How to Use AI for Self-Employment Tax: A Monthly Set-Aside Sheet

To work out how much to set aside for tax each month, build a sheet that adds up income, subtracts allowable expenses, projects the year's profit, applies your tax authority's current rates, and shows the result as a percentage of profit. Move that percentage of every payment into a separate savings account. Use AI to build and explain the sheet, not to supply the rates.

This is a method, not tax advice. Rates, allowances and deadlines differ by place and change every year, so every figure in the examples below is a placeholder invented to show the arithmetic. Replace each one with a number from your tax authority's current published page, and have an accountant review the result at least once.

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What the sheet does, in one picture

Four tabs, each with a single job:

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1. Settings: the rates and allowances for the current tax year, copied from the official page, with the page address and the date you copied them.
2. Income: one row per payment received, with date, client, amount, and a column for any sales tax or similar charge collected on behalf of the authority, which is not your income.
3. Expenses: one row per cost, with date, category, amount and an "allowable?" column (yes, no, partly).
4. Monthly: profit to date, projected profit for the year, estimated tax, the set-aside percentage, the amount to move this month, and the running balance of your savings pot against what the sheet says you owe.

The person in the example is a self-employed mobile bicycle mechanic who visits customers at home and invoices by the job. Income is uneven: quiet in winter, busy from spring. That unevenness is exactly why a flat "put aside a quarter" rule fails, and why the sheet projects a year rather than reacting to one month.

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Step one: copy the official rates into Settings

Find your tax authority's page on rates for the self-employed in the current year. Search for the authority's name plus "self-employed rates" and open the official page, not a summary on a third-party site. Copy five things into the Settings tab:

• the tax-free allowance for the year,
• each rate band: where it starts, where it ends, and its percentage,
• any social contribution charged on self-employed profit (in some systems this is the charge called self-employment tax): thresholds and rates,
• deadlines for filing and paying,
• whether payments on account (advance instalments) apply to you.

In the examples below, the placeholder rules are: an allowance of $12,000; a 20% rate on profit between $12,000 and $50,000, and 40% on profit above $50,000; a social contribution of 9% on profit between $8,000 and $50,000 and 2% above $50,000. Real systems have more moving parts, so treat these as a teaching model.

Now the AI's job. Paste the text of the official page into a chat and ask for a tidy settings table, so the assistant works from the page and not from memory:

Below is text copied from my tax authority's page on self-employed rates for the current year. Extract ONLY what the text states into a table with columns: item, threshold or band start, band end, rate or amount, and the exact sentence it came from.
If something I would need is not in the text (for example a contribution rate), write "NOT IN TEXT" and do not fill it from your own knowledge.
Text:
{{paste official page text}}

The "exact sentence" column lets you check each number in seconds. The "NOT IN TEXT" instruction is the one that matters, because a model's remembered rates can be a year old, or from a different system altogether.

Step two: capture income and expenses without retyping

Most self-employed people already hold both lists somewhere: a business bank account export, invoicing software, a pile of receipts. The least painful route is to download a bank statement as a spreadsheet file and let AI sort it.

Illustrative before and after. Three raw lines from a statement:

03/04  CARD PMT  TOOLSUPPLY LTD 4471     -86.40
05/04  FPI       J SMITH  INV 0212          +240.00
09/04  DD        PHONECO  MOBILE            -28.00

After one prompt (give the categories you use and ask for "personal or unsure" to be flagged rather than guessed):

The phone line shows the one habit worth building: mixed-use costs get a business-use percentage, set once with your accountant, never decided by the AI line by line. The same goes for anything that looks personal. For a longer routine on cleaning exports like this, cleaning messy data goes through the typical problems, and a mileage log has its own rules, covered in a mileage log for the self-employed.

Do not paste bank statements containing account numbers or other people's personal details into a chat tool you have not checked. Replace long references with short ones, or work from an export that has only date, description and amount.

Step three: the Monthly tab, with real arithmetic

Six months of illustrative figures for the mechanic (all invented, in dollars):

The projection formula is the heart of the sheet. A simple one: projected annual profit = profit to date divided by months elapsed, times 12. Here that is $23,000 / 6 x 12 = $46,000. (If your work is seasonal, use last year's pattern instead, or the trailing twelve months once you have them.)

Apply the placeholder rules to $46,000:

• Taxable profit: $46,000 - $12,000 allowance = $34,000, all inside the 20% band: $6,800.
• Social contribution: ($46,000 - $8,000) x 9% = $3,420.
• Total: $10,220, which is 22.2% of $46,000.

Round up to 23% to leave a small buffer. Each month's set-aside is then 23% of that month's profit: January $621, February $713, March $943, April $874, May $1,104, June $1,035, together $5,290 on $23,000. Check by multiplying: $23,000 x 0.23 = $5,290. The money goes by bank transfer into a savings account used for nothing else, on the day the payment lands, not at month's end when the balance has already been spent.

