How do I calculate mortgage repayments?

Monthly repayment = P × [r(1+r)^n] / [(1+r)^n − 1], where P is the loan amount, r is the monthly interest rate, and n is the number of monthly payments. Use our free mortgage calculator to get instant results.

How do I calculate BMI?

BMI = weight (kg) ÷ height (m)². A BMI under 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, and 30+ is obese according to WHO guidelines.

How do I calculate GST in Australia?

To add GST: multiply the price by 1.10. To remove GST from a price: divide by 1.10. Australia's GST rate is 10%.

How do I calculate compound interest?

A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate as a decimal, n is how many times interest compounds per year, and t is the number of years.

How do I calculate a percentage?

To find X% of a number: divide X by 100, then multiply by the number. For example, 20% of 350 = (20 ÷ 100) × 350 = 70.

How do I calculate the area of a circle?

Area = π × r², where r is the radius. For example, a circle with radius 5 has an area of π × 25 ≈ 78.54 square units.

How do I calculate loan repayments?

Monthly payment = P × r(1+r)^n / [(1+r)^n − 1], where P is the loan amount, r is the monthly interest rate, and n is the total number of payments. Our loan calculator does this instantly.

How do I calculate BMR?

Using the Mifflin-St Jeor equation: BMR (male) = 10W + 6.25H − 5A + 5. BMR (female) = 10W + 6.25H − 5A − 161. Where W is weight in kg, H is height in cm, and A is age in years.

How do I use the Pythagorean theorem?

a² + b² = c², where c is the hypotenuse (longest side). To find the hypotenuse: c = √(a² + b²). To find a missing side: a = √(c² − b²).

How do I calculate fuel efficiency in Australia?

L/100km = (litres used ÷ distance driven) × 100. For example, 40 litres over 500 km = (40 ÷ 500) × 100 = 8 L/100km.

25+ calculators · 7 categories

A calculator for everything.

Mortgages, triangles, calories, percentages — all in one place. Each calculator shows you the working so you actually understand the result.

7

Geometry

Area, volume & perimeter

5

Finance

Interest, mortgage & tax

5

Everyday

Units, BMI & tip splitting

3

Math

Percentages & Pythagoras

4

Health

Calories, BMR & heart rate

4

Science

Physics, chemistry & more

3

Date & Time

Age, duration & time zones

Geometry

Area, volume, and perimeter of 2D and 3D shapes — with step-by-step explanations.

Select a shape

Area

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Select a 3D shape

Volume

—

Select a shape

Perimeter

—

Finance

Interest, mortgages, tax, GST, investment returns and loan calculators.

Compound interest

Principal ($)

Annual rate (%)

Time (years)

Compounds per year

Final amount

—

Interest earned

—

Total growth

—

How compound interest works

Unlike simple interest, compound interest earns returns on both your original principal and all previously earned interest. The longer the time horizon, the more dramatic the snowball effect.

A = P(1 + r/n)^(nt)

1

P = Principal (starting amount)

2

r = Annual rate as a decimal (e.g. 5% → 0.05)

3

n = Times compounded per year (monthly = 12)

4

t = Number of years invested

Example: $10,000 at 5% for 10 years monthly → $16,470

💡 Tip: Monthly compounding earns more than annual — same rate, more frequent calculations = more interest on interest.

Simple interest

Principal ($)

Rate (% / year)

Time (years)

Interest

—

Total

—

Simple interest explained

I = P × r × t

Interest is calculated only on the original principal — it does not compound. Used for short-term loans, some bonds, and car financing.

Example: $5,000 at 6% for 3 years → Interest = $5,000 × 0.06 × 3 = $900

💡 Tip: Simple interest is always lower than compound interest for the same rate and period. Prefer simple interest when borrowing; prefer compound when investing.

Mortgage repayment

Loan amount ($)

Annual interest (%)

Loan term (years)

Monthly payment

—

Total repaid

—

Total interest

—

How mortgage repayments work

Each payment is split between interest (charged on the remaining balance) and principal (what you borrowed). Early payments are mostly interest; later payments reduce more principal — this is called amortisation.

M = P[r(1+r)ⁿ] / [(1+r)ⁿ − 1]

1

Bank charges interest monthly on your remaining balance

2

Your fixed payment covers that interest first

3

Whatever remains reduces your principal

💡 Tip: Even one extra repayment per year can cut years off your loan and save tens of thousands in interest.

