now this recipe is simple, it doesn't have honey, butter, and other items that may effect this number. Most of the time this flour to water ratio is good enough since the other ingredients are usually small amounts. If you have other ingredients in the recipe in larger amounts go on line and find what the hydration is of that ingredient so you can add this number to the water number when calculating. 

Step 2:
How much starter are you going to use in the recipe?
This is a personal decision you can use a portion of the recipe or 100%. There is an issue that you need to consider. First is your starter active and strong? If not it's best to add a bit of yeast and possibly gluten depending on your starter. To be honest usually my starter needs feeding when making pizza so I use a portion of it in the recipe and add a bit of yeast to help. Using a portion of the recipe with the starter allows the gluten in the new flour to counter act the possibly weak gluten in the starter.  So my decision for this recipe is 140g starter.

Lets take this same recipe and use your sourdough starter as part of the recipe. 
Sourdough Pizza Recipe
140g Starter (at 100% hydration)
190g water
233g hard white or hard red wheat flour
1/4 tsp (or omit)
1/4 tsp. salt

So if you want to use 140g of starter at 100% hydration how do you know how much flour and water is in the starter. At 100% there is equal weight of water and flour in the 140g of starter so 70g of water and 70g of flour.
From the basic recipe you reduce the water from 260g -70g = 190g. Then you reduce the flour from the basic recipe 303g - 70g =233g. Now you have the correct amount of flour and water using your starter.

My starter is at 75% hydration  x = the amt. of flour in the 140g of starter
140g = x + .75x
140g =1.75x
x = 140  = 80g flour
      1.75
So if there is 80g of flour in the 140g of starter there is 60g of water.
Using my starter the recipe would be

Sourdough Pizza Recipe
140g starter (at 75% hydration)
200g water
223g hard white or hard red wheat flour
1/4 tsp. yeast (or omit) 
1/4 tsp. salt

Hydration, in bread baking terms, means how much water or liquid there is in a given amount of flour. Hydration is figured out by the weight of the water as a percentage of the weight of the flour. Both ingredients are weighed and then the total amount of water by weight is divided by the total amount of flour by weight.

Most loaf breads are around 65-75 percent while most pizza doughs are 80-100% percent. Math is not my best subject so I keep this formula handy so I can look at it and just plug in the numbers when adjusting the hydration of doughs. Knowing how to do this will help you when you have issues with your bread baking or changing your recipe to a sourdough recipe.

How to figure the hydration of your dough
Basic Pizza Recipe
260g water
303g hard white or hard red wheat flour
1/4 tsp (heaping) yeast 3g
1/4 tsp.  (heaping) salt  3g

260g water = .858 x 100 = 86%
303g flour

Now this recipe is simple, it doesn't have milk, honey, butter, and other items that may effect this number. Most of the time this ratio is good enough since the other ingredients are usually small amounts. If the recipe has large amounts of other ingredients you will need to look up on the internet what the hydration is and adjust accordingly. A good example is milk, it is about 87% water and the rest is milk solids. So is your recipe has 260g of milk instead of water the actual hydration of the milk is 226g  (260g x.87) = 226g You will be using the 226g when figuring the hydration of the dough.

Flour milled from North American hard wheat may have a protein content of 11-14%. It will absorb much more water than, say, a French flour with a protein content in the 8-10% range. You will even see variations from one bag of wheat to another when baking. You may have to adjust the hydration of your dough. 

If you are working with a flour that has a  weaker gluten like Spelt. You may  want to reduce the hydration in the dough to get the desired results depending on what you are making for example like pizza.

This is an easy recipe that you can make with or without your sourdough starter. This recipe makes 1 medium pizza with a hand tossed crust. You can double it or change the thickness of the crust as you like. So here is the basic recipe without the starter.

 Basic Recipe No Sourdough Starter 
Oven: 400 - 425 degrees F.
Bake: In 2 steps: 1 (first the plain crust about 7-10 minutes)
                            2 (bake again after the toppings are added)
Recipe:
260g water
303g hard white or hard red wheat
3g (heaping 1/4 tsp.) yeast
1/4 tsp salt

Directions:
Mix by hand on in the mixer with the paddle attachment until all of the flour is combined. 
At this point you can put it into the fridge overnight for the next day baking if you like. If you are having for dinner start this at least 3-4 hours before serving.
If you are baking the same day you are making the crust take the dough out of the bowl and place on the counter. With wet hands pick up the dough and let it stretch towards the counter and fold over the dough. Do this a couple of times. 
Lay the dough on the counter and cover with the bowl and let it sit for about 15 minutes. Repeat this stretch and fold until the dough starts to hold some shape. It may take an hour or so.
Once you have developed the gluten you need to place it in the pan, or on parchment paper on the pizza screen. Stretch out the dough taking your time to rest the dough between stretchers  if it will not stretch out. 
Once it is the size you want it put a bit of olive oil over the dough and cover it with a piece of plastic to raise. 

Overnight Dough
If you put the dough into the fridge overnight remove a couple of hours before needing to bake. Do the same stretch and fold method a few times. You do not need to wait between the stretch and folds. The dough will be ready to put into the pan or on the parchment paper.