Math Formulas Every Adult Should Know (With Calculator Links)
The essential math formulas for everyday life — compound interest, percentages, area, BMI, and more. No textbook fluff, just what you actually use.
Most of the math you learned in school you'll never use again. But a handful of formulas come up constantly in adult life — buying a house, tracking fitness, understanding your salary, cooking for a crowd. Here are the ones worth keeping in your back pocket, with links to CalcHub tools that do the calculation for you.
Money Formulas
Compound Interest
A = P × (1 + r/n)^(n×t)P = principal, r = annual rate (as decimal), n = compounding periods per year, t = years.
This is the most important financial formula in existence. It tells you what your savings will grow to, what your debt will balloon to, and why starting to invest at 25 vs 35 makes such an enormous difference.
→ Compound Interest Calculator
Loan EMI (Equal Monthly Installment)
EMI = P × r × (1+r)^n / ((1+r)^n − 1)P = loan amount, r = monthly interest rate, n = number of months.
Every home loan, car loan, and personal loan uses this formula. Knowing it means you can verify what the bank tells you.
Percentage Change
(New − Old) / Old × 100Your salary went from ₹50,000 to ₹58,000? That's a 16% raise. Petrol went from ₹95 to ₹102? That's a 7.4% increase. You'll use this one weekly.
Tip/Discount
Tip = Total × (Tip% / 100) Discounted Price = Original × (1 − Discount% / 100)A 15% tip on ₹1,200? ₹180. A 30% discount on ₹4,500? You pay ₹3,150.
Health Formulas
BMI (Body Mass Index)
BMI = Weight (kg) / Height (m)²Under 18.5 = underweight, 18.5–24.9 = normal, 25–29.9 = overweight, 30+ = obese. It's imperfect (doesn't account for muscle mass) but it's a useful starting point.
Calories Burned Walking
Calories ≈ 0.035 × Weight (kg) × Time (min)Rough estimate — a 70 kg person walking for 30 minutes burns about 73 calories. The actual number depends on pace and terrain, but this gives you a baseline.
Heart Rate Zones
Max Heart Rate ≈ 220 − Age Target Zone = Max HR × (60% to 85%)For a 35-year-old: max HR ≈ 185, target zone is 111–157 bpm. Useful for anyone who exercises with a heart rate monitor.
Everyday Formulas
Area of a Room
Area = Length × Width (rectangle) Area = π × r² (circle)Buying carpet? Painting a wall? You need area. A 12ft × 14ft room is 168 sq ft. That's about 15.6 square meters.
Speed, Distance, Time
Distance = Speed × Time Time = Distance / Speed Speed = Distance / TimeDriving 250 km at 80 km/h? That's 3 hours 7 minutes. Simple but constantly useful for trip planning.
Cooking Scaling
New Quantity = Original × (Desired Servings / Original Servings)Recipe serves 4 but you need to feed 7? Multiply every ingredient by 1.75. This is just proportional reasoning, but having a calculator prevents errors when you're scaling multiple ingredients.
Fuel Cost for a Trip
Fuel Cost = (Distance / Mileage) × Price per LiterA 500 km trip in a car that does 15 km/l with fuel at ₹105/l costs about ₹3,500. Add 10% for city driving and AC usage.
Geometry You Actually Use
| Shape | Area Formula | When You Use It |
|---|---|---|
| Rectangle | L × W | Rooms, screens, plots |
| Triangle | ½ × base × height | Roofing, odd-shaped spaces |
| Circle | π × r² | Pools, round tables, pizzas |
| Cylinder volume | π × r² × h | Tanks, containers |
The One Formula That Matters Most
If you remember nothing else: compound interest. It governs your savings, your loans, your mortgage, your credit card debt, and your retirement. Understanding how (1 + r)^t works — that small percentages over long periods create exponential growth — is the single most financially valuable piece of math knowledge for everyday life.
Every formula on this page has a free calculator at CalcHub — no signup, no ads, instant results.
Related Guides
- How to Calculate Compound Interest — deep dive on compounding
- Understanding Amortization Schedules — loan payments explained
- Unit Conversion Cheat Sheet — quick reference for common conversions