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
Categories
25+
Calculators
∞
Calculations
Basic calculator
0
All categories
3

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
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
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
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
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
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
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
Result
—
Tip & bill splitter
Per person
—
Total bill
—
Tip amount
—
Tipping guide by country
Tipping culture varies widely around the world. Here's a quick reference:
🇦🇺
Australia: 10–15% is appreciated, never expected
🇺🇸
USA: 18–20% is standard, below 15% considered rude
🇯🇵
Japan: Tipping is considered impolite
🇬🇧
UK: 10–12.5%, check if service charge is already included
💡 Tip: An easy mental trick — 10% is just moving the decimal. Double it for 20%.
Speed, distance & time

Leave one field blank — it will be calculated from the other two.

Result
—
Speed, distance & time
Speed = Distance ÷ Time
These three quantities form a triangle — know any two and you can find the third. Rearrange the formula depending on what's missing:
1
Speed = Distance ÷ Time (km/h)
2
Distance = Speed × Time (km)
3
Time = Distance ÷ Speed (hours)
A 480 km trip at 100 km/h takes 4.8 hours (4h 48min)
💡 Tip: Enter time in decimal hours. 1h 30min = 1.5, not 1.3.
Fuel efficiency & cost
L/100km
—
Cost per 100km
—
Total fuel cost
—
Fuel efficiency explained
L/100km = (Litres ÷ Distance) × 100
Lower L/100km = more efficient. Here's how common vehicles compare:
⚡
Electric vehicle: 0 L/100km (use kWh/100km instead)
🌿
Hybrid car: ~4–5 L/100km
🚗
Small petrol car: 6–8 L/100km
🚙
Large SUV / 4WD: 10–15 L/100km
💡 Tip: Tyre pressure, speed, and A/C use can change your efficiency by up to 20%.
Discount & savings calculator
Sale price
—
You save
—
Calculating discounts
Sale price = Original × (1 − Discount% ÷ 100)
Discounts reduce the price by a percentage of the original. When multiple discounts are stacked, they apply sequentially — not additively.
30% off $200 → $200 × 0.70 = $140 (save $60)
Two 20% discounts stacked → not 40% off! It's $200 × 0.80 × 0.80 = $128 (36% off)
💡 Tip: "Buy one get one 50% off" is only 25% savings on the pair — compare per-unit cost, not headline discount.

Math

Percentages, percentage change, Pythagorean theorem, and powers.

Percentage of a 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
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.

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)
BMI
—
Category
—
BMI categories (WHO)
BMI = weight(kg) ÷ height(m)²
1
Below 18.5 — Underweight
2
18.5 – 24.9 — Normal weight ✓
3
25.0 – 29.9 — Overweight
4
30+ — Obese
⚠️ Limitation: BMI doesn't distinguish between muscle and fat, or account for age, sex, or ethnicity. A muscular athlete may be "overweight" by BMI but be very healthy. Use it as one data point, not a diagnosis.
Basal Metabolic Rate (BMR)
BMR (cal/day)
—
TDEE (maintenance)
—
To lose 0.5kg/wk
—
Mifflin-St Jeor equation
Male: 10W + 6.25H − 5A + 5
Female: 10W + 6.25H − 5A − 161
BMR (Basal Metabolic Rate) is the calories your body burns just to stay alive — breathing, circulating blood, brain function. You'd burn this many calories even if you lay still all day.
1
BMR = your bare minimum calorie need at rest
2
TDEE = BMR × activity multiplier (your actual daily need)
3
Deficit = Eat ~500 cal/day below TDEE to lose ~0.5kg/week
💡 Tip: TDEE is the number to focus on for weight management — it's BMR adjusted for your lifestyle. Eating at TDEE = weight maintenance.
Heart rate training zones
Max HR
—
Heart rate training zones (Karvonen method)
Target HR = ((Max − Rest) × intensity%) + Rest
This uses the Karvonen formula, which accounts for your resting heart rate for more personalised zones. Max HR is estimated as 220 − age.
1
Zone 1 (50–60%): Active recovery, warm-up
2
Zone 2 (60–70%): Fat burning, aerobic base — conversational pace
3
Zone 3 (70–80%): Aerobic endurance — moderate effort
4
Zone 4 (80–90%): Lactate threshold — hard but sustainable
5
Zone 5 (90–100%): Maximum effort — only for seconds
💡 Tip: A lower resting HR generally means better cardiovascular fitness. Elite athletes often rest at 40–50 bpm.
Exercise calorie burn
Calories burned
—
Cal per minute
—
MET-based calorie estimation
Calories = MET × weight(kg) × time(hours)
MET (Metabolic Equivalent of Task) compares the energy cost of an activity to sitting at rest (MET = 1). Running at MET 8 burns 8× more energy than rest.
1
Heavier people burn more calories doing the same activity
2
Higher MET = more intense = more calories per minute
3
These are estimates — actual burn varies by fitness level and form
💡 Tip: Exercise machines often overestimate calorie burn by 20–30%. Use these numbers as a guide, not an exact budget.

Science

Physics, chemistry, and engineering calculations with full formulas.

Newton's 2nd Law

Enter any two values — the third will be calculated.

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
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.

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.

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
DD / MM / YYYY
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
DD / MM / YYYY
DD / MM / YYYY
Days
—
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
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.