The formulas, in plain words

Ask the AI to write them from the layout, and then read each one against this description:

• Profit to date = SUM of income to date minus SUM of allowable expenses to date.
• Projected profit = profit to date / months elapsed x 12.
• Income tax = (MIN(projected, band limit) - allowance) x band-1 rate, plus MAX(0, projected - band limit) x band-2 rate, with the first term floored at zero.
• Contribution = (MIN(projected, upper limit) - lower threshold) x rate 1, plus MAX(0, projected - upper limit) x rate 2, floored at zero.
• Set-aside % = (income tax + contribution) / projected profit, rounded up to the next whole percentage point.
• Set aside this month = month's profit x set-aside %.

Test the sheet with the six-month figures above. If it returns 22.2% before rounding, it works. If it does not, the cause is nearly always a band boundary in the wrong place. Language models write these nested MIN and MAX formulas well, and also get a boundary wrong now and then, which is why the test case matters. If a formula you are handed makes no sense, getting an assistant to explain a formula line by line shows how to take it apart before you trust it.

When the year changes mid-way: a rising-income test

Now suppose in July the mechanic signs a contract with a local rental fleet and the projection jumps from $46,000 to $60,000. Rerun the sheet, using the same placeholder rules:

• Taxable profit: $60,000 - $12,000 = $48,000. The first $38,000 sits in the 20% band: $7,600. The remaining $10,000 is taxed at 40%: $4,000. Income tax $11,600.
• Contribution: ($50,000 - $8,000) x 9% = $3,780, plus ($60,000 - $50,000) x 2% = $200. Total $3,980.
• Combined: $15,580, or 26.0% of $60,000.

The set-aside moves from 23% to 26% (25.97%, rounded up). That is the very thing a flat rule misses. Because six months at 23% already left $5,290 in the pot, the sheet should also show a catch-up figure: the new percentage applied to profit to date, minus what is saved. On $23,000 at 26% that is $5,980, so the pot is $690 short. Saving 26% of the remaining months' profit will not close that gap by itself, so either move the $690 now or add about $115 to each of the six remaining transfers. A mechanic who knows in July will not meet a surprise in January.

Where the AI gets it wrong, and how to catch it

Four faults come up repeatedly with tax sheets, and none shows up unless you test.

1. Stale rates. The model fills a missing rate from memory. Fix: the "NOT IN TEXT" rule above, and a "source and date" cell beside every rate.
2. Cash versus invoice date. Some systems tax profit when invoiced, others when received, and small businesses are often allowed a choice. Ask your accountant which applies, then make the Income tab use the matching date.
3. Treating collected sales tax as income. If you charge sales tax or similar on behalf of the authority, it is not yours. The separate column keeps it out of profit. A sheet that lumps it in overstates profit and over-saves.
4. Personal spending hiding in expenses. A coffee machine is not an expense because it came from a business account. The "allowable?" column and a yearly review with your accountant guard against it.

A fifth, subtler one is the first year. In some systems, your first bill includes advance payments towards the following year, so the money needed in one month can be nearly double the "annual tax" figure you expected. Read the official page on advance or instalment payments and put the dates in the Settings tab. If the page is unclear, that is the sort of question to ask an accountant before the first deadline, not after.

A quiet month, a lumpy month: two tests for the sheet

A sheet that only works in steady months is not much use, so run two awkward cases through it before relying on it.

The quiet month. In July the mechanic is ill for two weeks. Income received is $1,900 against expenses of $800, so profit is $1,100. A naive reading says "I made little, so I owe little", and the monthly set-aside at 23% is only $253. The projection, though, now uses seven months: profit to date $24,100, divided by 7 and multiplied by 12 gives $41,314. The set-aside percentage drops slightly (tax $5,863 plus contribution $2,998 gives $8,861, or 21.4%, which rounds up to 22%). Keeping the 23% you already save, rather than cutting it for one weak month, leaves a cushion in place. The lesson is that one weak month moves the projection a little and the percentage less, which is exactly the smoothing you want. If you are tempted to skip the transfer because the month was thin, the sheet says why you should not.

The lumpy month. A customer pays a large invoice that covers three months of work in one go: $9,000 lands on 28 August. The cash view says August was spectacular, and the pot receives 23% of its profit. If the work was actually done across June to August and your accountant says income counts when received, the August figure is correct for tax and the projection will rise accordingly. If the rules say income counts when the work was invoiced, the date in the Income tab should be the invoice date instead. Either way, check the rule once, write it in the Settings tab, and do not let the sheet quietly mix both.

A third quick sum worth doing at the year's start is the "worst plausible year": halve your best month, multiply by twelve, and see what percentage comes out. If the answer is far from your usual set-aside, your pot is more exposed to a single customer than you think, and that is worth knowing before a bill arrives.

A weekly rhythm that keeps the pot honest

The sheet is only as good as the habit feeding it. A routine that takes fifteen minutes a week:

1. Friday: export the week's bank lines and paste them into a scratch tab.
2. Run the categorising prompt, and fix anything flagged "unsure".
3. Move the checked rows into Income and Expenses.
4. Read the Monthly tab: profit to date, projection, set-aside percentage, pot against target.
5. Transfer the week's set-aside into the savings account.

