GST, Budgeting and Household Expenses
Every time you buy something in Australia, 10% goes to the government as GST, but only on the pre-GST price, never on the GST-inclusive price again. Master the three GST calculations, build a budget across different time periods, and decode tiered utility bills.
Practise this lesson
Three printable worksheets that build from foundations to mastery, or build your own from any module’s questions.
Think First
Every time you buy something in Australia, a coffee, a pair of shoes, a phone, 10% of that price goes straight to the government as GST. But not everything attracts GST: fresh food, medical services, and some education costs are GST-free. If you've ever wondered why a supermarket receipt shows some items with GST and others without, this lesson explains the system. And when you move out of home, the bills don't stop at groceries, electricity, gas, water, council rates, insurance. How do people track all of this and make sure they don't spend more than they earn?
Learning Intentions
Key Terms
GST, Calculating In Both Directions
GST is a 10% tax added to the price of most goods and services in Australia, the key skill is moving confidently between pre-GST, GST-inclusive, and GST-component amounts.
There are three types of GST calculations you need to master:
- Adding GST: multiply the pre-GST price by 1.10. Example: $85 × 1.10 = $93.50.
- Finding the GST amount on a pre-GST price: multiply by 0.10. Example: $85 × 0.10 = $8.50.
- Removing GST from a GST-inclusive price: divide by 1.10. Example: $93.50 ÷ 1.10 = $85.00.
The ÷ 11 shortcut works because the GST component is 10/110 = 1/11 of the GST-inclusive price. Common GST-free items in Australia include: fresh food, most medical services, childcare, and some educational courses.
Three GST directions: Pre-GST → inclusive: ×1.10. Inclusive → pre-GST: ÷1.10. GST component from inclusive price: ÷11 (because GST = 10/110 = 1/11 of inclusive price). Never multiply an inclusive price by 0.10, that overcounts the tax.
Pause, copy the three GST rules: pre-GST → inclusive: ×1.10; inclusive → pre-GST: ÷1.10; GST component from inclusive price: ÷11 (not ×0.10, that overcounts because the price already includes the GST) into your book.
Budgeting, Surplus, Deficit and Balance
We just saw the three GST directions: pre-GST → inclusive (×1.10), inclusive → pre-GST (÷1.10), and GST component from an inclusive price (÷11, because GST = 10/110 = 1/11 of the inclusive price). That raises a question: GST is one single calculation, but in real life people need to track all income and all spending across a month or year to stay solvent. How do you test whether a budget is sustainable? This card answers it → a budget maps all income against all expenses in the same time period; surplus = income > expenses; deficit = expenses > income.
A budget is a plan that maps income against expenses over a defined time period, the goal is to ensure spending does not exceed earning.
A personal or household budget lists all income sources (wages, government payments, investment income) and all expenses (rent/mortgage, utilities, food, transport, entertainment, insurance) for a fixed period. The difference between total income and total expenses determines whether the budget is in:
- Surplus: income > expenses, money left over
- Deficit: expenses > income, spending more than earned
- Balanced: income = expenses
HSC questions often require you to convert all figures to the same time period before comparing. A common extension asks how to eliminate a deficit, either by increasing income or reducing specific expense categories.
Budget: surplus = income > expenses; deficit = expenses > income. Convert all figures to the same time period before summing. Annual surplus = monthly surplus × 12; weekly surplus = monthly surplus × 12 ÷ 52. A sustained deficit accumulates as debt.
Pause, copy the budget surplus/deficit formula (surplus = total income − total expenses when positive; deficit when negative), the time-period conversion rules (weekly × 52 = annual; monthly × 12 = annual), and the warning to convert all figures to the same period before summing into your book.
Household Expenses, Utilities, Rates and Bills
We just saw budget surplus and deficit, and that all figures must be converted to the same time period before comparing. That raises a question: utility bills like electricity aren't a single flat amount, they have an internal structure. How do you calculate the total for a bill that charges both a fixed supply fee and a per-unit usage fee? This card answers it → utility bill total = supply charge (daily rate × days) + Tier 1 usage (first N units × rate 1) + Tier 2 usage ((total − N) units × rate 2).
Household bills follow predictable mathematical structures, unit rates, fixed charges, tiered pricing, that require careful reading before any calculation.
