Why does the quarterly number feel impossible to guess?

Because you are being asked in April to predict December.

A contractor on r/selfemployed put it better than any tax article has, on 2 August 2026: "quarterly estimated taxes are still the part of this that makes me the most uneasy… A slow month followed by a big project month makes it genuinely hard to know what to set aside without either overpaying all year or scrambling in April." That is the whole problem in two sentences. Employees never face it because payroll takes a cut of every check automatically. You have to be the payroll department, and the payroll department has to guess.

The reply that actually solved it for people in that thread did not involve a better forecast. It involved giving up on forecasting: "every payment that lands, a flat cut goes straight to a separate account the second it hits, before i think of it as mine… i stopped trying to forecast quarters entirely."

That works because a percentage is self-correcting. Bill $4,000 in February and you sweep a small amount. Bill $31,000 in July and you sweep a big one. You never had to know in advance which month was which. When the quarterly date arrives you are not deciding what you can afford — you are moving money that has been sitting in a different account for weeks with somebody else's name on it.

The other half of the problem is that most people badly underestimate the size of the bite, because they are thinking about income tax and forgetting the bigger one. On $78,000 of profit, a married couple in the example loaded above pays about $8,100 of federal income tax and about $11,000 of self-employment tax. The tax nobody talks about is the larger of the two.

How much should you actually set aside?

Here is the honest version, run through the calculator above at four profit levels for a single filer with no other household income and no state tax, so you can see the shape of it.

Federal only, single, standard deduction, QBI deduction applied, nothing else. Add a state at 4.5% and every one of those percentages climbs by four and a half points. Add a spouse with a salary and it climbs again, because their income fills the low brackets first and your profit stacks on top at a higher rate.

Which is why "set aside 30%" is decent generic advice and a poor personal answer. At $25,000 of profit even with a state, 30% overshoots by more than half and you have parked money you needed for a truck payment. At $120,000 with a working spouse in a 6% state, 30% is cutting it fine. The whole reason this calculator asks about your spouse and your state is that those two inputs move the answer more than anything else on the page.

One more thing the table hides. Those percentages are of profit, not of deposits. If you collect $140,000 and spend $62,000 running the business, a 27.2% of profit answer is only 15.2% of the money coming through the door. Sweeping 27% of every deposit would be a very expensive mistake in the other direction. The calculator does both conversions for you, and the one to act on daily is the percentage of deposits.

What do the paid tools cost, and what do you get?

List prices as of August 2026, taken from the vendors' own pricing pages.

Sources, checked 8 August 2026: Intuit QuickBooks pricing pages (Solopreneur and Online Simple Start, after the 1 August 2026 increase) · Hurdlr pricing · Everlance pricing page and its free 1099 tax calculator. CPA fee range is a commonly reported spread, not a survey. Prices in this category move — check before you buy.

Two honest things about that table. First, the paid mileage and expense apps are not selling you tax math. They are selling you the part that is genuinely hard, which is capturing 300 small transactions and 8,000 miles over twelve months without sitting down to do it. If your profit number is a guess, a better calculator will not save you — better records will. Second, once your return has employees, an S-corp election, rental property, or a partner in it, none of this is enough and a good accountant is the cheapest thing you will buy all year.

Where is this calculator enough, and where is it not?

It is enough if you are one to three people, a sole proprietor or a single-member LLC that has not elected S-corp, you take the standard deduction, and your return is business profit plus maybe a spouse's paycheck. That describes most of the trades. For that shape of business the arithmetic on this page is the same arithmetic on Form 1040-ES, and you will land within a few hundred dollars of the real number.

It is enough to answer the two questions that actually get asked: what percentage do I sweep, and how much do I send on the fifteenth.

It is not enough for these, and it will not pretend otherwise.

• Kids and credits. The child tax credit, the earned income credit, education credits and dependent care all cut real money off the bill and none of them are modelled here. If you have children, your true number is lower than this page says, sometimes by thousands.
• Retirement and health insurance. A SEP-IRA or solo 401(k) contribution and self-employed health insurance premiums both come off before income tax. They are two of the biggest levers a profitable sole proprietor has, and this page ignores both. That means it errs high, which is the safer direction, but it also means you might be leaving money on the table.
• S-corp anything. If you pay yourself a W-2 wage out of your own company, the self-employment tax math on this page does not apply to you at all.
• The QBI wage limits. Above $201,750 of taxable income single, or $403,500 married filing jointly, the QBI deduction gets limited by W-2 wages you pay and property you own. This page will flag it and then send you to a professional, because modelling it honestly takes more inputs than anyone will type into a web page.
• Your actual state. One flat rate is a planning approximation, nothing more. Some states have brackets, some have a flat rate, some have local add-ons, and a few tax business income in ways that have nothing to do with your personal return. Nine states do not tax earned income at all.
• Being your books. This is a calculator, not a ledger. Use the free expense tracker to build the profit number you type into it.

How do you use it?

