My D800 is new and haven't got around yet to purchasing new memory card so temporarily using one of my old CF cards. Doing so I noticed something odd (at least to me). I shoot in Lossless compressed 14 bit. The file sizes are around 47MB. For sake of argument lets say they are 50MB. since there are 1000 MB in a GB the math says the card should hold 20 photos per GB. The card in question is an 8 GB card so theoretically it should hold 160 50 Mb files.
Why then does the camera show that when newly formatted it will hold only 100 photos?
My camera chronically underestimates the number of photos I can store on a memory card. I'm guessing that Nikon employs a very conservative algorithm to ensure that you can store at least as many pictures as they estimate. I typically get just over 1.6 times the number of photos the camera initially indicates, and I shoot lossless compressed NEF and small basic jpeg. In your case, this factor would translate to about 160 shots.
I suggest shooting until the card is filled a couple of times to get a more accurate feel for what it can actually handle.
Nikon is always very conservative with its in-camera remaining space calculation, when using one of the compressed file formats. Because the size of compressed image files varies, it is impossible to give a completely accurate estimate anyway.
Note that, as the card fills up, the figure will get closer to the real capacity.
I have seen this question asked before. The number of images the card can hold will vary depending on subject matter when using compression. When shooting RAW files, loss-less or uncompressed, the number of images a card will show available is the same for Nikon. I believe the reason for this is if for some strange reason you took 100 images with loss-less compression but the camera could not compress them at all, wouldn't happen, then you would only get 100 images on a 8Gb card. After each loss-less image is taken, the camera figures out remaining space and the number of images it can store based off of uncompressed RAW file sizes.
8Gb = 8,192mb/100 images = ~82mb per uncompressed RAW image.
Try setting loss-less and uncompressed, the amount of images shown should be the same. However, as other will tell you, with loss-less, you will get 1.5-2 times the images shown for any given memory card size.
Try taking 5 images, should only show 95 images left. But will actually show 97 or so left. Just tried mine. I use 16 GB cards and my D800 shows 200 images available after formatting cards. After taking 5 images, shows I have 197 left.
From above numbers with a given loss-less RAW of 40mb:
5 Loss-less images = 200mbs
8,192mb - 200mb = 7,992mb <--- Divide this number by ~uncompressed RAW file size of 82mb (figured from above)
7,992mb/82mb = ~97.46 images or 97 left.
If all your images were 40mb each, then a 8Gb card would hold about 204 images based off of the math shown. Now granted, images will range between 35-50Mb plus or minus a few Mb with loss-less, so these numbers are not exact.
Hope this helps a little with memory card space and the number of images available to take.
The amount of data saved by lossless compression is variable and depends somewhat on the complexity of the image. So it is likely that the camera estimates based on the size of an uncompressed NEF image - which is around 72 Mb iirc.
When you do get around to buying new cards it will be worthwhile to get something of at least 16Gb or 32 Gb capacity and the fastest you can afford.
I have several smaller card but only 1- 32GB card which I use the most as its also my fastest and newest card. My card when formatted says 400 photos with settings for shooting lossless compressed raw files.