How large does run need to be to keep the hens from destroying the grass?

[CKfarm22]

They do eat grass, but it is sometimes possible to have so much grass that it grows faster than the chickens can eat it. (That can require a lot of grass for not many chickens.)

Yet they’re still going to eat all of it sooner or later

 

[NatJ]

Yet they’re still going to eat all of it sooner or later

No, that is not always true, because grass GROWS. If you have enough grassy area and few enough chickens, the grass will grow faster than the chickens eat it. The real question is how large the area needs to be, and that depends on a bunch of factors (climate, amount of sun & shade, amount of water, etc.) One situation might need an acre per chicken, another situation might be fine with less than a tenth of that.

For example, I once had a dozen hens in a 1/4 acre pen that had good established grass, plenty of sunlight, and plenty of water. The chickens had access all day every day for more than a year. The grass required regular mowing, because it grew faster than the chickens could eat it. As best I could tell, the chickens did not kill any of the grass. There were no dead patches at all. The chickens certainly did take bites of the grass, but they never ate enough in one place to kill the plants, and they did not scratch up the grass plants because they preferred to scratch in the mulch under the trees beside the grass. (In some other conditions, that same dozen hens might need a much larger area to avoid killing the grass-- especially if there is a shortage of water, or a lot of shade. My grass was able to grow quite quickly for much of the year.)

 

[Disheygirl]

It looks very shady there. Most of my pen gets lots of sun so the shady part between the woods and the run stays grassy because they use the entire area.

Your chicken grass is nicer than my lawn!