Check out their 401(k), IRA, or Both? calculator which helps you determine just that. Likewise, their Retirement Calculator will help you calculate how much you'll need in retirement and what you should save to get there.
We've also added these calculators to our Investing (Retirement) resource page where you can view other nifty tools and calculators to help you on your way.
Note: Step One Investing has no partnership with Betterment and does not receive any financial compensation for reviews of the service.