1. Put in what you expect to collect and what you expect to spend. Use last year as the starting point and adjust for what you know about this year. Nobody's forecast is right; the point is to be roughly right rather than precisely nothing.
2. Set your filing status and add every other dollar on the return. A spouse's $45,000 salary can move your set-aside percentage several points, because it uses up your low brackets before your profit even starts.
3. Put a state rate in. Zero in the nine no-tax states. Everywhere else, last year's state tax divided by last year's state taxable income is close enough. Round up if you are unsure — being over on tax money is a much better problem than being under.
4. Open a separate account and sweep that percentage on the day money lands. Not weekly. Not monthly. The day it lands, before it feels like yours. A plain savings account at the same bank is fine, and it should be boring and slightly annoying to move money back out of.
5. Check the four quarterly amounts against safe harbor and pay whichever is smaller. If you had a much better year than last year, the prior-year safe harbor is a genuinely cheap way to stay penalty-free while keeping the cash in your business until April.
6. If your year is lumpy, run Step 3 and see what the annualized method would let you pay. Then decide if your records are good enough to defend it.
7. Print the worksheet in November and take it to whoever does your return. It shows your assumptions in one page, which is a much better conversation than "I think I made about a hundred grand."

How does the 2026 tax math actually work?

This is the reference section. Every figure below is from an IRS source, dated, and the whole return is worked longhand at the end so you can check the calculator against it.

Step one: self-employment tax, which is the big one

Self-employment tax is Social Security and Medicare for people without an employer to split it with. An employee pays 7.65% and their employer pays the other 7.65%. You are both, so you pay 15.3% — 12.4% Social Security plus 2.9% Medicare.

It is charged on 92.35% of your net profit, not all of it. That 7.65% haircut exists to imitate the employer half being deductible. So $78,000 of profit produces $78,000 × 0.9235 = $72,033 of self-employment earnings.

The Social Security half stops at the 2026 wage base of $184,500. Above that you only keep paying the 2.9% Medicare piece. If you also hold a W-2 job, your wages use up that base first, which is why this calculator asks about your own wages separately from the rest of the household's.

Then there is Additional Medicare tax: another 0.9% on earnings above $200,000 single or head of household, $250,000 married filing jointly, $125,000 married filing separately. Those thresholds are fixed in the statute and are not adjusted for inflation, so more people cross them every year. Unlike the rest of self-employment tax, this 0.9% is not deductible.

Finally, half of your self-employment tax comes off your income before income tax is figured — the same idea as the employer half being a business expense. On $11,021 of SE tax that is a $5,510 deduction, worth about $661 if you are in the 12% bracket and $1,212 if you are in the 22% one.

Step two: income tax on what is left

Adjusted gross income is business profit, plus your wages, plus everything else on the return, minus half the self-employment tax. From that you take the 2026 standard deduction: $16,100 single or married filing separately, $32,200 married filing jointly, $24,150 head of household.

Then the QBI deduction, section 199A, which is 20% of qualified business income and is the best deal a sole proprietor gets. Qualified business income is your net profit reduced by the deductible half of self-employment tax. It comes off taxable income but does nothing to self-employment tax. For 2026 the thresholds where the wage and property limits start are $201,750 for most filers, $201,775 married filing separately and $403,500 married filing jointly, and the limits phase in fully over the next $75,000 ($150,000 joint). The One Big Beautiful Bill added a new floor for 2026: if you have at least $1,000 of qualified business income from an active trade, the deduction is at least $400.

What is left is taxable income, and it runs through the 2026 brackets.

2026 brackets, standard deduction and section 199A thresholds from the IRS news release on tax year 2026 inflation adjustments and Revenue Procedure 2025-32, published October 2025. Social Security wage base of $184,500 for 2026 announced by the Social Security Administration in October 2025. Additional Medicare tax rate and thresholds from IRS Topic no. 560. Self-employment tax mechanics, the 92.35% figure and the four due dates from the 2026 Form 1040-ES. Underpayment interest rate of 7% for the quarter beginning 1 July 2026 from Rev. Rul. 2026-10, Internal Revenue Bulletin 2026-22. Annualization amounts and applicable percentages read off Schedule AI of Form 2210.

A full return, worked by hand

Married filing jointly. $140,000 collected, $62,000 of business costs, so $78,000 net profit. Spouse earns $45,000. No W-2 wages of your own. No state tax for this example. Standard deduction, no children, no retirement contributions.

