Memory cards are essential for phones, cameras, music devices and game devices these days. Figuring out what size of memory card to buy can sometimes be tricky.
If you are buying a card for a phone, for example, you'll probably want it to hold a few games and apps, and then lots of photos. A camera would need to have a card big enough to store lots of video or photos, in super-high quality.
It's hard to specify, exactly, that a card can hold a certain number of photos because it depends on the quality of the photos themselves.
For example, if you have a low-end phone that has a 4MP camera, then you can reasonably expect that each photo will be about 1.2MB in size, so a 32GB memory card can hold around 22,000 photos assuming there's nothing else on the card. You will probably want to put at least a few GB towards storing apps and the odd short video, but you can still be confident that you'll have room for around 10,000 photos before you run out of space.
If you have a better phone with a good 8MP camera, then the photos will be twice the file size, so you'll be able to store half as many photographs. That's still a lot of images!
The challenge comes when you look at real cameras, where you're more likely to be shooting in RAW format. RAW images are completely uncompressed you're essentially holding the data that the camera saw, which means that you can make a lot of changes to the image and "post process" it.
RAW images are much bigger in terms of file size. A 4MP RAW image is 12MB in size and most people don't use cameras that are so low quality these days. Even a relatively low end camera is probably going to be around 12MP which means a 36MB image. With that kind of quality, you can store 763 images on a 32GB card.
Most cameras are far higher quality than that too. If you bought a 22MP camera, then a RAW image would be 66MB, letting you fit just 416 on a 32GB memory card. Add a quick video or two, and you will soon run out of space!
So, what's the solution? Well, there are 64GB and 128GB SD cards available now that perform at high speed too so you don't need to worry about being able to write data to them quickly enough for the device to be able to keep up. With mobile phones, it makes sense to buy the highest capacity memory card that the phone can support since you can't easily swap between cards while using the phone.
If you want to buy a sd card for your GoPro Hero 3 or 4 action camera mounted on a quadcopter or a RC helicopter, then you would want to read this post about buying sd cards for GoPro cameras.
You would want to choose the sd card with a lot of care because when it comes to action cameras, performance is perhaps more important than storage capacity. More so if you are interested in time lapse photography.
With a camera, you can change cards more frequently and you can also just swap out the card when it fills up, or copy off the photographs when you are done with each shoot.
Storage is so cheap these days that we are becoming hoarders in our digital lives. We are loathe to delete anything, and instead we back up "to the cloud" and archive every moment of our lives. In some ways, this is a good thing. It means that we can collect memories and don't need to worry about losing things that we might one day look back fondly upon.
If you are in the market for a new memory card, make sure that it is fast enough for the device that you are using it in, and make sure that the device can actually read a card of that size. Slow cards are not suitable for use for recording video, because the card can't be written to at the same speed as the video is being recorded. High capacity cards might not work in older devices that have filesystems that can't understand such large spaces.
If you're not sure what kind of card you need, check the manual for your device. It should give you the technical specifications that will show you what size and speed of card is required. Sometimes, a faster card or a bigger card may 'work' but you won't get full capacity out of it and it won't perform at the stated speed. Sometimes, bigger cards might not work at all.
Try to buy the best possible card that your phone can take, because a small card or a very slow one will actually impair the end user experience by giving you a phone that is sluggish and unstable, and limiting the amount of games and apps you can run. Speed matters these days, especially now that phones are running 3D games and rich media applications that rely on transferring huge amounts of data quite quickly.