GST / Sales tax

Amount ($)

Tax rate (%)

Direction

Tax amount

—

Final amount

—

Adding vs removing GST

GST (Goods & Services Tax) is a consumption tax added to prices at point of sale. In Australia it's 10%; UK VAT is 20%; NZ GST is 15%.

1

Add tax: Price × (1 + rate) — e.g. $100 × 1.10 = $110

2

Remove tax: Price ÷ (1 + rate) — e.g. $110 ÷ 1.10 = $100

💡 Tip: Dividing by 1.10 (not subtracting 10%) is the correct way to find the pre-GST price — a common mistake!

Return on investment

Initial investment ($)

Final value ($)

ROI

—

Net gain

—

Understanding ROI

ROI = (Final − Initial) / Initial × 100

ROI tells you how much return you earned relative to what you invested. It's expressed as a percentage, making it easy to compare very different investments.

Example: Invest $10,000, end up with $15,000 → ROI = 50%

💡 Tip: ROI doesn't account for time — a 50% return over 10 years is far less impressive than 50% over 1 year. Use annualised ROI for fair comparisons.

Loan repayment

Loan amount ($)

Annual interest (%)

Term (years)

Monthly payment

—

Total repaid

—

Total interest

—

Personal loan calculator

Works for car loans, personal loans, and any fixed-rate instalment loan. The same formula as a mortgage — just typically over a shorter term.

M = P · r(1+r)ⁿ / [(1+r)ⁿ − 1]

1

Shorter term → higher monthly payments, but far less interest overall

2

Lower interest rate → dramatically lower total cost

💡 Tip: Even dropping your rate by 0.5% can save hundreds over the life of a personal loan. Always compare APR, not just the headline rate.

Everyday

Unit conversion, BMI, tipping, speed, and fuel efficiency calculators.

Unit converter

Math

Percentages, percentage change, Pythagorean theorem, and powers.

Percentage of a number

Percentage (%)

Of what number

Result

—

Percentages

Result = (% ÷ 100) × Number

Percent literally means "per hundred" — so 20% = 20 per 100 = 0.20 as a decimal. Multiply that decimal by your number to get the percentage of it.

1

Convert the % to a decimal: 20% → 0.20

2

Multiply: 0.20 × 350 = 70

20% of 350 = 70 · 15% of 80 = 12 · 8.5% of 200 = 17

💡 Mental maths: 10% is easy (move decimal left). Then halve for 5%, or double for 20%.

Percentage change

Original value

New value

Change

—

Direction

—

Percentage change

Change = (New − Old) / Old × 100

Shows how much a value shifted relative to its starting point. Positive = increase; negative = decrease.

1

Subtract the old value from the new: New − Old

2

Divide by the original: ÷ Old

3

Multiply by 100 to get a percentage

$200 → $250: change = (250−200)/200 × 100 = +25%

💡 Common error: Don't confuse percentage change with percentage points. An interest rate rising from 5% to 6% is a 1 percentage point increase, but a 20% relative increase.

Pythagorean theorem

Enter any two sides — the missing one will be calculated.

Side a

Side b

Hypotenuse c

Missing side

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The Pythagorean theorem

a² + b² = c²

In any right-angled triangle, the square of the longest side (hypotenuse) equals the sum of the squares of the other two sides. The hypotenuse is always opposite the right angle.

1

Find c: c = √(a² + b²)

2

Find a: a = √(c² − b²)

3

Find b: b = √(c² − a²)

The classic 3-4-5 triangle: 3² + 4² = 9 + 16 = 25 = 5²

💡 Real use: Carpenters use 3-4-5 to check if corners are square. Navigation, architecture, and GPS all rely on this theorem.

Health

BMI, BMR, calorie burn, heart rate zones, and body fat estimators.

Body Mass Index (BMI)

Weight (kg)

Height (cm)

BMI

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Science

Physics, chemistry, and engineering calculations with full formulas.

Newton's 2nd Law

Enter any two values — the third will be calculated.

Force (N)

Mass (kg)

Acceleration (m/s²)

Result

—

Newton's Second Law of Motion

F = m × a

Force (Newtons) = Mass (kg) × Acceleration (m/s²). This law explains why heavier objects need more force to accelerate at the same rate.

