Estimate My BAS

An Australian business owner hands you the quarter's total sales and total purchases, and walks away with their Simpler-BAS GST figures laid out and ready to enter: G1 (total sales), 1A (GST on sales), and 1B (GST on purchases), each with the calculation shown openly and the ATO source for the GST rate cited so every figure is traceable. This app prepares the numbers. It never lodges anything: you enter them into your own BAS yourself.

This is the Australian tax partner to cash-flow-forecast and profit-per-job. Those run for any business anywhere; this one is Australia-only and stays switched off until the owner's profile explicitly confirms Region: AU.

Step 0: The region gate (do this FIRST, before anything else)

BAS is an Australian tax instrument, so this app runs only for an owner who has explicitly confirmed their business is in Australia. Before you load any constants, gather any figures, or prepare anything, read the Region: line from the owner's profile (./CLAUDE.md).

test -f ./CLAUDE.md && grep -i "^Region:" ./CLAUDE.md || echo "NO_REGION"

Decide from that line alone:

• If the Region: line is exactly AU (Region: AU), the gate is open. Continue to Step 1.

• If the Region: line is anything else (blank, missing, a placeholder like <<< AU if your business is in Australia ... >>>, or any other value), politely decline and STOP. Do not load constants. Do not prepare any figures. Say plainly, in your own warm words, something like:

  BAS prep is an Australian tax tool, so I keep it switched off unless your profile confirms your business is in Australia. If that's you, just say so and I'll set Region: AU in your profile, then we can run it. If not, no worries, your cash flow and pricing tools all work the same wherever you are.

Never infer Region from anything else. The free-text "based in <city, country>" line in the profile is descriptive only: even if it names an Australian city (Sydney, Brisbane, Perth), that does NOT open the gate. Language, timezone, currency, and address never open it either. Only an explicit Region: AU line does. If the owner says "yes, I'm in Australia," the right move is to record Region: AU in their profile first (or point them at /start-here's region question), then run.

Step 1: Take in the quarter's figures

Once Region: AU is confirmed, gather what you need, and ask only for the pieces that are genuinely missing:

• Total sales for the quarter (the GST-inclusive total of all sales for the period). This becomes G1. You can take a figure the owner types in, or point at a cash-flow-forecast output or a spreadsheet they already have.
• Total purchases for the quarter on which they are claiming GST credits (the GST-inclusive total of business purchases). This drives 1B.
• Whether sales and purchases are fully taxable (the standard case) or include GST-free or input-taxed lines. Simpler-BAS divide-by-11 assumes fully taxable amounts; if a chunk is GST-free (some food, exports), say so and only apply the GST calc to the taxable portion.
• Which quarter it is (e.g. Q1 July-September), so the figure is labelled.

If a number is missing, ask one plain question for it rather than inventing it. Never put a made-up figure into a tax number the owner may rely on.

Step 1b: Read the figures back before you compute (one line)

A wrong figure here flows straight into a number the owner enters on a government form. Before any maths, play the numbers back in one tight line and get a yes:

Here's what I've got for [quarter]: total sales $X (GST-inclusive), total purchases $Y (GST-inclusive), all fully taxable. That right?

If they correct one, take it and read it back once more. Only once the inputs are confirmed do you compute.

Step 2: Load the AU constants (the GST rate and the BAS field map)

Get the GST rate and the Simpler-BAS field definitions from the region-gated constants loader, never from memory. The loader returns the AU constants only for region == "AU" and raises for anything else, so it is a second backstop on the gate:

python -c "from tools.regional import load_au_constants; import json; c = load_au_constants('AU'); print(json.dumps({'rate': c['gst']['rate'], 'calc': c['gst']['calc_method'], 'fields': {k: c['gst']['bas_fields'][k]['label'] for k in ('G1','1A','1B')}}, indent=2))"

Read the GST rate (c["gst"]["rate"]["value"], currently 0.10), the calc method (c["gst"]["calc_method"], divide-by-11 for a GST-inclusive amount), and the G1 / 1A / 1B field definitions from c["gst"]["bas_fields"]. Use these values; do not hand-type the rate.

Step 3: Prepare G1, 1A, 1B with the calculation shown

Compute each field openly so nothing is a black box. For fully taxable amounts, the GST inside a GST-inclusive price is the price divided by 11 (the constants' calc_method):

• G1 (total sales): the GST-inclusive total of all sales for the quarter, taken straight from the owner's figure. G1 = total sales.
• 1A (GST on sales): the GST collected on taxable sales = taxable sales / 11. Show the division: "1A = G1 / 11 = $X".
• 1B (GST on purchases): the GST credits on creditable purchases = taxable purchases / 11. Show the division: "1B = purchases / 11 = $Y".
• Net GST position (for the owner's information, not a BAS field on its own): net GST = 1A minus 1B. If 1A is larger, that's roughly what they remit; if 1B is larger, it points to a refund position. State it as a plain-English read, not as advice.

Round to sensible money and say you rounded. If any line is GST-free or input-taxed, exclude it from the divide-by-11 and say which portion you treated as taxable.

Step 4: Cite the provenance (so the figure is traceable)

In the output, show where the GST rate comes from, straight from the constants module, so the owner (or their accountant) can trace it:

• the GST rate's source_url (the ATO page) and its effective_from date, both read from c["gst"]["rate"].

State it plainly, e.g.:

GST rate: 10% (effective from 2000-07-01). Source: the ATO "how GST works" page (c["gst"]["rate"]["source_url"]).

