Find an average line of text in your book. Don't choose a line that's indented or short (such as dialogue). Count the letters, spaces and punctuation marks in the line. Divide this number by five to find the average number of words per line in the book.
Make a mark at the line where you are beginning to read. If you're just starting the book, this should be the first page. Set your timer for one minute and begin to read. After one minute, make a mark at the line where you finish reading.
Count the lines between your first and second marks. Multiply the number of lines by the average number of words per line to find your reading speed in words per minute. For example, if there are eight words per line on average and you read 42 lines, your reading speed is about 336 words per minute.