Things You'll Need
Sheet of 1/2-inch thick plywood
Use cinder blocks that are rectangular solids, approximately twice as long as they are wide or high. Don't use the thinner, square blocks often used for gardening. They're not the right dimensions for this project.
If you need a bed frame ... just the bed frame, but you don't have the money to buy one or the tools to build one, make one yourself with cinder blocks. Fans of the cinder-block-and-plank bookshelf will find this design for a platform bed familiar and even complementary to what they already have. These plans are for a low platform bed, as opposed to a high-rise platform bed--sometimes called a loft bed. Cinder blocks would not be appropriate for tall or high-rise platform beds.
Consult the following guide to determine how many cinder blocks you should use:
Twin beds are 38 by 75 inches and require four cinder blocks. Full (or double) beds are 53 by 75 inches and require five cinder blocks. Queen beds are 60 by 80 inches and require nine cinder blocks. King beds are 76 by 80 inches and require 13 cinder blocks. California king beds are 72 by 84 inches and require 13 cinder blocks.
Ask the associate where you buy the plywood--your local home improvement or hardware store--to cut it to match the dimensions of your mattress minus 1 inch from the length and 1 inch from the width. For example, a twin mattress measuring 39 by 75 inches would require a piece of plywood 38 by 74 inches. This will keep you from scraping your legs on the plywood.
Arrange four cinder blocks in a square corresponding to the corners of your mattress. Lay them on their sides so they are wider than they are tall. Though this step doesn't require any great precision, try to set them so they're about one inch in from where the edge of your bed will be.
Add extra cinder blocks as necessary. For a full mattress, place one block in the center (like the 5 on a playing card). For a queen mattress, place one block in the center and a block at the middle of each side. Set up a king mattress just as you did the queen, but with four additional cinder blocks forming a rectangle between the outer edge and the block in the center.
Set your plywood on the cinder blocks. Set your mattress on the plywood.