This is what makes the prepared figure auditable rather than a guess.

Step 5: Sanity-check the figures

Before you present, glance at the numbers and flag anything that looks off rather than presenting it silently:

• If purchases exceed sales by a wide margin (a large refund position) for a business that is normally net-positive, name it plainly as worth a second look.
• If a figure looks out of range for the quarter (e.g. a quarter's sales that look like a full year, or a number with a likely misplaced decimal), say so and ask the owner to confirm before they rely on it.
• If the owner mentioned GST-free or input-taxed sales but gave only one lump total, note that the divide-by-11 assumes fully taxable and that the GST-free portion should be carved out for an accurate 1A.

You are preparing figures, not auditing the business: a light, honest "this looks worth a check" is the right touch, never a made-up correction.

Step 6: Lay out the result

Present it as a clean read the owner can enter straight into their BAS:

## Your Simpler-BAS GST figures: [quarter]

| Field | What it is | Calculation | Amount |
|-------|------------|-------------|--------|
| G1 | Total sales (GST-inclusive) | as supplied | $… |
| 1A | GST on sales | G1 / 11 | $… |
| 1B | GST on purchases | purchases / 11 | $… |

**Net GST position:** 1A minus 1B = $… (for your information; enter G1, 1A and 1B on your BAS).

**GST rate used:** 10%, effective from 2000-07-01. Source: ATO "how GST works" (<source_url>).

**Assumptions** (the numbers this rests on):
- Quarter: [which quarter]
- Total sales (G1): $… [confirmed]
- Total purchases: $… [confirmed]
- Treated as fully taxable [or: GST-free portion carved out: $…]
- GST calc: divide-by-11 on GST-inclusive amounts
- Rounding applied: [how]

**You enter these. I don't lodge.** These are prepared for you to type into your
own BAS through the ATO (myGov / Online services for business) or your registered
agent. I have not filed, lodged, or submitted anything.

Frame it forward: this is the set of numbers the owner can enter with confidence, each one traceable to its source.

Step 7: Offer the reusable .xlsx (optional, never the win)

The read above IS the win: complete and usable as text. If the owner wants the prepared figures as a file they can keep with their records, offer to build a real .xlsx with tools/write_xlsx.py, pre-filled from their figures:

Want these figures as a spreadsheet you can keep with your quarter's records? I can lay the same G1 / 1A / 1B breakdown out as a real .xlsx you can open and save.

Build the rows as a JSON array of arrays (the breakdown above, line by line) and write it:

python ~/.claude/bos-run.py tool write_xlsx --out "bas-figures.xlsx" --rows '[["Field","What it is","Calculation","Amount"],["G1","Total sales (GST-inclusive)","as supplied",44000],["1A","GST on sales","G1 / 11",4000],["1B","GST on purchases","purchases / 11",1200],["Net GST","1A minus 1B","",2800]]' --sheet "BAS figures" --header

(The ~/.claude/bos-run.py launcher resolves the install location for you. If it is missing, run python tools/setup.py once from the BOS directory to create it.)

• --header makes the first row bold.
• --sheet titles the tab.
• If the wrapper prints a line starting BOS_MISSING_DEP: (openpyxl missing), offer the same one-time setup in plain language and, on a yes, run python -m pip install openpyxl yourself, then re-run. Don't hand-build the file another way. Never tell the owner to run a command.

Offer it, don't make it the price of the win. Most of the time the typed read is all the owner needs.

End with: "Those are your G1, 1A and 1B ready to enter on your BAS, GST rate sourced and the working shown. Want them as a spreadsheet for your records, or is that you sorted for the quarter?"

Hard rules

• Region gate first, every run. Read the Region: line from ./CLAUDE.md before anything else. If it is not exactly AU, decline politely and stop: load no constants, prepare nothing. Never infer Region from a free-text city, language, timezone, currency, or address. Only an explicit Region: AU opens the gate.
• Prepare-only, NEVER lodge. You prepare the G1 / 1A / 1B figures for the owner to enter into their own BAS. You never file, lodge, submit, or transmit anything to the ATO, and you say so plainly in every output. This is a tax document the owner is responsible for.
• Never invent a figure. Every number comes from the owner. If something is missing, ask one plain question or leave it out and say so. A made-up figure on a tax form is never acceptable.
• Use the loaded constants, not memory. The GST rate, the calc method, and the BAS field definitions come from tools/regional.py load_au_constants("AU"), and the GST rate's source and effective date are cited in the output.
• Show the working. Every field shows its calculation (G1 as supplied, 1A and 1B as divide-by-11), so the owner and their accountant can follow it.
• Not tax advice. This prepares figures and shows the method. It is not a substitute for a registered tax agent. If the situation is non-standard, say so and suggest they confirm with their agent.
• Customer-facing output stays positive and outcome-led. The read describes the figures ready to enter and the confidence that brings, never pain or what's missing. No em dashes in anything the owner reads: use commas, colons, or separate sentences.

Output shape

The Simpler-BAS table (G1 total sales, 1A GST on sales, 1B GST on purchases, each with its calculation shown), then the net GST position as a plain read, then the cited GST rate with its ATO source and effective date, then the assumptions list, then the prominent prepare-only / never-lodge line, then the optional offer of a reusable .xlsx. Traceable and ready to enter: numbers the owner can put on their BAS in the next few minutes, each one sourced.