I am trying to estimate the amount of yarn I will need for a spread. Is there some sort of rule as to how much yardage there is to the ounce, depending on the classification of the yarn (lace weight, sport, worsted, etc.)? I need 56-67 oz of lace or fingering weight yarn and want to know how many yards this is.

You opened a can of worms here. What you are looking for is know as grist or yards per pound. How good are your math skills? Do you have a comparable spread that you can weigh?
I think the first step is to take a couple of ball bands and get a yards per ounce for fingering and one for sport weight, and then multiply by the required amount in the pattern. That will give you your yardage. It is fairly effective to weigh a similar spread and use that weight as your requirement, I may need more details to help. Fingering weight what? cotton?Silk? Wool? You might try googling Yarn/yards per pound and see if there is a chart. the amount you mentioned runs between 3.5 and 4 pounds of yarn. that is a lot of fingering weight yarn.the chart I looked up said 10,000 yards per pound for fingering weight.
at the amounts you are looking at I think you could carpet a large room. now what was it you were thinking of knitting again?

There is a small meter you can buy that measures yarn. It is not very accurate though. I had one and sent it back because I found that it varied as much as 10% long or short depending on the yarn type and the speed at which it was drawn through the slot.

The yardage difference between fingering and lace weight yarn for the same weight can be tremendous. I knit a lot of socks with "fingering" weight yarn which typically runs at 440 yards/100 g but the lace weight yarn would run anywhere from 880 to 1200 yards/100 g. The 100 grams is roughly about 3.5 ounces.

I'm seeing a big discrepency here in what yardage Dance's chart shows for fingering and what KT finds as yardage for lace-weight, which is slightly finer than fingering. If 100g is approximately 4oz, than that means that 1200 yards times 4 is 4800 yards to a pound, a long way from 10,000 yards per pound in the chart.
So there is still some doubt here. I would think the best way to go about seeing how much yarn is needed for this coverlet is to make a swatch, a good-sized one, and measure it's weight and then how many of these swatches it would take to cover the area she has in mind. For example, if she made an 8x8 inch swatch and needed 50 of them to cover the area and each one weighed 1 oz then she would need 50 oz. And then get some extra for the unforseen stuff. And if the ball band says how many yards are in the skein she used, she can get an estimate of yardage needed if she wanted to.

My experience matches KT's: the lace weight yarns I've used have had about the same yardage as fingering weight at half the weight. I'm sure that's not always the case, but it will do for a general idea.

So a rough estimate based on Ugottalovecrafts' figures suggests something in the neighborhood of 8000 yards of fingering weight or 16000 yd lace weight.

I am planning on making a bed throw of undetermined dimensions. Based on two patterns I have, one in worsted weight, which needs 56 oz. of yarn, and the other in sport weight, which calls for 67 oz, I am trying to figure how how much yarn I need if I want to use fingering or lace weight.
The only store I have been to to shop for yarn is Joann's, and I don't think they have the quality yarn I want, so I have been looking on knitpicks.com and yarncountry.com for yarn comparing their prices, etc. Not sure yet what fiber to use, thinking of silk, but wondering if there is anything else similar to it.
Thank you all so much for your responses!

I honestly think that trying to guess how much fingering or lace weight you need based on the weight of yarn in the patterns you have isn't really going about it the right way. I hate to say it that way, but my concern is that actual yarn weight is rather subjective. One company could have 400 yards be a pound and another company could have 400 yards be 7/8 of a pound. Some of that has to do with the fibers and some to do with the dye. Dye does add weight, not lots but with the amounts you're considering, that could be important.
I would try something along the line of what I suggested. Get some yarn in the fiber and weight you are looking to use, make a big! swatch, and weigh it. Then determine how many of those squares you need to cover your area, multiply that by how much your swatch weighs and that's how much weight-wise you need. Or knit some set yardage, say 50 yards or something like that, in the pattern you intend to use, and knit until you run out. Measure the size of the swatch, again, see how many of those you need, and a little math and you know the yardage you need. Either of those has to be much closer to getting you a ballpark figure of what you'll need rather than guessing based on yarn that is so much different in weight. And then just to be safe, buy extra.

Ugottalovecrafts -- the fiber you select will also have an impact on the weight/yardage you will need. Another factor will be how the yarn is spun. I find that my merino wool, which tends to generally be more tightly spun than other wool, gives me less yardage for the same weight, even for a similar thickness of yarn. For a DK weight yarn that generally gives me around 110 yards/50 grams, I might only get 100 yards for merino.

The best thing to do is to look for some yarn that is readily available and then buy one skein and do a large swatch-- if you are doing the coverlet in squares like a traditional counterpane do one of the squares. Weigh the swatch and then use the swatch measurement to determine how many skeins you will need to get the coverlet of the desired size.

When you talk about using silk, there's generally what I call two types. There's the finely processed shiny slick silk, and there is the matte finished, somewhat sticky silk. Also, in my experience with silk, it will at some point, at some time (especially the shiny silk) STRETCH OUT -- and it won't shrink back much either -- even after a stint in the dryer. If you get the shiny, slippery silk, or even if you get rayon, be prepared to have your cover slip and slide around your bed.

And having made a worsted weight counterpane for a double bed which easily fits my queen size bed you are wise to try and stay away from worsted weight. That counterpane is heavy, heavy, heavy.

Another word for counterpane in worsted weight is afghan, and heavy is right. Think on sweater for bed. I agree that the make a substantial swatch, weigh and measure and multiply process would be the best bet. Out of the practical curiosity do you have pets who will be enjoying the coverlet? the finer the thread the more likely that it can snag shred or be damaged by enthusiastic pet use.

Thanks, I'll have to go to a yarn store when I can and see if I can find something that would work.
Is rayon similar to silk in texture? I want something that will be soft like silk, but if it is going to stretch out than maybe another fiber would be better.
No pets will be on the bed, so I won't have to worry about that.

I think of Rayon as having properties similar to shiny silk. Tencel, a related plant based fiber, is very soft as is bamboo. I've seen Tencel is relatively less expensive yarns -- something to think about in knitting a cover big enough for a bed.