How Big should my run/pasture be for 20 chickens and plenty of grass

My chicken run is 20x30 with the 7x8 coop inside of it.  I have 17 chickens.  Within two months they have totally destroyed the grass and we are down to the weeds they thought weren't tasty and dirt.  The grass was a foot high, and we've had plenty of rain since then.  If you have plenty of space, I would try to do two or three runs that are all accessable from the coop and rotate them so they have a chance to grow back.  I am actually working tomorrow on letting them into the pasture because the run is so done.  I do currently toss any weeds I pick into the run for greens, so that helps.

You will not be able to keep grass in the run. The chickens will have it destroyed in no time at all. 20 ftx20 ft is not going to be big enough for 20 chickens if you want the grass to remain. The best thing to do would be to make a run where the chickens can destroy the grass and ground all they want and then to let them out to free range during the day. That is the only way they are going to be able to eat grass and bugs. I have 20 acres that I let my chickens free range on and that works best for me. Of course, even though the chickens are not in the pens very often, the grass has long since been destroyed in their runs. 

Grass in the run, oh, that is adorable. 

Can you free range? 

Hi!  I have 9 chickens in a 50x50 run.  So far (2 years) they haven't been able to kill the grass, but I am prepared for it to happen.  For now, though, I still have to mow it about once a month.  I have decided to not complain!  So, if my chicken math is OK, I'd go with bigger than 100x100, but at that rate, you might just as well free range.  Good luck and let us know what you come up with!
 

My run is 25x25 and I have 10 chickens in there, and so far this year, the grass has faired OK.  We water it plenty (we're in a drought here in Utah) and it doesn't look too bad.  

I have my tractor coop and run which I can move but since the heat I've had it in only a few places and grass is history.  I only have 1/3 acre and part is forest with very tall trees.  I decided to pen off part of my back yard.  I already have a picket fence in the back so I put a portable fence from my house to the back fence, most of the area is under trees but not all. There are shrubs and vines and lots of tree leaves that have built up over the years.  Part of the lawn remains which is grass and clover with a few dandelions mixed in and I've managed to keep most of this green area still green.  The new yard is 50 feet by 30 feet and I have three chickens that spend 2 hours in the yard in the morning (5:30 am to 7:30) before I leave for work and again after I get home from 6:30 pm to dark when they roost.  I move the coop/run each evening to a new place and put down some fresh mini-flake bedding, clean the waterer and refill and top off or refill the layer crumbles which is raised up so they can't poop or scratch it out. 

They peck the grass tips and hit on the clover but not enough so far to kill it off to being gone but then there are only 3 chickens in this 30 x 50 foot area part time. I water the green area every night also.  I also toss in supplemental greens from the other parts of the property.  Next to the wooded part of their yard they work the thick leaves and mulched leaves from years of tossing them there (pre-chicken years) so much time is spent not eating the grass. 

I think if I had more chickens and didn't supplement the greens my 50 by 30 foot area would be bare most likely.  I plan to reseed the whole chicken yard in the fall with mixed grass seed and I'm looking to add green and red clover seed.   I can't let my chickens 'free range' and they are monitored when in the 'yard' as it is open on top and I know they could get out if they wanted to, but they stay busy with things.  There is nothing near to the fence they could use as a step up (and over). 

I think they are content with the set up and respect the fence edge and each night they go up to their coop/roost by themselves.

I have 20 chickens in a 30 x 40 run. Half is covered and shaded well by a large tree. The shaded area is slowly getting to dirt. They love to stay in the shade because of the heat. The other half still has lots of grass. They have been in there since March of this year. We have had lots of rain, so that is helping with the grass. I'm at a wait and see point as to how it will look when the winter comes. If they start to make mud and dirt of the whole run, I am going to section it off and rotate the areas. I can't really free range much because of random dogs wanting to visit. :/

I also put flat cardboard boxes in the dirt areas to give them a rest so little weeds will sprout. This also attracts crickets. The chickens stalk me when I rearrange the boxes, lol.