Beds and mattresses come in many different sizes and the sizes are slightly different depending on the country where it is made.
Make sure to know your mattress size so you can properly plan for the space in your bedroom.
Here is a list of common bed and mattress sizes in the United States. 
photo courtesy of Builders Design flickr.com/photos/buildersdesign/8713435263/
Crib and Toddler Bed Mattress Size
This mattress is for an infant or toddler.
27-1/4" wide by 51-5/8" long
Single or Twin Bed Mattress Size
This mattress is for one person of average height or shorter.
39" wide by 75" long
Twin XL Bed Mattress Size
This mattress is for one person. It accommodates adults better than a regular single or twin mattress since it is longer.
39" wide by 80" long
5" longer than traditional Single or Twin bed
Double or Full Bed Mattress Size
This mattress is for two people to share.
54" wide by 75" long
15" wider than Single or Twin bed
Queen Bed Mattress Size
This mattress is for two people to share.
60" wide by 80" long
6" wider and 5" longer than Double or Full bed
King, Standard King, or Eastern King Bed Mattress Size
This mattress is for two people to share.
76" wide by 80" long
16" wider than Queen bed
California King Mattress Size
This mattress is for two people to share.
72" wide by 84" long
12" wider and 4" longer than Queen bed
4" less wide and 4" longer than King bed
Mattress thicknesses typically range from 4 to 18 inches.
Notes:
1) If 2 box springs are used side by side under a king mattress (split box springs), these box springs are not equal to a Twin XL mattress (the extra long Twin bed mattress is 1 inch wider).
2) When purchasing comforters or duvet covers, Double/Full and Queen often use the same size.
3) If you want an antique bed, you will not find one in a Queen or a King size. Mattresses sold under the names Queen and King sized started being sold in 1958 by the Simmons Company. If you see an antique bed in one of these sizes, it was either modified to fit a standard mattress, or it is an antique replica - not a real antique.