Common household expense calculations in HSC questions include:
- Electricity bills: fixed supply charge per day + variable usage charge per kWh
- Water bills: fixed service charge + usage charge per kilolitre, sometimes tiered
- Council rates: usually a fixed annual amount calculated as a percentage of land value
- Insurance premiums: annual or monthly, sometimes with discounts for upfront payment
Tiered pricing means the first X units are charged at one rate, and additional units at a higher rate, this is structurally identical to tiered commission from earlier lessons.
Utility bill formula: Total = supply charge (daily rate × days) + Tier 1 usage (first N units × rate1) + Tier 2 usage ((total − N) × rate2). GST-inclusive total = pre-GST total × 1.10. Never omit the fixed daily supply charge.
Pause, copy the utility bill formula: Total = supply charge (daily rate × days) + Tier 1 usage (first N units × rate 1) + Tier 2 usage ((total used − N) × rate 2), and the final GST step if prices are given excluding GST into your book.
Choosing the Right Operation Quickly
We just saw utility bills combining a fixed daily supply charge and tiered usage rates, the most common error is forgetting the supply charge. That raises a question: an exam question mixes GST-inclusive prices, budget time periods, and bill tiers, how do you instantly choose the right operation before calculating? This card answers it → label each amount first ("inclusive price", "monthly expense", "daily supply charge") and use the decision table to map each label to the correct first operation.
Many mistakes in this topic happen before the arithmetic starts. If you identify the type of amount first, the correct operation usually becomes obvious.
| If the question says… | You likely need… |
|---|---|
| "before GST" or "exclusive of GST" | Multiply by 1.10 to get the GST-inclusive price |
| "including GST" or "GST-inclusive" | Divide by 1.10 to get the pre-GST price |
| "GST component" from an inclusive price | Divide by 11 |
| "weekly, monthly and annual figures mixed" | Convert everything to one common time period first |
| "daily charge plus usage charge" | Calculate both parts, then add |
| "first X units at one rate, remainder at another" | Split the usage into tiers before multiplying |
Identify the amount type before operating: 'pre-GST' → ×1.10 to add; 'inclusive' → ÷1.10 to remove or ÷11 for component; 'weekly/monthly/annual mixed' → convert to one period first; 'fixed + usage' → calculate both parts then add. Label amounts before calculating.
Pause, copy the amount-type decision table: "pre-GST" → ×1.10; "GST-inclusive" → ÷1.10 or ÷11; "weekly/monthly/annual mixed" → convert to one period first; "fixed supply charge + usage" → calculate both parts then add into your book.
GST calculations, both directions
(a) A plumber charges $340 before GST. What is the GST-inclusive price?
(b) A laptop is advertised at $1,628 including GST. What is the pre-GST price and the GST component?
$$\text{GST-inclusive price} = \$340 \times 1.10 = \$374.00$$
Multiply pre-GST price by 1.10 to add 10% GST in one step.
$$\text{Pre-GST price} = \$1{,}628 \div 1.10 = \$1{,}480.00$$
The price of $1,628 already includes GST. Divide by 1.10 to remove it.
$$\text{GST component} = \$1{,}628 \div 11 = \$148.00$$
Use ÷11 to find the GST component from a GST-inclusive price.
$$\$1{,}480 + \$148 = \$1{,}628\ \checkmark$$
Pre-GST + GST component = GST-inclusive price. The check confirms both values.
Budget construction, surplus and annual position
The Chen family has the following monthly finances. Income: wages $5,840, rental income $620. Expenses: mortgage $2,100, groceries $780, utilities $310, transport $420, insurance $195, entertainment $380, clothing $150.
Calculate their monthly surplus or deficit and their annual position.
$$\text{Total monthly income} = \$5{,}840 + \$620 = \$6{,}460$$
Sum all income sources for the month.
$$\text{Total monthly expenses} = \$2{,}100 + \$780 + \$310 + \$420 + \$195 + \$380 + \$150 = \$4{,}335$$
Sum all expense categories systematically to avoid missing any.
$$\text{Monthly surplus} = \$6{,}460 - \$4{,}335 = \$2{,}125$$
Income exceeds expenses → surplus. The family is living within their means.
$$\text{Annual surplus} = \$2{,}125 \times 12 = \$25{,}500$$
Multiply monthly surplus by 12 months to find the annual figure.