1. Self-employment earnings: $78,000 × 0.9235 = $72,033.00
2. Social Security: $72,033.00 is under the $184,500 base, so × 12.4% = $8,932.09
3. Medicare: $72,033.00 × 2.9% = $2,088.96
4. Additional Medicare: earnings of $72,033 are under the $250,000 joint threshold, so $0.00
5. Self-employment tax: $8,932.09 + $2,088.96 = $11,021.05
6. Half of it, deductible: $11,021.05 ÷ 2 = $5,510.52
7. AGI: $78,000 + $45,000 − $5,510.52 = $117,489.48
8. Less standard deduction: $117,489.48 − $32,200 = $85,289.48
9. QBI: $78,000 − $5,510.52 = $72,489.48. Twenty percent of that is $14,497.90. The taxable-income cap is 20% × $85,289.48 = $17,057.90, which is higher, so the deduction is $14,497.90
10. Taxable income: $85,289.48 − $14,497.90 = $70,791.58
11. Income tax, joint: 10% of the first $24,800 = $2,480.00. Then 12% of $70,791.58 − $24,800 = $45,991.58 → $5,518.99. Nothing reaches 22% because the bracket starts at $100,800. Total $7,998.99
12. Total federal: $11,021.05 + $7,998.99 = $19,020.04, or $4,755.01 a quarter
13. Tax the business itself creates: with no business income, the couple's $45,000 salary alone would produce $1,280.00 of federal tax ($45,000 − $32,200 = $12,800 taxable, all in the 10% bracket). So the business adds $19,020.04 − $1,280.00 = $17,740.04
14. Set-aside as a share of deposits: $17,740.04 ÷ $140,000 = 12.7%. As a share of profit: $17,740.04 ÷ $78,000 = 22.7%

Load the page with a 4.5% state rate on top, as it loads by default, and the state adds $3,510 on the $78,000 of profit, pushing the sweep to 15.2% of deposits and 27.2% of profit. That single input moves the answer by two and a half points of every deposit, which is why "just save 30%" is not an answer.

Safe harbor: the rule that stops the penalty

The 2026 Form 1040-ES says you must pay estimated tax if you expect to owe at least $1,000 after withholding, and that you avoid the underpayment penalty if your payments and withholding come to the smaller of:

• 90% of the tax on your 2026 return, or
• 100% of the tax on your 2025 return — but 110% if your 2025 AGI was over $150,000, or over $75,000 if you file married filing separately.

As a contractor on r/selfemployed put it: "Safe harbor only protects you from the penalty… it's 100% of last year only if your prior-year AGI was under $150k. Above that it's 110%." Both halves of that sentence matter. The 110% catches people out constantly, and so does the word "only" — safe harbor is a penalty shield, not a payment plan. If you earn double this year and pay the prior-year safe harbor, you will be penalty-free and you will still write a very large cheque in April.

The useful way to read it: prior-year safe harbor is a ceiling on what you must pay now. In a year that is going better than last year, it lets you keep cash in the business through the season and settle up in April with no penalty. In a year that is going worse, 90% of the current year is the smaller number and you should use that instead. The calculator picks the smaller of the two for you and shows both, because which one wins flips depending on the year you are having.

What the penalty actually costs

It is not a fine. It is interest, charged on each installment from the day it was due until the day the money arrives, at the federal underpayment rate. That rate was 7% for the quarter beginning 1 July 2026, and it is reset quarterly at the federal short-term rate plus three points.

Do the arithmetic on a real miss. You should have sent $3,000 on 15 June and you send it on 15 December instead. Six months at 7% annual is roughly $3,000 × 0.07 × 0.5 = $105. That is the whole penalty. It is worth avoiding and it is not worth panicking over, and it is definitely not worth skipping a payment entirely because you cannot make the full amount. Send what you have on the date. Interest only runs on the part you were short.

The annualized method, for a year with a shape

Four equal payments are the default, not the law. Schedule AI of Form 2210 lets you compute each installment on the income you actually had by that point. The periods and the numbers printed on the form:

Look at the second row for a second. The June payment covers April and May only — two months, not three — and yet by then you must have paid 45% of the year's tax. Landscapers, pool guys, HVAC shops and anyone else whose money arrives between May and September get hit by that one every year, which is exactly what the annualized method is for. Under it, a spring where you earned very little produces a small first installment legitimately, instead of an even quarter you cannot fund.

The cost is paperwork. You file Form 2210 with Schedule AI, and you need books good enough to show income by period. If your records are a folder of photographs of receipts, four equal payments is the safer trade.

The mistakes that cost people money

• Saving a percentage of profit but sweeping it off deposits. Twenty-eight percent of profit and 28% of deposits are wildly different amounts. On the numbers loaded above, one is $21,840 and the other would be $39,200.
• Forgetting the spouse. Your profit stacks on top of their salary. Run it without them and your set-aside percentage comes out several points too low.
• Treating a 1099-NEC as the income number. Your income is profit, not the total of the 1099s. Every deductible dollar you spent running the business comes off first, which is why the expense tracker earns its keep.
• Paying on invoices instead of collections. Almost everyone is on the cash method. The December invoice paid in February is next year's problem.
• Skipping a payment because you cannot make the whole thing. Interest runs on the shortfall only. Partial beats nothing every time.
• Spending the tax account. The reason it works is that the money leaves. A separate account at a separate bank, with no card attached, is not paranoia — it is the entire mechanism.
• Forgetting the state. Federal estimated payments go to the IRS; state estimated payments go to your state, on their own schedule and their own forms. Two payments, not one.

Common questions