Radius of a zip code

Hello all,

I was trying to find all zip codes within a given zip code or radius.

I have map points and Latitude and Longitude in my zip table.

I remember seeing a post or two referencing this but can't see to find
it.

I've tried the following with no luck:

-- 20 Miles
--select 20 * 360.0 / (7900.0 * atan2(1,1) * 4.0);
select * from zip_code where map_loc @
circle(map_point('dallas','tx','75201'), .290105212724467 ) order by
city

Anyone that has this experience, can you validate this for correctness?

Thanks in advance,

Andy