Side Income Tax Set-Aside Calculator — Rex's Toolbox

Rex says

Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.

Try a scenario

Click to load — tweak from there.

Inputs

Expected side income($/yr)

Deductible business expenses($/yr)

Self-employment tax rate(%)

Marginal income tax rate(%)

State income tax rate(%)

Typical payment size($)

Result

Set aside this share of every payment

33%

Hold back per typical payment

$500

Estimated quarterly instalment

$2,000

Total estimated tax

$8,001

Self-employment tax portion

$2,882

What you actually keep

$12,399

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How to use this

1Enter expected side income ($/yr).
2Enter deductible business expenses ($/yr).
3Enter self-employment tax rate (%).
4Enter marginal income tax rate (%).
5Enter state income tax rate (%).
6Enter typical payment size ($).
7Read your set aside this share of every payment on the right — it updates as you type.
8Hit Share to keep the scenario or send it to someone.

About this calculator

Side income arrives without withholding, which means the tax bill arrives all at once later. This calculator estimates the percentage of each payment to move into a separate account: self-employment tax on net profit plus income tax at your marginal rate, after deductible business expenses. It returns the set-aside rate, the dollar amount per payment, and an estimated quarterly instalment, so you can automate the transfer the moment money lands. Treating that transfer as non-negotiable is what separates people who enjoy side income from people who dread April.

FormulaNet profit = gross − expenses. SE tax ≈ net profit × 92.35% × SE rate. Income tax ≈ (net profit − half of SE tax) × marginal rate. Set-aside % = total tax ÷ gross.

Worked example

Using the values the calculator loads with:

Inputs

Expected side income: 24000 $/yr
Deductible business expenses: 3600 $/yr
Self-employment tax rate: 15.3 %
Marginal income tax rate: 22 %
State income tax rate: 5 %
Typical payment size: 1500 $

Results

Set aside this share of every payment: 33.3%
Hold back per typical payment: $500.08
Estimated quarterly instalment: $2,000.33
Total estimated tax: $8,001.30
Self-employment tax portion: $2,882.43
What you actually keep: $12,398.70

What each field means

Inputs

Expected side income ($/yr): The expected side income used in the calculation, measured in $/yr. Starts at 24000 $/yr so you have a working example on load.
Deductible business expenses ($/yr): The deductible business expenses used in the calculation, measured in $/yr. Starts at 3600 $/yr so you have a working example on load.
Self-employment tax rate (%): The self-employment tax rate used in the calculation, measured in %. Starts at 15.3 % so you have a working example on load. Accepted range: 0–30 %.
Marginal income tax rate (%): The marginal income tax rate used in the calculation, measured in %. Starts at 22 % so you have a working example on load. Accepted range: 0–60 %.
State income tax rate (%): The state income tax rate used in the calculation, measured in %. Starts at 5 % so you have a working example on load. Accepted range: 0–15 %.
Typical payment size ($): The typical payment size used in the calculation, measured in $. Starts at 1500 $ so you have a working example on load.

Results

Set aside this share of every payment: Returned as a percentage and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Hold back per typical payment: Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Estimated quarterly instalment: Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Total estimated tax: Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Self-employment tax portion: Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
What you actually keep: Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.

FAQ

Is this a substitute for a tax professional?

No. This is a planning estimate to size a set-aside, not a filing. Brackets, thresholds, deductions like QBI, and state rules change annually and vary by situation — confirm your actual liability with a qualified preparer before relying on a number.

Do I have to pay quarterly estimated tax?

In the US, generally yes once you expect to owe a meaningful amount beyond withholding, and underpayment penalties apply if you fall short. One common workaround for people with a day job is to increase W-2 withholding to cover the side income instead of filing quarterlies.

What counts as a deductible expense?

Costs that are ordinary and necessary for the work — software, equipment, a portion of home internet, professional fees, mileage for business travel. Keep contemporaneous records; a deduction you cannot evidence is a deduction you cannot defend.

Why is the self-employment tax base 92.35% of profit?

The rule allows a deduction for the employer-equivalent portion before the tax is applied, so the base is net profit multiplied by 0.9235. It is a small reduction but it is built into the standard calculation, which is why it appears here.

Accuracy and limitations

Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.

Related tools

Consulting Day Rate Calculator

What to charge per day to hit your income target.

Personal Financial Runway Calculator

How many months your savings cover you if income stops today.

Savings Goal Date Calculator

Find the month you hit a savings target at your current contribution and rate.

Cite this calculator

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APA

RevenueLab. (2026). Side Income Tax Set-Aside Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/side-income-tax-set-aside

HTML

<p>Source: <a href="https://www.revenuelab.fyi/toolbox/side-income-tax-set-aside" target="_blank" rel="noopener">Side Income Tax Set-Aside Calculator — RevenueLab</a> (2026).</p>

Markdown

Source: [Side Income Tax Set-Aside Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/side-income-tax-set-aside) (2026).