Is it okay to use "Sure thing" as a reply to my boss?

I was wondering whether sure thing — when I mean certainly — is too casual in a business environment, or whether it could be used under certain circumstances.

(I’m not a native speaker of English.)

There are two ways a boss might interpret that remark.

(1) It's too informal; that person should speak more formally.

(2) I like that person's zeal for getting the job done.

This varies from business to business, of course, and even from boss to boss. In general, though, when someone has a reputation for being a top performer who does high-quality work, they can often get away with more informal expressions in the workplace.

If someone works for me, and I know they are dependable and reliable (they get things done right and on-time), then something like “Sure thing, boss” is like sweet music in my ears. I know I don't have to worry about the task any longer.

However, if this person's performance is inconsistent – if the person is someone who frequently "drops the ball" – then that same expression might grate on me. Sure, I'll appreciate their enthusiasm, but it will only remind me how much I wish we had someone more dependable on the payroll. Even though it's just an informal expression, you still don't want to assure your boss that something is a sure thing unless it is assuredly a sure thing!

So, remarks like this depend on a number of factors: the nature of your business, the company culture, your relationship with your boss, your supervisor's temperament, and your own performance and reliability. If you're new at your job, I'd ease into it over time.

"Sure thing," although informal, is synonymous with "certainly." If you and your boss have an informal type of relationship (a good barometer may be if you're on a first name basis with him/her), then you could use it. If you want to respond formally, use "certainly." If you're unsure of the accepted etiquette, just use "yes;" its meaning is completely neutral and clear.