1

1 Newton = force to accelerate 1 kg at 1 m/s²

2

Earth's gravity = 9.81 m/s² — so a 70 kg person weighs 686 N

3

Leave one field blank — the calculator finds it from the other two

Push a 5 kg box at 2 m/s² → F = 5 × 2 = 10 N

💡 Tip: Weight is a force (W = mg), not a mass. Your "weight" in Newtons = your mass × 9.81.

Kinetic energy

Mass (kg)

Velocity (m/s)

Kinetic Energy (J)

—

Energy (kJ)

—

Kinetic energy formula

KE = ½ × m × v²

Kinetic energy is the energy an object possesses because of its motion. Crucially, it scales with the square of velocity — doubling speed quadruples kinetic energy.

1

60 km/h crash: 1,000 kg car has ~139 kJ of energy

2

120 km/h crash: same car has ~556 kJ — 4× more dangerous

1,000 kg at 27.8 m/s (100 km/h) → KE = 0.5 × 1000 × 27.8² ≈ 385 kJ

💡 Tip: This is why speed limits matter so much — a small increase in speed causes a large increase in crash energy and stopping distance.

Ohm's law

Enter any two values — the third will be calculated.

Voltage (V)

Current (A)

Resistance (Ω)

Result

—

Power (W)

—

Ohm's law

V = I × R

Voltage (V), Current (I), and Resistance (R) are the three fundamental quantities in electrical circuits. Know any two and you can find the third.

V

Voltage (Volts) — electrical pressure, like water pressure in a pipe

I

Current (Amps) — flow of electrons, like water volume flow rate

R

Resistance (Ohms Ω) — opposition to flow, like pipe narrowness

Power = V × I (Watts). A 60W bulb at 240V draws 0.25A

💡 Tip: Higher resistance = less current for the same voltage. This is why thin wires can overheat — high resistance = more heat generated (P = I²R).

Density, mass & volume

Enter any two values — the third will be calculated.

Density (kg/m³)

Mass (kg)

Volume (m³)

Result

—

Density formula

ρ = m ÷ V

Density is how much mass is packed into a given volume. Objects float when they're less dense than the fluid they're in — that's why ships float and ice cubes rise to the top.

💧

Water: 1,000 kg/m³ (the reference point)

🪵

Wood (oak): ~700 kg/m³ — floats!

🔩

Steel: ~7,800 kg/m³ — sinks

🌬️

Air: ~1.2 kg/m³

💡 Tip: A ship is mostly hollow air — its average density (total mass ÷ total volume including hull) is less than water, so it floats despite being made of steel.

Date & Time

Age, time between dates, day of week, and duration converters.

Age calculator

Date of birthDD / MM / YYYY

As of date (default: today)DD / MM / YYYY

Age

—

Total days

—

Next birthday in

—

Age calculation

Age is counted in complete years, months, and days elapsed since your birth date — accounting correctly for leap years and varying month lengths.

1

Enter your date of birth

2

Leave "As of date" blank to calculate age as of today

3

Or enter any past/future date to find age at that moment

💡 Fun fact: If you were born on 29 February (leap day), you only technically have a birthday every 4 years. The calculator handles this correctly.

Days between two dates

Start dateDD / MM / YYYY

End dateDD / MM / YYYY

Days

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Weeks

—

Months (approx)

—

Date differences explained

Counts exact calendar days between two dates. Days are the most precise unit; weeks, months, and years shown are approximations.

1

Order doesn't matter — the result is always positive

2

1 week = exactly 7 days; 1 month ≈ 30.44 days (average)

3

Use cases: project deadlines, lease lengths, time until an event

💡 Tip: For billing, check whether your contract counts the start day, end day, or both — "inclusive" vs "exclusive" counting can differ by 1 day.

Duration converter

Value

From

Time conversions

Everything converts through seconds — the base unit of time. All other units are just multiples of seconds.

1

1 minute = 60 seconds

2

1 hour = 3,600 seconds

3

1 day = 86,400 seconds

4

1 year = 31,536,000 seconds (365-day approximation)

💡 Note: Months (30-day) and years (365-day) are approximations. For calendar-accurate date math, use the "Days between dates" calculator above.