Electricity bill, tiered pricing and GST
An electricity bill shows: supply charge $1.04 per day for 90 days; usage charge, first 500 kWh at 28.6c/kWh, remaining usage at 34.2c/kWh. The household used 720 kWh over the 90-day period.
Calculate the total bill (excluding GST), then find the GST-inclusive total.
$$\text{Supply charge} = \$1.04 \times 90 = \$93.60$$
Fixed daily charge × number of days. This applies regardless of electricity used.
$$\text{Tier 1 usage (first 500 kWh)} = 500 \times \$0.286 = \$143.00$$
The first 500 kWh are charged at the lower rate of 28.6c = $0.286 per kWh.
$$\text{Tier 2 usage (remaining)} = (720 - 500) \times \$0.342 = 220 \times \$0.342 = \$75.24$$
Total usage (720) minus first tier (500) = 220 kWh at the higher rate.
$$\text{Total before GST} = \$93.60 + \$143.00 + \$75.24 = \$311.84$$
Sum all three components: supply charge + tier 1 usage + tier 2 usage.
$$\text{GST} = \$311.84 \times 0.10 = \$31.18$$
GST is 10% of the pre-GST total. Electricity bills attract GST in Australia.
$$\text{GST-inclusive total} = \$311.84 + \$31.18 = \$343.02$$
Or equivalently: $311.84 × 1.10 = $343.024 ≈ $343.02.
Budget comparison after converting time periods
A student has weekly income from casual work of $420 and receives a monthly allowance of $260 from home. Their expenses are: rent $180 per week, groceries $95 per week, phone $38 per month, transport $42 per week, and insurance $624 per year.
Convert everything to a weekly budget and determine whether the student has a weekly surplus or deficit.
$$\text{Weekly allowance} = \$260 \times 12 \div 52 = \$60.00$$
Convert the monthly amount to annual, then divide by 52 to get a weekly figure.
$$\text{Total weekly income} = \$420 + \$60 = \$480.00$$
Both income figures are now in the same weekly time period.
$$\text{Weekly phone cost} = \$38 \times 12 \div 52 \approx \$8.77$$
$$\text{Weekly insurance cost} = \$624 \div 52 = \$12.00$$
Convert monthly and annual expenses to weekly amounts.
$$\text{Total weekly expenses} = 180 + 95 + 42 + 8.77 + 12 = \$337.77$$
Sum all expenses now expressed in weekly terms.
$$\text{Weekly surplus} = \$480.00 - \$337.77 = \$142.23$$
Income is greater than expenses, so the student has a weekly surplus of $142.23.
Multiple Choice
5 random questions from a replayable lesson bank, feedback shown immediately
Short Answer Questions
SAQ 1. A tradesperson quotes $1,760 including GST for a job. Calculate the pre-GST price and the GST component.
Show sample solution
Pre-GST price: $1,760 ÷ 1.10 = $1,600.00
GST component: $1,760 ÷ 11 = $160.00
Check: $1,600 + $160 = $1,760 ✓
SAQ 2. A household has fortnightly income of $2,240. Their fortnightly expenses are rent $820, groceries $310, transport $145, and phone $96 per month. Determine whether they have a fortnightly surplus or deficit.
Show sample solution
Fortnightly phone cost: $96 × 12 ÷ 26 = $44.31
Total fortnightly expenses: $820 + $310 + $145 + $44.31 = $1,319.31
Fortnightly surplus: $2,240 − $1,319.31 = $920.69 surplus
SAQ 3. An electricity bill has a supply charge of $0.98 per day for 91 days and a usage charge of 31.5c per kWh. The household used 540 kWh. Calculate the total bill before GST and the GST-inclusive total.
Show sample solution
Supply charge: $0.98 × 91 = $89.18
Usage charge: 540 × $0.315 = $170.10
Total before GST: $89.18 + $170.10 = $259.28
GST-inclusive total: $259.28 × 1.10 = $285.21
Revisit Your Initial Thinking
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Boss Battle
Challenge the boss using your GST and budgeting skills. Pool: Module 3 lessons 1–9.