The actress, who plays pub landlady Liz McDonald on the ITV1 soap, agreed to tie the knot with McEwan after he popped the question during a holiday in Spain last month.
Callard believes that their relationship was strengthened by her battle with serious depression last year. At her lowest moment, she considered suicide while staying in The Priory clinic - but McEwan alerted nurses that she had a secret stash of pills for a planned overdose.
Speaking to The Mirror, the 53-year-old explained: "I can't keep it secret any longer. Jon and I are getting married! I am so in love, and so ecstatic to be engaged. Jon and I just fit. I wouldn't be here if it wasn't for him. It sounds dramatic, but he definitely saved my life.
"I'll be the luckiest woman alive to call Jon my husband. I've been married three times before and never had a big do. But this time my wedding's going to be massive!"
Reflecting on last year's troubles, she added: "It wasn't just the fact that Jon rumbled me to the nurses that saved me from ending it all. It was much more than that. He stood by me, through everything. He never gave up on me, never gave up trying to understand my torment."
The couple's wedding ceremony is due to take place in October. Callard is planning to have 18 bridesmaids - including her daughter Rebecca and soap co-stars Kym Marsh and Katherine Kelly.