For federal taxes, if you send the IRS 20% of your gross income each quarter, then come tax filing time you'll be fine. I've been doing this for almost 20 years, and at filing time I'm always well within $1000 of my tax liability. So I don't worry about any underpayment penalties. That 20% does include the additional 15.3% self-employment tax too. When you file your federal return, all the business deductions you're allowed to take will reduce your taxable business income - usually by quite a bit. Thus, sending the IRS 20% of your *gross* business income will be just fine.
For Hawaii, they don't have a sales tax "per-se". Instead, you pay a General Excise Tax (GET) on the gross business income.What the rate is, depends on the type of business. The tax rate is 0.15% for Insurance Commission, 0.5% for Wholesaling, Manufacturing, Producing, Wholesale Services, and Use Tax on Imports For Resale, and 4% for all others. For more details, see https://files.hawaii.gov/tax/legal/taxfacts/tf2015-37-1.pdf
For state income tax, which is separate from GET, the tax rate for 2022 runs from 1.4% to 11%, depending on your income. See https://www.tax-rates.org/hawaii/income-tax to get an idea of what you can expect to pay the state, based on your projected business income.
Note also that depending on what county you're in, you can also pay up to an additional 4% give or take, to that county.
For a non-resident starting a business in HI, I generally recommend you seek local professional help for that first year so that you get things set up and done correctly. Doing it wrong can (and will) cost you dearly down the road. Makes the cost of professional help for that first year well worth it. Especially in HI, as it's the highest taxing state in the nation when you combine GET with the state income tax.
My wife ran a daycare business when I was stationed at Hickam. After paying all the taxes and licensing fees, it was not worth it.
That ranges anywhere from 1.4% to a high of 11% for the 2022 tax year. See the FAQ at https://www.tax-rates.org/hawaii/income-tax. This FAQ does include the GET tax rate for the different income brackets.
