Basic Maths Formulas Every Adult Should Know
School maths textbooks tend to disappear from our lives quickly, but the underlying formulas don't. Whether you're managing a budget, quoting a job, calculating tax, or working out how much tile to buy, these are the core formulas that come up again and again in real life.
This is your practical reference guide. Bookmark it, save it, and come back whenever you need it.
1. Percentage
Amount = (Percentage ÷ 100) × Number
Use it for: test scores, discounts, tax rates, pay rises, and statistics.
Example: You got 42 out of 60 → (42 ÷ 60) × 100 = 70%
Full guide: How to calculate percentage - a simple guide
2. Average (Mean)
Use it for: tracking monthly spending, comparing grades, sports statistics, and benchmarking.
Example: Weekly grocery bills of $120, $95, $140, and $110 → (120 + 95 + 140 + 110) ÷ 4 = $116.25 average
3. Profit and Loss
Profit % = (Profit ÷ Cost Price) × 100
Use it for: selling items online, pricing freelance work, and evaluating small business performance.
Example: You buy a second-hand item for $60 and sell it for $95 → Profit = $35, Profit % = 58.3%
4. Simple Interest
Where Rate is a decimal (5% = 0.05) and Time is in years.
Use it for: comparing savings accounts and quick loan estimates.
Example: $2,000 in savings at 4% for 2 years → 2000 × 0.04 × 2 = $160 interest earned
5. Compound Interest
Unlike simple interest, compound interest earns interest on interest, making it much more powerful over time.
Example: $2,000 at 4% compounded annually for 5 years → 2000 × (1.04)5 = $2,433.31
6. Area Formulas
- Rectangle: Area = Length × Width
- Triangle: Area = (Base × Height) ÷ 2
- Circle: Area = π × r² (π ≈ 3.14159)
Use it for: home renovations, garden beds, paint coverage, tiling, and flooring.
Example: A rectangular room 4.5m × 3.8m → Area = 17.1 square metres
7. Speed, Distance, and Time
- Speed = Distance ÷ Time
- Distance = Speed × Time
- Time = Distance ÷ Speed
Use it for: road trips, running pace, and shipping estimates.
Example: Driving 320km at 100km/h → Time = 320 ÷ 100 = 3.2 hours = 3 hours 12 minutes
8. GST and Tax Calculation
Removing GST: Original Price = Price incl. GST ÷ 1.10
Use it for: invoicing clients, quoting prices, and checking receipts.
Example: Service fee of $350 + GST → $350 × 1.10 = $385
9. Unit Price (Price Per Unit)
Use it for: grocery comparisons and wholesale buying decisions.
Example: 500g for $4.20 vs 750g for $5.85 → $4.20 ÷ 500 = $0.0084/g vs $5.85 ÷ 750 = $0.0078/g. The larger pack wins.
10. Tip Calculation
Example: $85 bill with 15% tip → 85 × 0.15 = $12.75 tip. Total = $97.75.
Full guide: How to split a bill with friends (and calculate tip)
A Note on Using These Formulas
These formulas are tools, not rules to stress over. The goal is to understand the logic well enough that you can adapt them to your situation. If the numbers are large or the steps are multiple, there is no shame in using a calculator. That's exactly what they're for.
The most important skill is knowing which formula to apply; the arithmetic itself can always be handed off to a calculator.
Related Reading
Before you go, these posts will help you put today's topic into practice:
- How to calculate percentage - a simple guide - a deep dive into the most commonly used formula.
- How to split a bill with friends (and calculate tip) - put formulas 1 and 10 into practice immediately.
- 5 everyday situations where a calculator saves you money - real-world applications for the formulas above.
Final Thoughts
Keep this guide somewhere you can find it. Maths doesn't get easier by avoiding it, but with the right formula and a reliable calculator, it does get faster.