As a full-time freelance writer, I'm a self-contained small business. I write for 40-plus hours a week, get paid for what gets published, and pay myself a salary through my business entity.

It's a lovely, albeit challenging, lifestyle in which I have both enormous flexibility and tremendous responsibility. Nobody can fill in for me if I'm out, sick, or otherwise unable to work, but I can trade business hours for evenings and weekends if I so choose.

The challenge -- and it's one that faces many people who have a sideline they like more than their real job -- came around five years ago when I had just started pursuing this path. I was working as the editor of two local daily newspapers and writing on the side (generally very early in the morning).

I had begun working for The Motley Fool, and things were promising, but nothing was promised. If I could write full time, things would probably happen faster, but they could also flame out, leaving me with no job and no sideline. 

A woman drives a car.

You need to really understand the numbers before you make a side business, like driving for a ride-share service, your full-time job. Image source: Getty Images.

What to consider?

In my case, I had a wife and son and a mortgage and made slightly more money than my wife did at the time. I was also burning out quickly, as being a good daily newspaper editor at a poorly staffed paper was a very demanding job.

I made the leap without doing any of the things I'm about to tell you to do. That's partly due to self-confidence and at least partly due to realizing I couldn't keep up the pace I was maintaining for much longer.

What I should have done was conduct a thorough examination of my income, savings, and monthly budget. I could have then compared that to how much work was likely being offered to me as soon as I made the leap.

No matter what your side business is, you need to understand how much money you need each month and what it might take to get that. If you make $200 driving for a ride-share service on Friday and Saturday nights, can you make anywhere near that on weekdays or nights?

Talk to people in the field and get not only realistic numbers for income but also for costs and how long it took them to ramp up. You, for example, may have too many houses to clean or too many people's taxes to do along with your regular job but not enough to deliver the income you need. That's not a total red flag, but before you take the leap, you need to have a plan for bridging that gap.

Bet on yourself

Don't ignore the numbers and leave a job to make your side hustle your full-time gig if things simply don't line up. If, however, you see a logical path to get where you want to go and you have the short-term resources to back your own play, then make it happen.

That's not to say you should walk into your boss's office and quit now. Instead, make sure you have a proper emergency fund and maybe a little extra for problems you don't foresee. It's very rare for things to go exactly as planned, but you can survive disasters if your plan assumes they will come.

Go in confident but be ready for the worst. If something bad doesn't happen, then you will be in a strong position right from the start.