I think the usual rule of thumb is a rose needs 1 inch of rain per week, and if your question is how many gallons of water does that amount to, per rose, per week, the answer is a bit hazy. One inch of rain places one half a gallon of water on every square foot of soil in your garden. If an average rose occupies a 3'x 3' area of garden space (9 square feet), then maybe adding a half gallon of water to each of those 9 square feet (for a total of 4 1/2 gallons of water per rose) would come close to duplicating the effect of 1 inch of rain.

Why are watering recommendations so complicated? I guess that rose growers know exactly how many gallons they water each bush and how often. I guess they do not sit down to calculate rain fall each time. So why is that given in terms of inches of rain? Will people in Arizona or Cal, wait for rain? They have to water most of the time...

Those are good "rules of thumb". Temper them with other factors worth noting:

--type of soil: clay retains moisture far better than sand.

--relative humidity of the air: the low humidity of the
desert southwest removes much more moisture from a plant than does the sultry heat of the southeast.

--air movement: a steady wind removes moisture from plants.

--mulch: a thick layer of mulch improves moisture retention in the soil. However, mulch might also pack down and repel water if not fluffed up periodically. Always keep mulch a few inches from the base of the plant, to avoid encouraging disease in that area.

One inch of water penetrates approx 4" of soil (depending on soil). You want to get down at least 8 inches--that's why the two inches of water.

Roses do not like to have their roots sitting in water, so if you have clay soil, be careful. Watering once a week is adequate for most roses in most places (obviously, baby bands need more and old established roses can go without for a month).

Deep watering once a week promotes strong, deep roots. Watering more often keeps the roots on the surface and makestheroots more vulnerable and the plant less stable.

The math for all of us that have been out of school for a little too long (and apologies to those using the metric system):

There 231 cubic inches per gallon.

Figure out how big an area you want to cover--say a rectangle length 36" x width 24". Multiply length x width (in this example, 36 x 24 = 864 square inches.)

Divide cubic inches per gallon (231) by the area you want to cover (864 in this case): 231/864 = 0.267 inches. This is how deep a gallon of water would cover a 24" x 36" area. A little over a quarter of an inch. So in this case, it would take about 4 gallons over the 24 x 36 area to give you an inch of water applied. (If you want an EXACT number of gallons, divide 1 by your result--1/0.267 is 3.75 gallons of water over this area will give you one inch. I don't think the roses will care if you aren't exact.)

If you want to cover a circular area, to find the approximate area in square inches, multilply the diameter (the distance across the circle in inches) by 0.785. Then divide 231 by the result to find out how much a gallon covers.

Mike_Rivers has given the correct answer, as he always does. 1/2 gal per square foot. With pots, you can eyeball an inch or half-inch on the pot rim.

I have to say, with respect, the statement that 1" penetrates 4" of soil is not helpful. 1" penetrates 12"-14" of sandy soil, even if it is dry. With water-retentive soil, it depends on how moist the soil is. If clay is so dry that plants wilt in the shade and die, then you would need 2" to moisten it to any depth. However, if the clay soil is moist enough to support growth, then 1" is a deep watering.

Mikeber, I do keep up with rainfall and factor it into my watering, don't you? I water by hand and know roughly how long it takes my hose to deliver a gallon.