newskye,

you will need 36 cubic feet for each bed to fill them to the top.

4x6=24 cubic feet if they were one foot deep...then another 12 cubic feet per bed for the additional 6 inches of depth...does that make sense?

Rene

I'm useless at calculating quantities, but I've bought enough soil for my beds through the years to suggest you don't buy topsoil. It's usually poor quality, and full of weed seeds. Find a place locally that sells in bulk( by the truckload) and find out what sort of flower bed mixes they sell. Sometimes it's a 50/50 mix of topsoil with compost, or soil/compost/sand- it depends on what kind of soil you have in that area. It will definitely cost more( like 50% more) do do it this way, but your plants will do so much better-it's like night an day. With great soil you'll water less, have less insects and diseases---it's ALL ABOUT THE SOIL!!!!!! Am I on a soapbox or what?
Robin

I have to echo what Robin said - DON'T get regular topsoil - spend the extra money - you will never regret it and your plants will thank you over and over again. As to how much - I am bascially clueless also. We buy in bulk - a truckload at a time but then I am gardening an acre+ and when we use up one truckload we order another. Our mix is manure/sand/soil conditioner and costs about $250 a truckload - we end up with a pile roughly 4'x8'x6'......

Lynne

Rene is right on the money with the calculation. Put all dimensions in feet and multiply length x width x height.
4 x 3 x 1.5 = 36 cubic feet.

When you buy soil in large quantities, it is often sold in cubic yards. To get those, divide by 27

36 / 27 = 1.33 cubic yards

If you are buying it by the ton, you can use 1 ton per cubic yard for a rough conversion. So you would need 1.33 tons of topsoil for each bed. Multiply by 3 beds and get 4 tons (4 cubic yards) of topsoil. Some suppliers have a minimum amount.

I do agree with Robin. A lot of topsoil sold by excavators is pretty sad stuff and would need a lot of amending before it was good for a garden. Also, some topsoil may have heaven-knows-what in it, and may even be harmful. Try to work with a supplier you trust.

ThinMan

Thanks everybody... I went ahead and ordered one ton... I do agree with you about low quality topsoil, truly, it IS about the soil, but I just can't afford the good stuff so I bought what I could afford. Called "sandy loam," it doesn't seem that bad to me... it smells good, it clumps together nicely in the hand and breaks apart easily... but of course I'll amend as much as possible as I go along. I plan to trench compost starting immediately and mulch liberally, and then start off with plants which aren't too picky.

While it's already been covered I just had to chime in because I do this for a living.
6' x 4' = 24
24 x 3 beds = 72SF
72 x 1.5' depth = 108CF
108/27 = 4CY
+20% for settling = 4.8CY
That is what I would estimate(actually, I would bump it up to 5).
There are all sorts of soil mixes out there, but for the sake of simplicty I would suggest that you do 12" of a soil mix with 6" of compost rototilled into it.
Ideally, in raised beds such as what you are creating, you should use a mix of topsoil, pine fines, compost, and sand. Maybe some peat moss, too.
Have fun playing in the dirt!