On the first day of each quarter, rerun the Settings check against the official page in case rates or thresholds moved, and save a dated copy of the sheet. If you invoice on time, the cash side is calmer; polite reminders for unpaid invoices and invoices and chasers for freelancers cover that part.

The same sheet for a bakery, an HVAC contractor and a solar installer

A home baker selling at markets has cash receipts and lots of small purchases. The Income tab collects market takings by day, and the expenses are ingredients, packaging, stall fees and a share of home utilities, the last of which needs the business-use percentage agreed with an accountant. The set-aside percentage is likely to be lower in her first year if profits sit below the allowance, and the sheet shows that without anyone guessing.

A self-employed HVAC contractor has large material purchases and a van. The sheet needs a vehicle row, with business mileage kept in a log, and an equipment tab for bigger purchases that may be written off in a special way. Do not let AI decide how an asset is treated; record the purchase and ask your accountant what the rules allow.

A self-employed solar installer earns in irregular, large lumps. Projecting annual profit from three good months overstates it. Use trailing twelve months, or enter known contracts for the rest of the year and project from the pipeline. The set-aside percentage should then be recalculated after each large payment, because one big job can move a person into a higher band.

Which AI tools help, and what each is for

Any of the main assistants can build and check this sheet: ChatGPT, Claude and Gemini all take pasted text and files, and all write spreadsheet formulas. Google Sheets and Excel also have assistants built in, which are handy for filling a formula down a column. ChatGPT Plus and Claude Pro each list at $20 a month billed monthly, and the free tiers are enough for a build like this. Whichever you use, keep the private parts of your finances to the minimum the task needs, and read its data-controls page before uploading statements.

The division of labour is the same in every tool. AI is good at tidying a messy export, writing formulas, explaining a term, and drafting questions for your accountant. It is poor at knowing today's rates, at deciding what is allowable, and at spotting what is missing from your records. The first group goes in the sheet; the second stays with the official page and a person qualified to give tax advice.

Questions to bring to the accountant

An hour with an accountant goes further when you arrive with the sheet and a short list. Ask the AI to draft it from your own sheet, then trim it to the questions you really have:

• Which of my expense categories are allowable, and at what business-use percentage?
• Do I report income when invoiced or when received?
• Do advance payments apply to me, and what are the dates?
• Does my projection look sensible, and is a 23% set-aside enough?
• Is there anything I should be recording that I am not?

The sheet turns that conversation from "I have a box of receipts" into "here are my numbers, please correct them", which is faster and, for most accountants, cheaper. If you would like help building or reviewing a routine like this around the tools you already use, that is the kind of thing my AI implementation consultation covers, and you can reach me through the contact page. A sensible next read on the wider question is what AI can safely prepare for business taxes.

Questions before you trust a set-aside sheet

Can AI tell me how much tax I owe?

It can calculate from rates you give it, but it cannot know your full position, and it may quote out-of-date rates from memory. Use it to build the sheet and explain terms. Take the rates from your tax authority's current page, and ask an accountant to confirm the result before you file.

What percentage should a self-employed person save?

There is no single figure. It depends on profit, allowances, your other income and the rates that apply to you. Rules of thumb such as a quarter of profit are only a starting guess: in the placeholder example the answer moves from 23% to 26% when one contract arrives. The sheet shows how to calculate your own figure instead.

Should I set aside from income or from profit?

From profit, because tax is charged on income less allowable costs. A simple method is to save a percentage of each payment received and then true it up monthly against profit. Setting aside from gross income overshoots in costly months and undershoots in cheap ones.

What if I do not have enough saved when the bill comes?

Contact the tax authority before the deadline. Many offer instalment arrangements, and acting early is treated more kindly than silence. Do not use a credit card to pay the bill without comparing the interest against any instalment plan on offer. An accountant can advise on your options.

Further reads

• How to Find AI Savings Line by Line in Your Profit and Loss — Read your profit and loss line by line.
• How Accountants Use AI to Explain Tax to Clients in Plain English — How accountants explain tax in plain words.
• Tax Software With AI: What a Small Business Should Check — What to check if you move from a sheet to tax software.
• Sole Trader MTD: What AI Can and Cannot Do With Quarterly Updates — Quarterly digital record-keeping and what AI can and cannot do.
• Why Is AI Bad at Maths? What to Check in Quotes and Invoices — Why every total in the sheet needs checking by hand.
• Salon Commission or Booth Rent? Model Both on Your Numbers With AI — Work out the break-even rent for a salon chair at three occupancy levels, build the sheet with ChatGPT and check employment status first.
• CIS Monthly Return: Software, Accountant or an AI-Built Sheet? — What you need to file a CIS monthly return, the April 2026 nil-return change, and how software, an accountant and an AI-built sheet compare.
• AI Tools and AI Development: The Complete 2026 Guide — the AI hub, including every tutorial in the AI-for-business series.

Sources: general bookkeeping practice; the reader's own tax authority pages for rates and deadlines; ChatGPT and Claude pricing pages, checked 5 November 2026.