I found some information on the web on pouring concrete that helps answer your question. Follow the link to Estimating Quantities of material needed. The example is in metric units but the procedure is independent of the units.
The instructions are to multiply the required volume by 1.65 to get the total volume of dry material needed, in your case that is 7.5 1.65 = 12.375 cubic yards. You are using a 5:1 mixture so one sixth of this volume needs to be cement and five sixth is sand/gravel. Thus you need
1/5 12.375 = 2.475 cubic yards of cement and
4/5 12.375 = 9.9 cubic yards of sand/gravel.
I hope your bags of cement have a volume measurement on them. If not then there is a web site in the UK that has the density of Portland Cement as 1506 kg/cu.m. Write back if you need to use this density and you have trouble with the conversion.
Lou wrote back,
This is Lou I asked the question regarding how much cement would I need
for a 7/1/2 yard pour. Your calculation shows 2.475 yds
Cement is available in either 1/2 Cu ft or 2/3 cu ft bags
Am I correct that if using 1/2 cu ft ft bags, I would need 44.55 bags?
22.275 cu ft
X 2 bags cu ft
There are 3 feet in a yard and hence 3 3 3 = 27 cubic feet in a cubic yard so the calculations should be
2.475 27 = 66.825 cu ft and hence 133.65 bags.
This seems like a lot even though 7.5 cu yards is a large volume. (Are you sure your pour will be 7.5 cu yards?)
I decided to do some more research but the more I read on mixing concrete the more confused I got. There are some online calculators that I found but they work in metric units. (7.5 cu yards is approximately 5.7 cu meters.) There is one called mixing concrete that you can use.
Another site has
The VERY approximate quantities of materials used to generate 1m³ of a C20 (1:2:4) equivalent concrete are.....
* Ordinary Portland cement (OPC) - 350 kg
* Fine aggregate (sand) - 700 kg
* Coarse Aggregate (gravel) - 1100 kg
* Water - 190 kg (190 litres)
I also found a Portland Cement Association page with the recommendation
Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. A good general rule to use is the rule of 6's:
* A minimum cement content of 6 bags per cubic yard of concrete,
* A maximum water content of 6 gallons per bag of cement,
* A curing period (keeping concrete moist) a minimum of 6 days, and
* An air content of 6 percent (if concrete will be subject to freezing and thawing).
But they don't say how large a bag is.
I have mixed a lot of concrete myself using a 3.5 cu ft mixer but I have never tried to measure the volume of concrete you get from a bag of cement. My best suggestion at this point is for you to ask someone, perhaps at the supply store where you are going to buy the cement, how much concrete you get from a bag of cement using a 1:5 mixture.
Sorry I can't be more help,