Will, 46, shared a touching throwback picture on Facebook yesterday.
In the caption, he worked out how many candles he had watched her blow out and how many times he had told her he loved her during the 20 years they had been together, reported People magazine.
"This was taken at your mom's house 20 years ago. That's a long time ago!!! So I decided to do some math...," Smith captioned the picture.
"I have sung happy birthday to you 20 times and I have bought you 19 birthday presents (I was mad that one year). I have watched you blow out 693 candles (737 after tonight [sic]). I've told you 'I love you' at least 8,285 times. And of the nearly 3.96 Billion women on the planet - there is only 1 that I want to spend the rest of my life with. Happy Birthday, my Love!"