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Fox proof = 19g which is approx 1mm thick wire. This is usually welded mesh (square) but I have seen hex chicken wire quoted as this guage.
But then it would prob cost the same.
I know a lot of people do use chicken wire and risk it if they can supervise their hens during the day but then foxes also get daring and wait for your back to be turned.
Our run is mostly welded mesh, but part of it is chicken wire over about 3ft. They are in the back garden but we accept the risk. The ducks would see any fox off I think :roll:
To be truly fox proof, you'd need about 6ft high, with the top 6"-12" angled outward as per security fences and the bottom 9-12" buried, and/or a skirt round the bottom edge or slabs.
And even then, they may find a way in.
Is your main problem budget ? (ours was/is).
Rob